How to measure how much solar we need?

pianotuna wrote:
Hi Almot,

Wow, only 28 amp-hours from 1000 watts of panels? I'm floored. On Dec 21 I get about 17 amp-hours from 256 watts of panels.

Yes. You know about PVwatts calculator. This is what it shows for Seattle in December. Enter DC Rating KW=1, Panel Tilt=0, and the result is 11 KWH a month. Divide into 31 days and 12.7V, and you will get 28 AH a day.

28 AH is average daily. With 1 good day per 3 or 5 bad days you will indeed get 70 AH or more, on a good day, from 1000W panel. And almost nothing on bad days. The thing is, the area is really rainy and cloudy in winter, not too many good days, so the average comes out very low. They could try and ride out the dark spell on batteries, but with a battery bank of usual RV size this won't be easy - and then they might not have enough time to re-charge on a bright day before it starts raining again.

I am looking at this all from TT point of view, and you are right, with Class C there is a benefit of charging on the road if you move often. So in the end this might be possible, but still difficult. It depends on how much the OP move around and how good they are at energy conservation.

Edit - PS:
As BFL have established empirically, solar harvest depends on the area, and the calculator apparently agrees with BFL ;) . For example, same 1000W panel, according to calculator, will harvest on the average 70 KWH a month or 178 AH a day in December in Tucson AZ. Which agrees with my observations of 80-90 AH on 490W panel in North Baja in December. This is 6 times more than in Seattle. Weather pattern, i.e. number of sunny days (or sunny hours per day), does matter.

How those snowbirds in AZ with 70-80 AH a day - this is on the darkest month of season - still have to run a generator on most days, is beyond my understanding. The wives are probably to blame again, no other natural or unnatural causes come to mind :)