Solar & battery
Off-Grid Solar Calculator
Size an off-grid system: array, battery and generator backup for the worst month, not the average.
Result
Off grid is sized on the worst case rather than the average, which is why it costs several times a grid-tied system of the same daily output. Almost every off-grid installation includes a generator, because sizing for a two-week December overcast is far more expensive than running one for a few days. Reducing the load is nearly always cheaper than generating more.
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About this calculator
Why is off grid so much more expensive?
Because there is no grid to fall back on. A grid-tied system can be average-sized; an off-grid one has to survive the worst week of the year, which means a much larger array, a large battery and usually a generator as well.
How many days of autonomy do I need?
Three is the usual design figure. More gets expensive fast; less means the generator runs often. Climate decides it: consecutive overcast days matter more than annual sunshine.
What is the cheapest way to cut the cost?
Use less. Propane for cooking and water heating, efficient refrigeration, and LED lighting cut the load enough to shrink the whole system, and a kWh not needed is far cheaper than a kWh generated.
The formula
array kW = daily kWh / (worst-month sun hours x derate); battery = daily kWh x autonomy / depth of discharge
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