Tagged 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!

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.

A small experiment with solar

100 watt panel with wood mount
100 watt panel with wood mount

Being carbon-conscious, naturally inclined to tinker, and seeing the falling costs for components, I was curious to know whether I could put together a small solar PV system on my own.  Having experienced a few blackouts this summer and expecting more in the future, I was also curious about providing a small amount of backup power for essential items.  Here’s the story of my first foray into off-grid renewables.

This post at Do The Math (an excellent blog you should read regularly) in which Tom Murphy describes his small, off-grid system really got me started on the whole thing.  I’m not a physics professor, but after reading it and doing some additional google searching, it seemed easy enough for a lay-person armed with a small amount of reading.  A valuable resource (also provided by Tom’s blog) is the Solar Living Sourcebook, available at your local library, which provides all the basics on what solar PV is, how it works, important safety tips, and options for setup.  I also learned a few things from various youtube videos and generally google searching.

The system I put together is 12 volts, which seems very common for small, off-grid installations.  It’s basically only six things: a solar panel, a charge controller, a battery, an inverter, and assorted wires and fuses.  The solar panel provides the electrons, the charge controller controls how those electrons flow to the battery (and makes sure it doesn’t overcharge), the battery stores electrons, and the inverter turns the battery’s 12 volt DC power into 110 volt AC power so it can be used with regular household electronics.  The wires and fuses connect things together and provide safety.

Panel unboxing
Panel unboxing

You can now purchase relatively affordable panels from Amazon or Home Depot in many wattages and sizes.  I chose a 100 watt panel that seemed to receive good reviews and a website that suggested that the company might be around for a while.

Charge controller showing all systems go!
Charge controller showing all systems go!

The other pieces of the system (inverter and charge controller especially) come in a huge range of prices.  After some reading, I decided that it might be better to spend a little more on a charge controller, as many people had complaints about cheap versions, and keeping your battery well-maintained is important (the function the charge controller plays).  I purchased a 30 amp controller from Morningstar, which I think could power up to 300 400 watts of panels if I expand the system in the future.  The battery is rated at 80 amp hours, and is sealed lead acid.  I purchased it from a local battery store, and its a discount version.

80 ah battery
A bad photo of the 80 ah battery

So what can this thing power you ask? That’s a function of how much the panel produces, how much the battery stores, and how much amperage I can draw at one time from the battery and inverter.

According to some assumptions I pulled from NREL’s PVWatts tool, the panel might generate 400 watt hours per day (100 watts X 4 hours equivalent of 100% production) in the peak season and maybe 210 in the low season (November), although I’ve seen higher numbers in other places.  The battery is large enough to store all that daily production and more (80 amp hours X 12 volts = 960 watt hours).  In fact, it would probably take one and a half to two two and a half days of sun to fully charge the battery.

Even in the winter, the daily production of the panel would probably be enough to power a few lights (the LED variety), an efficient laptop, a fan, and a small TV for a few hours.  It won’t run a hotplate, anything but the smallest air conditioner, a heater, or a refrigerator, at least not continuously.  The battery and the inverter could probably handle it (one of these things), but the panel wouldn’t be able to keep up.  As far as a back-up power source, this set up would power my refrigerator for about 12 8 hours, and our 8.8 cubic foot chest freezer for about 24 16 hours.  That’s assuming a fully charged battery, the panel couldn’t keep up with the draw from those appliances for more than a day.  These are just my estimates, I don’t have any real world results yet, but will report back soon.  Right now I’ve got just the chest freezer plugged into the inverter and I’m going to time how long until I get the low battery warning.

Things I’ve learned so far:

  1. It’s all the other stuff that costs money.  At this stage, the panel itself only accounts for 20% of the cost.  If I added two three more panels (which would probably max out the charge controller) to economize, the panels would still only be 40% 51% of the cost.
  2. I need scale to “save” money.  Right now, my costs per watt are about 76% higher than what I have been quoted to put a grid-tied, full size system on my roof.  If I maxed out the charge controller with two more panels and got another battery, I could bring my costs in line with the pros (again, on a per watt basis).  Whether this would continue to scale up I kind of doubt, since batteries get expensive and I would get into more serious electrical work pretty quickly.
  3. I need another battery (or two) for large stuff.  High amperage appliances, like a vaccuum, seem to be within the wattage range of the inverter, but my battery is only 80 Ah.  The internet tells me I should only run things that are 10-12% in amps of that capacity to avoid shortening the batteries life, and indeed I got a low battery warning when trying to run the shopvac.
  4. You should think hard about where to locate a panel before you embark on this kind of project.  I’m still squeamish about getting into roofing for fear I will cause a leak, and others in my household disagree about the aesthetics of a home-built wooden frame.  My goal in the long run is to get this on a roof somewhere.
  5. In the future, our homes should probably run direct current (DC) rather than alternating current (AC).
  6. Solar panels aren’t just for tree-huggers.  If you’ve ever searched youtube for videos about solar back-up systems, you’ll be a lot less surprised about news items like the Atlanta Tea Party teaming with the Sierra Club to promote more solar.  Many of the instructional videos I watched were clearly made by folks of the conservative persuasion who were into solar because they feared the grid would go down or they weren’t comfortable being beholden to utilities/the government.  Maybe there is more common ground here than we thought.

 

How much should utilities pay for distributed solar power?

Close-up of completed project - Gibbs Dairy goes solar

In the energy and climate circles, there is a lot being written lately about the threat to the traditional utility model from distributed, renewable energy sources.  David Roberts has been running a series describing the problem and looking for solutions.  Chris Nelder also has a good read on the topic.

One of the key issues is the idea that utilities want to avoid “stranded assets”, or infrastructure they still have to pay to maintain with a shrinking pool of customers.  As some customers get more power from solar, sales of electricity shrink, leaving utilities with the same distribution infrastructure to maintain using less revenue.  Some utilities, the latest being a municipal utility in San Antonio profiled by David Roberts, argue they shouldn’t pay customers the “market” rate for electricity their customers generate with rooftop solar, but instead should pay them a wholesale rate, or the same as they pay for other electricity on the grid.

The thinking here is that paying the wholesale price will put renewable energy on an even playing field, and help keep the old utility model more financially whole, since wholesale prices are typically much lower than market prices.  For example, the 5-year average wholesale price for electricity in the grid area that serves Minnesota was $53.62 per MWh for the period ending in 2010, according to FERC.  This is for the “peak” time of day, meaning the afternoon, which is also the time solar is most productive.  That’s equal to roughly 5 cents per kWh, which is the unit at which typical household sales are measured.  Last month I paid about 11 cents per kWh to Xcel before taxes, fees and other charges like WindSource.

At 5 cents/kWh, rooftop solar would take a very long time to pay off.  Many fewer people would likely choose to install it.  However, those in the renewable energy world will tell you that 5 cents/kWh doesn’t pay the owner of a system for some of the benefits solar energy has over wholesale electricity.  We should actually be looking at a “value of solar” that includes not just the wholesale energy price, but reimbursement for other values.  There is movement right now in Minnesota to legislate that a true “value of solar” be computed for future projects.  So what other value does solar energy have that utilities might value?

For one, it can be more efficient.  Whenever you transmit electricity or long distances, you lose some due to resistance (heat).  EIA estimates these loses at 7% nationally and 7.4% in Minnesota.  That means utilities are generating more kWhs than are needed to make up for the losses, and thus the customer is paying more for each kWh.  If you’re generating power very close to where you use it, you minimize these losses and the extra generation.  Distributed solar energy should actually be valued 7% above wholesale prices by a utility if you think it will reduce these line losses.  If you include that 7% bump, 5 cents becomes almost 6 cents per kWh.

The other value is the reduced environmental cost of solar generation.  There is plenty of discussion about what the optimal cost of carbon should be, and it all depends on what you adopt as your discount rate.  Here is a must-read on discount rates, also by David Roberts.  If you think that climate change will have a net drag on the economy in the future, your discount rate is likely low, and the optimal cost of carbon gets up into the $50 to $100/ton range.  Carbon levels per unit of electricity produced vary quite a bit across the county, but in Minnesota and parts of the upper Midwest, they averaged 0.738 metric tons per MWh in 2009 (the latest year for which EPA has data).  At that rate, a high carbon tax might add between 3.5 and 4.5 cents per kwh.

If you add all this up, (an economically optimal price on carbon, savings from transmission losses, and a wholesale price consistent with the 5-year peak average), you get a value of solar energy between 9.5 and 13 cents per kWh.  That’s at or above the market rate I’m paying in Minnesota right now.  Check out my extremely messy spreadsheet if you want to see the math.

Keep in mind there are other values of solar energy I haven’t considered in my calculus.  The Minnesota House legislation includes the savings from delaying capital investments in distribution infrastructure, savings from not having to build more generation, fuel price hedge value savings (not having to bet on fuel costs), and the value of local employment generated by manufacture and installation of solar energy.