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.
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.
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.
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:
- 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
twothree more panels (which would probably max out the charge controller) to economize, the panels would still only be 40%51% of the cost.
- 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.
- 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.
- 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.
- In the future, our homes should probably run direct current (DC) rather than alternating current (AC).
- 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.