Off-grid is a different question from backup
Everything else on this site asks how long you can last without the grid. Off-grid asks whether your generation matches your consumption, every day, forever.
That changes what matters. In backup, capacity is king — how many kilowatt-hours you have stored. Off-grid, the binding constraint is almost always generation, and battery capacity is just the buffer that carries you from one sunny day to the next.
Which is why the sizing exercise runs in the opposite direction: start with what you use, then size the array to produce it in the worst month, then size the battery to bridge the gap between good days.
Step one: measure, do not estimate
This is the step people skip and it is the one that decides whether the system works.
Off-grid consumption is usually counted in kilowatt-hours per day. A frugal small dwelling runs on 2 to 4 kWh; a normal suburban household transplanted unchanged uses 20 to 30 and will bankrupt any realistic array.
| Household | Daily use | Array needed in summer |
|---|
| Frugal cabin — LED, laptop, pump, propane fridge | 1-2 kWh | Roughly 300-500 W |
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| Small off-grid home — electric fridge, well pump, tools | 3-6 kWh | Roughly 800-1,500 W |
|---|
| Full household, propane heat and cooking | 8-12 kWh | Roughly 2,000-3,000 W |
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| Anything with electric heat or air conditioning | 25 kWh and up | Not a realistic off-grid design |
|---|
Array figures use the 400-500Wh per 100W of panel per good summer day figure used across this site. Winter output in most of the United States is a fraction of that, which is what the next section is about.
Step two: size for December, not July
This is the arithmetic that catches people. Solar output in midwinter at northern latitudes can be a quarter of the midsummer figure or less, once you account for short days, low sun angle and weather.
An array that comfortably covers 4 kWh a day in July may deliver 1 kWh in December. You have three ways to close that gap and every real system uses some mix of them.
Oversize the array. Effective, expensive, and means dumping enormous surplus for eight months of the year.
Tilt for winter. A steeper panel angle favors the low winter sun at some cost to summer output — the angle page covers the trade, and off-grid is the case where winter bias usually wins.
Run a generator. The pragmatic answer. A few hours a week in the darkest months is far cheaper than the array that would avoid it.
Step three: the battery is the buffer
Battery capacity off-grid is sized in days of autonomy — how long the system runs with no generation at all.
Two days is the common design target. It covers a run of overcast weather without forcing a generator start, and it does not commit you to the cost of a week's storage you will rarely use.
So a household using 4 kWh a day wants roughly 8 kWh of usable storage, which is where expansion batteries and multi-unit systems start being the realistic answer rather than a single box. The 3000Wh class page covers the bottom of that range.
What to make non-electric
The cheapest kilowatt-hour is the one you do not use, and off-grid that principle is worth more than anywhere else because you are paying for generation rather than consumption.
- Heat. Wood, propane or oil. Electric heat off-grid is not a design, it is a mistake — the space heater arithmetic shows why.
- Cooking. Propane. An induction cooktop is a two-kilowatt load for the sake of convenience.
- Water heating. Propane on demand, or solar thermal, which is a different and much more efficient technology than photovoltaic.
- Refrigeration. This one is worth keeping electric — a modern efficient fridge is a few hundred watt-hours a day and a propane absorption fridge is finicky.
Where this differs from a cabin
A weekend cabin has a completely different problem: intermittent use, a system that sits idle for months, and the freedom to simply not go when the weather is bad.
Off-grid living has none of that slack. The system runs every day, is maintained by you, and has to work in February. The off-grid cabin page covers the intermittent case, and the cabin generator page covers starting after a long sit.
The short version
- Generation is the constraint, not storage.
- Measure your daily kilowatt-hours before buying anything.
- Size for December. Winter output is a fraction of summer.
- Two days of autonomy plus a generator is the design that actually pencils.
- Heat, cooking and hot water should not be electric.
