How to design an off-grid solar system
Off-grid systems are sized by their worst month, not their average. That is the whole difference.
A grid-connected system can be undersized without consequence — the grid covers the shortfall. Off-grid there is no shortfall to cover, and a system that fails in December has failed, however well it performed in June.
Step-by-step
- List every load with its power and hours of use.
- Enter your location's monthly solar resource.
- Read the array and battery required, month by month.
- Look at the worst month. That is the design case.
Why the annual average is dangerous
December in northern Europe can produce a fifth of what June does. A system sized on the annual average is roughly half what December requires, and the failure arrives at the coldest, darkest point of the year.
The month-by-month view is not a refinement here. It is the calculation.
Reduce loads first, always
Off-grid, every watt of load requires panels, battery, and charge controller capacity to support it. The multiplier is large — often five to ten times the appliance's own cost over the system's life.
Efficient appliances, LED lighting and a well-insulated building are not economies here; they are the design. Electric heating off-grid is usually infeasible, and this is why off-grid homes overwhelmingly use wood, gas or a heat pump sized against a very different calculation.
Autonomy and generators
Days of autonomy is how long the battery runs with no generation. Three to five days is typical, and sizing for a fortnight of December cloud is prohibitively expensive.
Which is why most off-grid systems include a generator. It is not a failure of design — it is a rational decision to cover the last few per cent of the year with fuel rather than with a battery bank that sits unused for eleven months.
Frequently asked questions
Why size for the worst month?
Because there is no grid to make up a shortfall. A system sized on the annual average fails in winter, at the point of the year when failure matters most.
Do I need a generator?
Most off-grid systems include one. Sizing the battery and array for the very worst run of weather costs far more than covering those few days with fuel.
Can I run electric heating off-grid?
Rarely. Heating loads are large and coincide with the season of least generation. Off-grid homes generally heat with wood, gas, or a carefully sized heat pump.
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