From energy

The view from the roof of the Minneapolis Convention Center, which holds a 600 kW solar PV system

How much energy could Minneapolis get from solar?

Solar PV seems to be the current darling of the renewable energy world.  But how much “resource” is really out there?  How much should cities rely on the development of local solar resources to meet their climate and energy goals?  What trade-offs should urban cities make between desirable things like tree canopy and maximizing solar energy resources?  GIS tools and new data resources can help begin to answer that question.

Counties and states are beginning to produce LiDAR data more regularly, which provides the building block information needed to analyze solar resources on buildings and elsewhere (see my previous post for a brief intro to LiDAR, or see here).  Minnesota happens to have LiDAR for the whole state, and Minneapolis has a climate action goal that references local renewable development, so I’ll focus there.

So how much solar electric potential does Minneapolis have?  Enough to supply 773,000 megawatt-hours (MWHs) each year, at the upper bound.  That would mean covering every piece of rooftop with good sun exposure and appropriate pitch (southeast to southwest facing or flat) with the best modern PV panels.  It would also mean solar installations on 68,351 structures, consisting of over 2.3 million individual panels.

773,000 MWhs would represent about 18% of Minneapolis’ total annual electricity consumption (based on 2010 figures).  It would be the equivalent of reducing 392,684 metric tons of CO2 (also based on 2010 figures), which is equal to the emissions from the energy usage of almost 36,000 average American homes each year.

Explore the results for individual buildings in Minneapolis.

There are some limitations to this calculation, and some additional interesting findings, but first a brief description of how I came up with these numbers.

Methodology

Annual solar insolation values shown in a black-white color ramp.
Annual solar insolation values in the Wedge neighborhood shown in a black-white color ramp.

I briefly covered how to calculate solar potential in a previous post, and the process for this analysis was similar.  I was able to get my hands on the solar insolation raster for the whole city thanks to the excellent work of some students in Dr. Elizabeth Wilson’s capstone class at the University of Minnesota’s Humphrey School.  Solar insolation represents a measure of the total energy from the sun reaching any particular point (each square meter in this case) on a building, tree, earth, etc.  To calculate this, ArcGIS has a complex tool called Solar Radiation Analysis.  It takes in to account things like how trees shade buildings, and how the sun moves across the sky at different times of year based on the latitude of a particular point on earth.  It spits out a measure of solar energy hitting that location over the course of a year,  measured in watt-hours per square meter. This gives you a good idea of where exactly on each building a suitable spot might be for a solar PV system.

LiDAR data can also be used to calculate the slope of roofs, another important piece of information to understand solar potential.  This allows a user to pick out areas of flat or south-facing roofs.

Red hatched areas represent areas of rooftop that are good for solar
Red hatched areas represent areas of rooftop that are good for solar

Finally, Minneapolis supplies building footprints, so I knew approximately what was a roof. I confined my analysis to building roofs, assuming we don’t want any of our precious open space filled with solar panels.  I also buffered the roof edges, since I’m told OSHA requires some open space between the panels and the roof edge for safety, at least for flat roofs.  I also considered 1,000 watts to be the minimum size that would warrant an installer to climb onto a roof.

Combine all this with some assumptions about the space needed for installations on flat and sloped roofs (the students helped with that too) and information on the size and power output of panels, and you get a measurement of the total “good” roof area and associated potential energy production from each roof.

That’s enough how-to, here are more interesting findings.

Findings

The 100 buildings (0.14 percent of the total building with solar) with the largest solar potential would provide 14 percent of the total production, or over 109,000 MWhs annually. The 1,000 buildings (1.4 percent of the total buildings with solar) with the largest solar potential would provide 43 percent of the total production, or over 333,000 MWhs annually. Targeting these structures for further analysis and possibly incentives would probably make sense to achieve the largest economies of scale for installation costs.

Buildings symbolized by their total solar energy potential
Buildings symbolized by their total solar energy potential – warmer colors represent higher potential

The 100 highest-potential buildings are geographically concentrated in roughly three areas: the northeast industrial area – roughly north and east of the U of M campus, the Lake Street/Greenway Corridor, and extending from the North Loop along the river into north and northeast Minneapolis.  Unsurprisingly, these are areas that still have many large, flat-roofed warehouse and industrial buildings.  If Minneapolis wants to maximize its solar resource, we may want to think about the trade-offs in redeveloping these areas or developing high density near them that may shade existing rooftops.

Commercial, industrial and single-family residential structures (based on parcel data) each account for almost exactly 23% of the total roof-top solar potential in the city.  The next largest potential was among apartment properties at 9%, and duplexes at 7%.  While the top three were evenly split potential-wise, single-family residences with good solar potential included over 46,000 structures, while commercial and industrial together was about 4,300.  See economies of scale note above.

The fact that 46,000 residential structures have good solar potential means that lots of homeowners, even in leafy Minneapolis, could be empowered to go solar.  This would be a more powerful political constituency than a small number of commercial property owners.  Obviously some would face the trade-off between more trees and their benefits and electricity from solar.

Suburban areas are much more likely to approach energy production equal to energy usage.  With its high density commercial core, Minneapolis uses a lot more energy than it can produce on its roofs.  Residential structures are also smaller and more shaded than many suburban areas.  This isn’t necessarily a bad thing, as density brings many other environmental benefits, like the ability to use transit cost-effectively.

Limitations

Xcel Energy limits the size of solar installations they allow to be connected to their system.  An interconnected solar PV system cannot be designed to produce more than 120 percent of the customer’s total usage from the previous year.  Many homes in Minneapolis, and possibly low-energy warehouse buildings, could accommodate systems larger than that.  This analysis limited system size only based on roof/sun conditions, and not electricity usage in the structure since that wasn’t known.  In some cases, this means this analysis over-represents solar potential.

This analysis includes no information on roof age or structural integrity.  Some flat-roofed buildings aren’t structurally able to accommodate solar without expensive retrofits.  Residential structures may need to have old roofs replaced before putting on a solar energy system (which are typically designed to last 20 years).  Some structures, like parking ramps and stadiums, would require additional structural supports to be added before a solar energy system could be added.  These factors could all further limit solar potential on Minneapolis buildings.

There was a geometry problem I couldn’t solve in GIS.  While I could calculate the size of a roof area that got good sun and had the correct slope, I couldn’t quickly figure out how many solar panels of a certain shape (defined length and width) fit in that area.  I only used total square footage divided by the square footage of a standard solar panel.  Internet forums are filled with many people better at GIS than I discussing this problem (but not providing me with easy solutions).  If anyone reading this wants to take a crack at it, let me know in the comments.

100 days of solar

100dayssolar
Electricity production in the first 100 days

Today I noticed that my solar charge controller has been running for 100 days (it logs this among many other data points).  Here are some highlights from the first 100 days:

  • The system has produced 32 kWhs from two 100-watt panels.  This is roughly 2% of the total electricity consumption we saw over the same period last year.
  • Converting from DC current to AC current at low wattages is wildly inefficient.  I usually run the wifi router and cable modem continuously off the battery and I lose about 40% of my produced energy to the inverter.  It is much happier running closer to its peak (1000 watts).  We should probably convert to DC.
  • Something happened to my charge controller settings when I converted to 24 volts.  Although the controller was still charging, I lost about 10 days worth of data (hence the gap in the chart) and wasn’t able to communicate with it over that time.  A firmware reboot fixed this.
  • Although very cold, clear days are when the panels perform their best, the sun just doesn’t shine for that long each day in January and February in Minnesota.  The panels being on the ground doesn’t help either.  Just from the middle of March to the middle of April I’ve about doubled my daily output.
  • All that said, this chart doesn’t really show total potential of the panels on a given day.  If I didn’t use much of the battery the day before, panel production the next day was curtailed by the controller to avoid overcharging the battery.  I’m trying to match the loads I put on the battery with the “capacity” of the season, but that’s sometimes tricky.
  • I recently learned we were accepted into the Minnesota solar rebate program for 2014!  So with the help of a friendly solar installer, we should have a 2.8 kW grid-tied system installed sometime this year. Along with the grid-tied panels, the installer will be adding two panels on the roof dedicated to battery charging.  Now I just have to wait…
Solar insolation in January

Mapping Minnesota’s solar resource

Boston, New York City, Denver, Cambridge and other cities have created solar potential maps to help their residents understand that solar photovoltaic systems are viable in dense urban areas, and to demonstrate the potential that exists on rooftops.

Of course, I had to try this myself.

Minnesota produces LiDAR data, which is basically micro-scale elevation data produced by flying a plane back and forth in a grid and shooting the ground with lasers a bajillion times.  Skilled/obsessive GIS users can clean from this data information that can be used to make a fairly accurate model of everything on the ground (buildings, trees, etc).  GIS software also makes it easy to produce daily, monthly or annual solar insolation maps.  By taking the position of the buildings and trees, knowing the latitude, and projecting how the sun moves across the sky throughout the year, the software calculates a total amount of solar radiation that will hit a point after shading, angle and other factors are taken into account.

KingfieldSolarAfter much tinkering, the Kingfield Solar Energy Potential map was born.  The extreme density of the LiDAR data limits how large an area I could process (there were 4.9 million individual data points in this one small section of Minneapolis), but you get the idea.  This map shows the area of each roof that might be appropriate for solar, how many panels could fit in that area, and an estimate of the annual production from those panels.

Some roofs are wholly inappropriate for solar, whether due to tree or building shading, orientation or size.  But there is significant potential.  If solar was installed on every appropriate piece of roof in this one-quarter square mile area, it would produce an estimated 2.2 megawatt hours of electricity each year, and avoid 2.9 million pounds of carbon dioxide emissions.

DSC02868

Counting every watt hour

This little device is an ethernet to wi-fi adapter. It connects my solar charge controller to my home wi-fi network so I can make fancy graphs.  It uses 1.2 watts per hour.  I know this because I measured its usage using a watt meter.  I do this with everything I power from the solar batteries.

I have a hunch that this is what solar does to you, makes you compulsive about energy use.  Even if (when?) I have a large grid-tied system, I imagine myself checking the daily output, and constantly thinking about how to reduce my usage to match.

On very cloudy days, this little thing has used over 45% of the energy produced by the panels.  I unplugged it.  For now, graphs only on special occasions.

 

Home solar: adding MPPT and marvelous data

MPPT charge controller switched on!
MPPT charge controller switched on!

As part of my plan for the eventual expansion of my off-grid solar energy system, I recently added a new charge controller with Maximum Power Point Tracking (MPPT).  Besides being much more efficient, this controller is capable of producing reams and reams of wondrous data, and is network-connected, meaning I can geek out on battery voltage and array current from anywhere in the house!  The charge controller I had was great, but it wouldn’t handle anything beyond a few more small panels.  Now I should be able to go all the way up to 750 watts of panels (my goal).  So, thanks Santa!

While installing the controller, I also took the opportunity to install a breaker box, which should bring me closer to code, and upgrade to larger diameter battery cable, which should reduce efficiency losses.

The MPPT advantage

MPPT is a fancy way of saying the charge controller is able to send significantly more energy to the batteries from the same panels.  How much more? After only a few days of testing, I estimate 40 – 60% more than the Pulse-Width Modulation (PWM) controller on days when the battery is low.  (If you want to know the details of how MPPT works, I found this explanation helpful.)

Here’s some actual data from my system which I think illustrates the MPPT advantage well:

1-5 plain graph

The blue line is the amps, or current, coming from the panels.  The red line shows the amps the controller is putting in to the battery.  It’s higher!  The magical MPPT doohicky converts excess voltage into amperage (remember, amps X volts = watts) so less of your panel’s potential is wasted.  On this particular day, I estimate the charge controller may have been able to wring an extra 100 – 150 watt-hours from the panels.

There are other interesting things going on here, so here’s a little annotation:

1-5 annotated graph

Here’s the next day, when the battery starts out the day almost totally full.  It was very sunny.

1-6 amps graph

The controller limits the array current and current to the battery significantly because the battery is almost fulled charged.  The gentle downward slope in the amperage is a function of battery charging called absorption.  Less current is pushed into the battery as it reaches capacity.

I can track hundreds of days of watt-hour production, so I’ll do another update when I can show some seasonal changes.  How I yearn for the days when the panels get more than 4 hours of sun per day!

Xcel Energy: social cost of carbon is $21 per ton

Old news, but still worth posting. In October, Xcel Energy filed a report with the Public Utilities Commission defending the cost overruns of upgrading the nuclear power plant in Monticello. Via the Star Tribune:

Xcel filed the report in response to the state Public Utilities Commission’s pledge in August to investigate the Monticello investment. The company said that even with the cost overruns, the project benefits customers — saving an estimated $174 million through the remaining 16 years of its license.

Yet that cost-benefit number relies on a “social cost” comparison between keeping the nuclear plant, which emits no greenhouse gases, vs. generating electricity from a plant that does emit them. State law says utility regulators should consider the cost of greenhouse gas emissions, though they’re not currently regulated. Without carbon-emissions savings, the Monticello upgrade would be a losing proposition, costing customers $303 million extra over its life, according to Xcel’s filing.

In interviews, Xcel executives defended the investment, saying they would make the same decision today, even though the utility world has changed since 2008, when the project began. Natural gas, now a favored fuel for power plants, is low-priced thanks to the fracking boom. And electricity demand has lagged since the recession, dampening the need for new plants.

“If we didn’t have our nuclear plants, we would be taking a big step backward in terms of our CO2 accomplishments,” said Laura McCarten, an Xcel regional vice president.

If you dig into the dockets (CI-13-754), you can find that Xcel’s modeling assumptions include a price on carbon of $21.50 per metric ton starting in 2017.

Regardless of your feelings about nuclear power, a utility stating that the externalities of carbon should be priced when making energy planning/financing decisions is significant. The use of a ‘social cost of carbon’ (SCC) metric at the federal level has (not shockingly) been the point of some contention.  The Office of Management and Budget’s SCC is $35/mt in 2015 versus Xcel’s $21 in 2017.

Theoretically, we should start to see this figure or something similar used in all future energy planning decisions (Sherco, cough, cough) in Minnesota.  Unless of course, Xcel was only being selective in order to justify recovering this very large expense (and spare the shareholders).

It would be an interesting exercise to apply this Minnesota SCC to land use and transportation infrastructure and planning decisions.

leverBigCorners

Planet levers we can pull at home

leverBigCorners

At Ensia, Jon Foley explores how we can break the cycle of climate inaction:

Frankly, we cannot afford to waste more time in a state of denial, saying that maybe this time our national leaders will wake up and take the problem seriously. We need to look for leadership and solutions elsewhere.

More importantly, we need to match our climate solutions to situations where leadership is still effective. We need to find targeted, strategic opportunities to reduce emissions, matching solutions to effective leadership.

But just where are those targeted opportunities?

In the search for effective climate solutions, we need to look for what I call planet levers: Places where relatively focused efforts, targeted the right way, can translate into big outcomes. Just like a real lever, the trick is to apply the right amount of force in just the right place, with little opposition.

In the search for planet levers to address climate change, we should look for ways to significantly cut emissions that don’t require grand policy solutions, such as carbon taxes or global cap-and-trade schemes, or the approval of the U.S. Congress or the United Nations. We need practical solutions to substantially cut emissions that work with a handful of nimble actors — including a few key nations, states, cities and companies — to get started.

Focusing on cities presents a particularly good set of levers to address climate change. Cities represent a nexus point of critical infrastructure — for electricity, communications, heating and cooling, and transportation — that are already in desperate need of improvement, and shifting them toward low-carbon “climate smart” technologies is a natural progression. Done right, most of these investments would improve the health, economic vitality, efficiency and livability of cities. Most important, most cities largely avoid the partisan gridlock of our national (and some state) governments, making them an excellent place for making progress.

I agree with Jon that cities are a good place to focus, not only because they have “functioning governments” that aren’t deadlocked, but because they have some key policy levers that can be pulled without a great deal of opposition, without getting a huge number of actors involved (creating potential for gridlock or slow movement), and that could have significant emissions impacts in a short time period.

Here are some of the local climate levers I think we can lean on locally, mostly at the city level.

Community choice aggregation (CCA)

The deregulation of electric utility markets is usually associated with some bad outcomes.  However, it can have positive benefits as well.  Since July of this year, over 58,000 residents and over 7,000 small business customers in Cleveland have received a 21% savings on their electricity bill AND received electricity from 100% green sources (50% wind, 50% hydro) through the Cleveland Municipal Aggregation Program.

This type of program is made possible by the fact that in deregulated electricity markets, cities can act as bulk purchasers for all or many of their community’s electrical customers.  This large buying power allows cities to negotiate good terms – like low rates and high renewable percentages.  These programs also don’t require the dismantling or purchasing of local investor-owned utilities.  Six states allow CCAs, and to date eight cities have used this authority to secure cleaner, more affordable power for their residents.  Most allow customers to opt-out and stay with their existing utility if they choose.

A program that requires electric customers to basically do nothing and could reduce the city’s greenhouse gas footprint 3% seems like a pretty good lever.

Note: state legislation is required to make CCA a reality.

Community solar (solar gardens)

Most people in Minnesota (some say only a third) have a roof that is good for collecting solar energy.  Shading, orientation, structural integrity, and ownership structure are just a few of the potential barriers to putting solar on roofs.  Matching the demand for solar with the supply of best locations, developed at a large scale for efficiencies, is something community solar or solar gardens can do.  These programs could be a powerful climate lever.  According to Midwest Energy News:

Minnesota's first community solar project in Rockford, MN. Image courtesy Wright-Hennepin Cooperative.
Minnesota’s first community solar project in Rockford, MN. Image courtesy Wright-Hennepin Cooperative.

The idea is to let customers who can’t or don’t want to install solar panels on their own rooftop instead buy individual panels in a nearby solar development. The electricity generated by a customer’s panels is credited to their utility bill as if they were installed on their home or business.

New legislation makes this possible in Minnesota.  In Colorado, where the program has been in place since 2012, 9 megawatts of solar was sold out in 30 minutes.   That’s roughly the equivalent of 3,000 single family home-sized systems.  Time will tell if this demand by project developers translates into strong demand by consumers.

Solar gardens generally require state policy change (except in the case of a municipal or cooperative utility), but don’t require thousands of people making individual installation decisions, hiring contractors, finding financing, etc.  A smaller number of experienced installers can do big projects with (theoretically) lower costs, supported by community interest.  Customers can buy-in to solar projects at whatever level they choose (usually bound by a minimum and maximum) but can skip all the installation headaches.

Capturing waste heat from the sewer

This one is my favorite.  There is a large supply of wasted heat flowing directly beneath our feet all day because we’ve literally flushed it down the drain.  One estimate says we’re flushing away 350 billion kWh of energy each year.  That’s more than 35 Minneapolis’ worth of energy every year.

Neighborhood Energy Utility, City of Vancouver
Vancouver’s Southeast False Creek Neighborhood Energy Utility

Sewer waste heat recovery systems, or “sewer thermal”, work just like ground-source heat pumps to pre-condition air or water before they are used for heating and cooling (don’t worry, no sewer water or gas gets into your air conditioner).  In the Olympic Village neighborhood of Vancouver, sewer waste heat provides 70% of the annual energy demand of a district heating system (natural gas provides the rest).  National Geographic has a good overview of the growing attention being paid to sewer thermal.

All major cities have large sewer mains collocated with the highest density development. Tapping this waste heat resource would require digging up those pipes, but it can be done much more easily in conjunction with large new redevelopment projects.  And generally, there are few actors: wastewater utilities control the pipes, cities control the right of way.

Making energy use transparent

According to the EPA, the commercial and residential sectors were responsible for 40% of US greenhouse gas emissions from the burning of fossil fuels (which is itself responsible for 79 percent of emissions) in 2011.  And in most major cities, it’s the large buildings (usually commercial buildings) that are associated with half or more of the energy consumption and associated greenhouse gas emissions.  Making these buildings more energy efficient could be a significant climate lever, but that requires knowing how they are performing now and motivating action from their owners and managers.

Nine cities in the US (and many more internationally) are addressing building energy use by making energy usage information more transparent.  Building rating and disclosure policies (typically enacted by cities) require large buildings to use widely adopted benchmarking tools to measure their energy performance, and generally require them to disclose this information, along with a score, to the public.

NYCenergyuse

In New York City, one million residents can now see how much energy and water their apartment buildings consumed.  In total, over 2 billion square feet of real estate in New York City is now benchmarking building energy and water performance each year.  This information isn’t just for tenants, building owners and managers, real estate professionals, and energy service providers can all use this information to improve the performance of the building stock.  In 2012, in their first report on benchmarked buildings, New York City estimated that:

If all comparatively inefficient large commercial buildings were brought up to the median energy use intensity in their category, New York City consum­ers could reduce energy consumption in large buildings by roughly 18% and GHG emissions by 20%. If all large buildings could improve to the 75th percentile, the theoretical savings potential grows to roughly 31% for energy and 33% for GHG emissions. Since large buildings are responsible for 45% of all citywide carbon emissions, this translates into a citywide GHG emissions reduction of 9% and 15% respectively. Much of this improvement could be achieved very cost-effectively through improved operations and maintenance.

An EPA study also showed that buildings doing benchmarking reduce their energy usage.  An analysis of 35,000 large buildings over three years showed that these buildings showed a 7 percent average energy savings.  Many of these policies are very new (NYC has only reported results for two years), so time will tell how increased public scrutiny of energy performance influences energy use.  But ask any building professional, and they will tell you that the first step to improving efficiency is measuring what is currently being used.

LED streetlights

Streetlights typically account for a significant portion of the electricity used by a city government enterprise.  For Minneapolis, its 31 percentNavigant says up to 40% can be typical.  Water treatment (for drinking) and wastewater treatment are two other major sources of energy use for cities or regional government entities.

Streetlight retrofits can often be done by a city itself, if they own the lights, or by the utility, which is also sometimes the owner.  Retrofits can be quick (a few years), and the paybacks, both in greenhouse gas emissions and cost, can be significant.

LA's Hoover Street before and after LED lighting retrofit. Image via Los Angeles Bureau of Street Lighting
LA’s Hoover Street before and after LED lighting retrofit. Image via Los Angeles Bureau of Street Lighting

Los Angeles recently completed the world’s largest swap-out, replacing 140,000 lights.  Los Angeles estimates it will save $7 million in energy costs and $2.5 million in avoided maintenance costs (LEDs can last as long as 20 years, versus the standard lights 6).  The project will be paid off in seven years.  New York City, Las Vegas, Austin Texas, San Antonio, and Eden Prairie, Minnesota are all switching.  For cities in the metro that don’t own their own lights, Xcel Energy is testing 500 LED lights in West Saint Paul, and could set a new rate for cities once the test is complete.  Initial results are showing energy use is cut in half, while the quality of the light has improved.

These are some examples of “levers” I think can be pulled relatively quickly, and without a great deal of political wrangling. And maybe more importantly, they can be done at the local level, usually by cities.  Cities are demonstrating they can and will move on climate, breaking what Jon calls the “cycle of climate inaction”.

There may be other strategies which are essential to addressing climate change, but which require engaging many more stakeholders and/or take significantly more time (an example might be residential building energy retrofits).  These strategies may be just as critical, often because they may address issues besides energy and climate – like environmental equity.  But if we want to work on a timetable that’s anything close to what they experts call for, we should identify and prioritize these short timeframe, high-impact levers we can pull at home.

Power surge

solar farm

My back yard solar farm has doubled in size to a massive 200 watts!

If you remember from the last post, the wooden frame was half empty (who built it like that?), and was feeling really lopsided without another panel.  The frame is now full (Amazon is a marvel of the modern age), so any further expansion will have to happen on the roof or elsewhere.

Untitled
Parallel wiring.

Under optimal conditions I think this will produce 480 watt-hours per day.  For reference, that’s about 60 percent of the power needed to run our chest freezer.  These panels are connected in parallel, meaning the voltage (12) is the same, but the amperage is doubled (to about 10 amps).  I ordered a cheap ammeter, which I’m excited to hook up so I can see real time results.

The last few weeks have been very cloudy, so I think output has been down.  The next week doesn’t look that great either.  When you’re a solar farmer, you start caring about the weather a lot more.

After adding some additional wiring, I now have access to a solar-powered power strip in the living room, rather than just having access in the basement where I could run an extension cord from the inverter.  Now I’m regularly running some lights and the stereo from the battery.  The cable modem and router are small, consistent loads, so I may move those downstairs to become battery-powered.  I’ve become somewhat obsessed about matching load to the production, since I don’t want to “waste” any electricity produced (and also don’t want to run my battery below 50%).

Stay tuned for another update when the ammeter arrives.

On estimating solar output and voltage drops

12.9 volts?! I want 18.9!
12.9 volts?! I want 18.9!

When I started tinkering with off-grid solar one of the first questions I asked myself was “how long is it going to take to charge this battery?” Or, similarly, “how much power will I produce in a day”?  Initially, the answer seemed easy: I’ve got a 100-watt panel, Minnesota gets about 4 hours of peak sun on average per day, so I’ll get 400 watt hours per day!  A 960 watt hour battery should be charged in two and a half days!

Wrong.  Way off.  Maybe more like 250 watt hours per day, and more like four days to fully charged.

I realized this quickly when attempting to charge a fully depleted battery.  In reality, it took into the fourth day for the charge controller to switch to battery maintenance mode (showing it was completely full).

Under perfect operating conditions and when grid-tied, you may actually get close to that nameplate 100 watts.  However, when your system is connected to a battery the voltage drops.  Many charge controllers (except for the really good expensive ones) will match the voltage of the panel to that of the battery to facilitate charging, which is almost always a lower voltage than the panels potential peak.  The rest of that potential is wasted.  So while my panel will produce 5.29 amps at 18.9 volts under optimum conditions (5.29 amps x 18.9 volts = 100 watts), when connected to my battery, it will probably only produce 5.29 amps at between 11 and 13 amps (5.29 amps x 12 volts = 63 watts).

Panels are built this way on purpose to make sure power can continue to flow to the battery even during overcast conditions when voltage may drop a little (gotta make sure that water flows downhill!).  Grid-tied panels don’t have this problem, since the grid can usually accommodate your voltage, and in a grid-tied system you’ll probably opt for one of the fancier MPPT controllers (see link above).

So, in summary, I’m probably getting 65 – 70% of my nameplate wattage (by design), and a realistic estimate for charging a fully-depleted 80 amp-hour battery from my 100 watt panel is 3.5 – 4 days.

Another takeaway for this amateur: it’s about amps, not watts.  The websites where you shop for panels always have the watts in large font, but the small print tells you the optimum operating current, or amps.  This number times hours of sun gives a much better estimate of the output (5.29 amps x 4 hours = 21 amp hours per day) for a battery-tied system.

Home solar update

Dead battery!
Dead battery!

The battery was totally dead this morning (11.4 volts) after about 16 hours powering the freezer.  The charge controller indicated the panel was charging the battery for about 4-5 of those hours, but it was extremely cloudy.  I turned off the inverter and reconnected the freezer to grid power (it’s nice to have a backup to the backup!)  After charging all day today, the battery is up to about 60%.  I hope I didn’t do any permanent damage to the battery.

My estimate of the charge time was pretty close, but my estimate of running time for the freezer was pretty far off.  I don’t think I calculated any loses from the inverter, which the internet tells me can be 15% or more.  It was also a hot day, hotter than when I measured my freezers usage initially, which could have had an impact. Next time I’ll definitely be measuring the total watt hours used (I forgot to hook up the meter) so I can try to estimate what was lost to the inverter.

One more day of charging before I do any more experimenting.