The short answer

A weekend cabin drawing a few kWh a day needs about 4–6 panels and a 10 kWh battery. A modest off-grid home at ~10 kWh/day typically wants 15–20 panels (5–8 kW) and 20–40 kWh of storage; a large all-electric house, more again. Against the same load, a grid-tied system would need roughly a third of the panels and no battery at all. The gap isn't waste — it's the price of having no safety net.

Average vs worst-case

A grid-tied system is sized to your annual average. It can afford to be — when the panels fall short on a dull winter day, the grid quietly makes up the difference, and on a bright summer day the surplus is exported. The grid is a giant battery you never paid for. Off-grid has none of that. The system has to stand alone on the gloomiest stretch of the year, so it's sized on your worst month's sun hours, not the average. As one rule of thumb puts it, winter peak sun hours are what separate systems that work in January from systems that don't.

That single shift is why the panel count balloons. In a temperate climate the worst month delivers only about half the sun of the yearly average, so an array that would be ample on paper has to roughly double to cover December — and then carry a reliability margin on top for the cloudy days even the worst month average hides. The seasonal chart in the tool shows the consequence: an array big enough for winter spends all summer producing far more than the house can use.

Days of autonomy & the battery

The battery answers a different worst-case: not a weak day, but a run of sunless days. "Days of autonomy" is how long the bank must carry your load with effectively zero solar input, and the sizing is simple — daily load × autonomy days ÷ usable depth of discharge. The norm is 2–3 days for most setups, stretching to 5–7 days for a permanent remote home; a weekend cabin can get by with less. Climate sets the target: Phoenix rarely goes more than a day or two without strong sun, while a place like Seattle can string together seven cloudy winter days.

Chemistry then decides how big that bank physically is. LiFePO4 safely uses 80–90% of its capacity and lasts 3,000–6,000 cycles; old-style lead-acid should only be drained to 50% and lasts 400–600 — so for the same usable energy, lead-acid needs nearly double the nameplate capacity and replacing far sooner. That's why lithium iron phosphate has all but taken over serious off-grid storage, despite the higher sticker price.

The oversizing trade-off

Here's the uncomfortable truth of off-grid design: a system built for the worst week is wildly oversized for every other week. All that summer surplus has nowhere to go — there's no grid to export to — so once the batteries are full the charge controller simply throttles the panels and dumps it. You pay for a January-sized array and waste most of what it makes in July.

Chasing the last few percent of reliability with yet more panels and battery is the most expensive part of the whole exercise. So most off-grid homes don't try — they go hybrid, adding a propane or diesel generator to cover the rare extended gloom. The generator is cheap insurance that lets you size the solar for "almost always" instead of "absolutely always." One more catch worth knowing: your inverter must be rated for your peak simultaneous draw — a pump, a fridge starting and a microwave at once — not your average, so it's sized at 1.5–2× your largest expected surge. And every watt you cut from the load saves several panels and a chunk of battery, which is why off-grid living rewards efficiency more than any grid-tied home ever does.

Designing for the worst day, not the average one

Grid-tied solar is an efficiency play — shave the bill, lean on the grid. Off-grid is a resilience play, and that changes the whole mindset: you're not optimising for the typical day, you're engineering against the worst one. The economics are unforgiving at the edge — going from 95% to 100% solar-only reliability can nearly double a system — which is exactly why the smartest off-grid builds stop at "almost always" and let a generator cover the rest. What's changed the game is cheap panels and LiFePO4: storage that used to be heavy, fragile lead-acid sulphating its way to an early grave is now lithium that cycles for a decade, making true independence practical where it used to be punishing.

So the real off-grid question isn't "how many panels" — it's "how bad a week am I willing to design for, and where do I hand off to a generator." Get that line right and the rest is arithmetic. The companion number is the storage itself: how many solar batteries you need.

Common questions

Game: off-grid facts

Five quick rounds on sizing for the worst week.

Method & sources

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