That's assuming perfect sun exposure on a roof or ground moving with no shadowing at any time of the day and at the optimal tilt. Pvwatts says that usable ener

That's assuming perfect sun exposure on a roof or ground moving with no shadowing at any time of the day and at the optimal tilt. Pvwatts says that usable energy from a 1480W system at 43.6N in Boise ID might be more like 77kWh a month in December, the shortest sunlight month of the year. An off grid system generally needs to be calculated to survive December. January will be a few percent better as days start getting longer. Realistically, to get through entire week long periods of snow and overcast, if I had a load that was 50kWh a month I would want a 75-80kWh a month predicted PV production system to make it worry free in December.