A free off-grid power sizer built for real systems, not theory. Pick your platform, load in what you actually run, and get a battery bank, solar array, controller and inverter — with every formula shown, hard warnings when your electrical architecture will cost you, and a link you can save or share.
Your setup changes the whole calculation. Pick the closest.
Check what you run. Adjust watts & hours/day to match your gear — defaults are realistic, duty-cycled starting points.
Paste your spec into a forum for a sanity check, or save the link to come back to this build.
Most free calculators hide their assumptions. Here's every number this tool uses and why — so you can check it, argue with it, or adjust it for your own situation.
Every checked appliance is watts × hours/day, summed into watt-hours (Wh). AC appliances are additionally divided by inverter efficiency, because running them through an inverter costs real energy. Defaults are duty-cycled where it matters — a compressor fridge rated 50W doesn't run 24 hours, so its default hours reflect realistic cycling, not nameplate.
Quality pure-sine inverters run roughly 85–93% efficient under typical partial load. We use 0.90. This is applied to AC loads only; native DC loads skip it entirely, which is exactly why DC-first builds are more efficient.
LiFePO₄ can technically deliver close to 100% of rated capacity, but repeatedly emptying a bank shortens its life. You pick: 80% (long life — the conservative choice for a bank you want to last a decade), 90% (balanced, our default), or 100% (max capacity, smallest bank, shortest life). The bank is sized as daily Wh × days ÷ DoD ÷ voltage.
A panel's rating is measured in a lab at 1000 W/m² and 25°C cell temperature. Real installations lose to heat (roughly 10–15% on a hot roof — panels lose about 0.3–0.4% per °C above 25°C), wiring and mismatch (3–5%), controller conversion (~5%), soiling (2–5%), and imperfect angle. Stacked, that's about a 25% haircut, so we assume 75% of nameplate. Array = daily Wh ÷ (sun-hours × 0.75), rounded up to the next 25W.
"Sun-hours" means peak equivalent hours — not daylight hours. A day producing scattered light all afternoon might only be 4 peak-sun-hours. The presets are honest ranges: 3 = poor (northern winter, frequent overcast), 4 = average (a defensible year-round default for much of the US), 5 = good (sunny regions, decent season), 6 = desert southwest at its best. Size for your worst month, not your average, if you intend to stay put through winter.
Controller amps = array watts ÷ system voltage × 1.25. The 1.25 covers two real effects: panel voltage rises in cold weather (which is when controllers get destroyed), and edge-of-cloud lensing can briefly push output above nameplate. Sized up to the next 5A.
Continuous is your simultaneous AC load with 15% headroom, rounded to a real product size. Surge is tracked separately as the single largest startup spike among your selected loads — motors and compressors can pull 2–3× running watts momentarily, and an inverter that can't absorb that shuts down even though the continuous rating looked fine.
Current = (AC watts ÷ 0.90 + DC watts) ÷ system voltage. Thresholds: above 100A you're into heavy cable and should be deliberate about it; above 150A the cost of copper, the voltage drop and the heat make a higher-voltage architecture the better engineering answer. These aren't code limits — they're the practical points where builds get expensive and unpleasant.
Modern rigid panels land around 18–19 watts per square foot (a common 200W panel is roughly 11 sq ft). Real roofs lose area to vents, fans, AC units and walkways, so we add 15% mounting overhead. Within 12% of your stated space is flagged as a tight fit rather than a failure, because careful layout can often recover that margin.
Stations are sized on usable capacity (we assume 90% of nameplate is practically available) and recommended against real product capacities rather than an arbitrary number. Refill time accounts for portable-panel derate (70% — foldable panels rarely sit at ideal angle). We also flag when the panel you'd need exceeds the solar input limit common on mid-size stations, because that's a spec people discover after buying.
It doesn't do wire gauge or fuse selection (those depend on run length, routing and temperature — see Safety), it doesn't model shading or panel string configuration, and it doesn't replace a qualified installer for permanent structures. It gets you an honest, defensible starting spec so you're shopping with numbers instead of guesses.
You've got your spec. The Gear hub is the shortlist that matches it — power stations, batteries, panels, controllers and connectivity, with the reasoning shown.
See the matched gearMissing appliance, a number that looks wrong, a feature you'd use — send it and it gets built.