How many days of autonomy to build for

Two to three days for a home, more for anything critical. That is the standard answer and it is a reasonable one.

What it leaves out is the shape of the trade. The third day of autonomy costs as much to build as the first, and gets used perhaps twice a year.

What a day of autonomy actually buys

Autonomy is how long the bank carries your loads with no meaningful charging. It is sized against the worst week, not the average one, because the average week never causes the problem.

The shape of your weather matters more than the headline number. A climate with reliable sun and occasional single dull days is a two day problem. A northern winter that can sit grey for five days running is a different question entirely, and so is a monsoon season.

So the honest starting point is not a recommendation, it is your own record. How long has the longest dull stretch been where you are, in the season you actually need the power?

The cost curve is a straight line

Here is the part the guidance usually omits. Battery cost scales linearly with autonomy, because another day is simply another day’s worth of cells.

A bank covering 1 kWh a day for two days needs 2,500 Wh at an 80% depth of discharge. Three days needs 3,750. Five days needs 6,250, which is two and a half times the two day bank for a reserve that may be called on once or twice in a year.

Meanwhile the usage curve is the opposite shape. Day one gets used constantly, day two often, day three occasionally, day five almost never. Run both versions through the solar battery calculator and the per-day cost of reserve stays flat while the value of each extra day falls off a cliff.

Where a generator changes the arithmetic

That mismatch is the entire argument for a generator in an off-grid system.

Two days of battery plus a modest generator covers the same outages as five days of battery, usually for a fraction of the money. What you give up is silence, fuel and being present to start it, which matters if the house is unoccupied for stretches.

The usual pattern for a permanently occupied off-grid home is two to three days of storage with a generator for the rare longer gap. For a weekend cabin that sits empty, more storage and no generator often makes better sense, because nobody is there to pull a cord.

Size the array against the bank

Autonomy is only half a system. The other half is whether the array can refill the bank once the sun comes back.

An array sized for average daily production will take days to recover a deeply discharged bank, so a long outage leaves you limping for a week afterwards. A reasonable approach is to size the array so it can cover the daily load plus a meaningful share of recharge on a decent day.

This is also where chemistry asserts itself again. Lead acid that is left partially charged for long periods sulphates and loses capacity permanently, so an undersized array quietly destroys the bank it was supposed to serve. Lithium iron phosphate is far more tolerant of sitting part charged, which is one of its less advertised advantages.

Winter is the design case

Solar output in December in a northern climate can be a quarter of July’s, and the loads are usually higher because lighting and heating run longer.

A system sized on annual averages will work beautifully for eight months and fail in the two that matter. If full winter sizing is out of budget, the right move is to plan for the shortfall deliberately, with a generator, a seasonal load reduction, or honest acceptance that January runs lean, rather than finding out the first time it happens.

The number most people land on

For a full-time off-grid home in a temperate climate with a generator in the shed, two to three days is the usual answer and it is a good one.

Push past that only when you have a specific reason you can name. Medical equipment, a site nobody can reach in winter, a business that cannot stop. Those justify five days or more. Anxiety about the dark does not, and it is expensive, because every battery in that bank is ageing on its shelf whether or not it is being cycled.