Solar Power Calculations

Yes, our current solar system is dying...yesterday was a mostly cloudy day, and when I tried to charge the cell phone in the late afternoon, the batteries were drained almost immediately and the inverter screamed at us.

So, we did the calculations to find out how much power we need per day (in watt hours). For example, the cell phone is .2A which means it draws (at 110V alternating current) 22 watts. It takes about half and hour to charge so that means we need 11 watts for that. The Apple laptop needs 1.5 Amps, again at 110 for 165 watts. It takes longer to charge, but it doesn't need to be charged every day so I allowed 82.5 watts per day for that. Our total daily watt hours turned out to be 245 Watt hours.

Step 2 is to figure out how big a PV system we need to produce this amount of power. For this we take the 245 watt-hours and divide by (3.9 peak sunlight hours *.77) equals 79.28 watts array size needed. The .77 factors in an 85% battery efficiency and a 90% charge regulator efficiency. The Kyocera 135 (our candidate new panel) gives us about double that!

Finally, we need to see how much battery capacity we need. Ideally, we would like enough power stored to last through three days with no charging possible. The number of ampere-hours is (245 * 3)/(12V * .42) or 146 amp hours. The Surettte S460 has 460 amp-hours - so we are way ahead of the game there, too.

We did all the calculations with a book published by Natural Resources Canada in 2002: Photovoltaic Systems, A Buyer's Guide. It was oddly satisfying to get through all this and to actually better understand the system.

It will be great to be able to read comfortably and even sew in the evenings!

The solar4power website has more information about PV systems.