How do I power an off-grid shed or cabin?
Scenarios Updated 2026-08-01
Three separate sizing questions, in this order. Add up the watt-hours you use in a day; that sets the array. Decide how many cloudy days you must survive without sun; that sets the battery. Find the largest single thing you will switch on and its start-up demand; that sets the inverter.
Getting them in the wrong order is the classic mistake: people buy an inverter big enough for a well pump and a battery sized for a laptop, or an array that covers the annual average and never the December week that actually matters.
Step one: the daily watt-hour total
What a small off-grid building actually consumes
Published figures for the loads a shed or cabin usually has. Lighting and communications are almost free; refrigeration is the first real cost; anything that heats or pumps is in another class again.
Multiply by the hours you use it. A whole cabin of these is a rounding error against a fridge.
See the record →Continuous, so it is one of the larger daily totals despite the small wattage.
See the record →Usually the largest steady load in a small off-grid building, and the one that sets the battery.
See the record →Brief, but it is the load that decides the inverter rather than the battery.
See the record →A submersible motor’s locked-rotor demand. Every machine in our portable library refuses this start, which is the honest answer about pumps and portable power stations.
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Step two: days of autonomy, not hours
A cabin battery is sized by how long you can go without generation, not by peak power. Multiply your daily watt-hour total by the number of consecutive poor-sun days you are willing to lose, then divide by the usable fraction of the battery — not by its nameplate. On the record we use as the worked example throughout this site, BLUETTI publishes both coefficients, so the usable figure is derivable; most brands publish neither, and their pages here print no runtimes at all for that reason.
Step three: the biggest thing you switch on
The load that decides the inverter
A live run of a well pump against the largest portable machines in our library. This is the point where a shed system stops being a portable power station and becomes an installed inverter with a proper motor rating.
Refused on the start, not the run. Nothing in the portable class clears a submersible pump’s locked-rotor demand.
Show the working →The largest machine we hold, and still short of the published start.
Show the working →That is a genuinely useful "no". If your building has a submersible pump, the sizing conversation is about an installed system with a motor-rated inverter, a soft starter or a pressure tank arrangement that lets the pump run less often — not about which portable power station to buy. Lights, communications, refrigeration and tools are a different matter and sit comfortably in the portable class.
A shed is not a balcony
The two most common small-solar questions look similar and are almost opposites. A balcony system is grid-tied, small, and constrained by law and by mounting; a shed system is off-grid, constrained by the worst month of sun and by whatever motor you own, and answers to nobody but the code official. The shed gets to choose its array angle, which is the single largest advantage it has. If your building is grid-connected, most of this page still applies to sizing but the legal chapter of the balcony article applies to you instead.