The Field Calculators → Power
Power
Solar array & battery sizing
Size it on December, not on June. This is the calculator that decides whether your system works in the month you need it.
Result
Daily energy use3000 Wh/day
Peak sun hours — WORST month2.5 hrs
System derate75 %
Battery bank voltage48 V
Days of autonomy2
Usable depth of discharge80 %
Array size needed
1,600 W
About 4.0 panels at 400 W each.
Battery bank
156 Ah at 48 V
7.5 kWh of usable storage.
If you'd sized on summer instead
727 W
You'd be 873 W short in December — which is when it matters.
Charge controller current
42 A min
25% headroom over the array's output at bank voltage.
Panel tilt for winter
Latitude + 15°
Steeper sheds snow and catches the low winter sun.
What this will not do
- Size on December. A system sized on the annual average dies on January 9th, in the dark, with the house cold. This is the single most common and most expensive solar mistake.
- Measure the load, don't estimate it. Every appliance lies on its label, and standby draw is where the budget actually goes. A $15 plug-in meter for a week is the best money in the system.
- Charging lithium below freezing permanently damages it. Use LiFePO4 with a low-temperature charge cutoff, and treat the cutoff as mandatory equipment rather than a feature.
- Cold also *helps* panels — output rises as cells get colder. The December problem is daylight hours and sun angle, not temperature.
The full build and the reasoning are in Chapters 40, 41 of the guide.