Once the replacement battery arrived, it was possible to use the power captured by the Harbor Freight 100 watt solar kit. (Item #63585) The E-Flite Power Meter tracks the cumulative solar energy pumped back into the battery every day. Over several sunny days with minimal cloud cover, the daily tally ranged from 8 amp-hours to nearly 10 amp-hours. On an overcast day, the daily tally struggled to reach 4 amp-hours. (Reminder: the solar panels are not optimally placed to face the sun in these experiments.)
At the battery voltage range of 12 to 13 volts, this means a sunny day gives us about 125 watt-hours of electricity. (12.5 volt * 10 amp-hour) An overcast day’s output drops to about 50 watt-hours. (12.5 * 4) Since the sunniest and most productive times for solar largely overlaps with the most expensive times of the time-of-use electricity rates, we can try our best to make this solar array look good, by comparing against the highest rate of 35 cents per kilowatt-hour.
Even when using that expensive rate, a sunny day’s generation only works out to a tiny bit over 4 cents of grid electricity. A cloudy day couldn’t quite make up to 2 cents worth. Rough estimates point to a meager 10 dollars a year of savings on the electric bill.
Fortunately, we’re not doing this for money, and there is room for improvement as well. The solar array can be better aligned with the sun which, from earlier experiments, we know will make a huge difference. But a more immediate concern is the fact only a few items around the house can directly use DC power. There aren’t enough cell phones and tablets to consume 125 watt-hours a day.
In order to make solar power more useful, we’ll need an inverter to take the battery’s 12 volt DC power and turn it into household 120 volt AC.