OK, here's some actual numbers to follow along with. These are fictitious, just to show how to do the calculations.
Let's say I have the following energy usages from my power bills:
January: 9.87 kWh
February: 8.43 kWh
March: 8.81 kWh
May: 7.88 kWh
June: 8.01 kWh
July: 9.15 kWh
August: 10.03 kWh
September: 9.39 kWh
October: 7.47 kWh
November: 8.74 kWh
December: 8.99 kWh
I divide these all out by the number of days in the month to get:
January: 9.87 kWh/31 days = 0.3184 kWh/day = 318.4 Wh/day
February: 8.43 kWh/28 days = 0.3011 kWh/day = 301.1 Wh/day
March: 8.81 kWh/31 days = 0.2842 kWh/day = 284.2 Wh/day
April: 7.56 kWh/30 days = 0.252 kWh/day = 252 Wh/day
May: 7.88 kWh/31 days = 0.2542 kWh/day = 254.2 Wk/day
June: 8.01 kWh/30 days = 0.267 kWh/day = 267 Wh/day
July: 9.15 kWh/31 days = 0.2952 kWh/day = 295.2 Wh/day
August: 10.03 kWh/31 days = 0.3235 kWh/day = 323.5 Wh/day
September: 9.39 kWh/30 days = 0.313 kWh/day = 313 Wh/day
October: 7.47 kWh/31 days = 0.241 kWh/day = 241 Wh/day
November: 8.74 kWh/30 days = 0.2913 kWh/day = 291.3 Wh/day
December: 8.99 kWh/31 days = 0.290 kWh/day = 290 Wh/day
OK, now I can see that my maximum daily usage is in January and August, so I'll just work with those. I go to Weather
and see that on average, in January I get 19 sunny days of 10.5 hours a day and in August I get 24 sunny days of 13.5 hours per day.
Now I re-run my calcs for those two months. I want to be a little conservative, so I'll assume 40% efficiency on a dreary day:
January: 19 sunny days and 12 dreary days
19+(12*0.40) = 19+4.8 = 23.8 sunny days in January
23.8 sunny days * 10.5 hours per day = 249.9 hours of sunshine in January
August: 24 sunny days and 7 dreary days
24+(7*0.40) = 24+2.8 = 26.8 sunny days in August
26.8 sunny days * 13.5 hours per day = 361.8 hours of sunshine in August
In January I used 9.87 kWh and I get 249.9 hours of sunshine; that's 0.0395 kWh/h = 39.5 watts.
In August I used 10.03 kWh and I get 361.8 hours of sunshine; that's 0.0277 kWh/h = 27.7 watts.
So I now know that January will be my hardest month. I use more power in August, but I also have more sunshine available... so all of my future
calculations will be made using January numbers. (This will probably be typical for most people in the northern hemisphere.)
Now, I add in my buffer and I get 39.5*1.2 = 47.4 watts... I want a 50 watt solar panel.
Now, I looked back at the previous winters online and I have decided I want to be able to get through a three-day streak of bad weather. OK, that's
going to be 72 hours, right? Nope, it will be 72+13.5 = 85.5 hours... don't forget that three days includes 4 nights, you have to add in those extra
13.5 hours of nighttime.
My January power usage was 318.4 Wh/day. 318.4/24 = 13.27 watts. Let's say I plan to use 12 volt lead-acid batteries, so 13.27 w/12 v = 1.11 amps. You
have to supply that for 85.5 hours, so that's 1.11*85.5 = 94.9 amp-hours of reserve power I will need. That's two 50 amp-hour batteries, right? Wrong!
Don't forget the buffer. 94.9*1.2 = 113.9 amp-hours of reserve power, so that's either three 50 amp-hour batteries, or maybe two if you can get 60
amp-hour batteries. Remember that the capacity depends on the discharge time... in this case, that would be over 85.5 hours.
You will likely not find an exact capacity at that that exact discharge rate; use the closest discharge rate. Also, keep in mind that more reserve
capacity is preferable to less; more reserve capacity means less complete cycling and therefore longer battery life, plus it allows you to last a
little longer should you get a longer-than-expected period of overcast skies. In the above scenario, I would opt for three 50 amp-hour batteries.
All of this is to size the solar panel and the batteries. The next step which takes into account the maximum
power usage is where the power
draws for the various appliances come into play.