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Off-Grid Autonomy Calculator

Work out how long a battery bank runs a given load, and how much array is needed to replace it each day.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.0.0

Market
Imperial · sales tax

Autonomy

25.5 hours

Check your inputs

Generation exceeds consumption on an average day at the stated sun hours.

Usable energy
12.75 kWh
Days at this load
1.06 days
Daily consumption
12 kWh
Daily generation after losses
12.04 kWh
Daily surplus or shortfall
0.04 kWh

At the values currently entered, the autonomy works out to 25.5 hours. Note that battery nameplate (kwh) and continuous load (kw) sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Daily array yield = rated DC watts × peak sun hours × system derate; NREL PVWatts methodology
  • Peak sun hours is the daily insolation in kWh/m² and varies by location and season — it is a site input, not a constant

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Frequently asked questions

Why size on the worst month?
Because an off-grid system has no fallback. Peak sun hours in a northern winter can be a third of the summer figure, so a system sized on the annual average runs out in December — the month you least want it to. Size on the worst month, or accept a generator for that period and size on something between.
What is a peak sun hour?
An hour at 1000 W/m², the reference irradiance panels are rated at. A site with 3.5 peak sun hours receives 3.5 kWh/m² across the day, spread over more actual daylight at lower intensity. It is a way of collapsing a whole day's varying sunlight into one number that multiplies cleanly against a panel rating.
Why does the surplus matter if the battery is full?
It largely does not, and that is the point people miss. A full battery cannot absorb surplus, so generation above what the load takes is simply lost. Oversizing an array for the worst month means throwing away most of the summer output — which is fine, and cheaper than the battery that would store it.