Power Shift: Sizing the PV Array

By , March 21, 2015

In the post, Power Shift: Apples and Oranges, I described auditing our power usage to arrive at an estimated average daily electricity use: 1438 Watt hours (Wh).

To estimate how many of what size solar panels we would need to satisfy this need, we turned to Backwoods Solar, one of the most trusted off-the-grid living resources in our neighborhood. I used their catalog as a text book for the estimate.

I found several different methods of computing array size in various titles in our Homestead Reference Library, but the Backwoods method seemed easiest, and I know they stake their well deserved reputation on it.

Following their formula, we divided our Wh/day by 4-5 hours of average useable sunlight per day. I split it right down the middle for our use, at 4.5. We multiplied this number by 1.5 to add 50% to the figure to account for solar panel rating loss.

We then multiplied by the percentage indicated on their region map. This calculates the percentage by which one should increase one’s solar panel power to compensate for regional weather. Sunny Arizona has 0%. The rainy western portions of Washington and Oregon, and the Canadian border region from Michigan to New Hampshire need 100%. Unfortunately, as so often happens, the map only covers the contiguous Lower 48. I find this odd, considering how much business the company does with Alaska, and likely Hawaii as well, where I understand there’s a significant number of off the grid homes.

At any rate, I used 100% for us as well, doubling the last figure. That gave me the number of watts I’d need to produce with solar panels: 959. Or, just under 1 KW.

I expanded my spreadsheet to list the common wattage of large non-industrial solar panels, ranging from 210 to 320 watts, and to calculate how many of each panel it would take to meet our need. After that, I shopped around for the best prices I could find for good quality panels in that range.

I hoped to take advantage of the proliferation of large grid-tie modules. Grid-tie systems, solar and wind installed in on-grid homes to produce power that utility companies are required to buy from the consumer in most states, are booming, which is largely responsible for the dropping prices mentioned in the earlier post, Power Shift: Affordable Power. Three large panels rather than 4 smaller ones might save some money, and would reduce the amount of wire to connect strings and the work of doing so.

However, those larger panels generally get used in large arrays, requiring large and expensive charge controllers. I found I had to lower my expectations a bit to get a complete system that better fit our space and budget. I’ll describe the system we designed in an upcoming post.

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