About Solar Sizing Calculator

Free, single-purpose calculators for sizing solar and off-grid systems. One job per page, the maths in the open, and no sign-up.

Why These Exist

These tools started life as working spreadsheets. Array size, battery bank, cable runs and fusing were being worked out the same way over and over, on vans, cabins, outbuildings and roofs, so each calculation was rebuilt as a page that does the job in one box, shows the formula, and does not ask for anything in return. Every calculator here comes from that: a repeated problem that deserved a permanent answer.

The published answers to most of these questions are either a rule of thumb with no working shown, or a sales page with a form attached. Neither tells you where the number came from. Every page here shows the formula, a worked example you can check by hand, and the assumptions the answer rests on.

What every page does

Answers one question, with no preamble above the inputs
Shows the formula it used, not just the result
Carries a worked example you can check by hand
States its assumptions and where the constants come from
Works in metric and imperial, and names the standard where they differ

What Is On The Site

Every tool is a single page that does one job. Browse them all in the tools directory.

Sizing the system

From what you use to what you need to buy

Solar system size in kW, panel count and roof area
Battery bank capacity in Ah and kWh, with days of autonomy
Charge controller and inverter ratings
Off-grid load, appliance by appliance

Wiring and protection

The DC side, where the mistakes are expensive

DC cable size against ampacity and voltage drop
Voltage drop on 12V, 24V and 48V runs
PV fuse and breaker sizing to NEC 240.6 and 690.8
AWG to mm2 conversion, with the full wire chart

Output and payback

What it makes, and what it is worth

Panel output with cell temperature handled separately
Series and parallel array wiring, including cold Voc
Solar payback, discounted LCOE and net present value
Plug-in solar savings under the UK 800 VA rules

Who It Is For

Installers and electricians

Sizing a job on site without opening a spreadsheet, with the standard and the clause named on the page.

Van, boat and cabin builders

The off-grid tail the generalist calculator sites handle badly: small banks, long DC runs, real constraints.

Homeowners and self-builders

Checking a quote against the arithmetic before signing it, and finding out what a system is actually worth.

Why Use These Calculators

The maths is shown

Every page carries the formula and a worked example you can check by hand. If you disagree with the answer you can see exactly where it came from.

Standards are named

NEC, AS/NZS and IEC clauses are cited where they apply, and region-tagged where they differ, rather than blended into one silent assumption.

Independently reviewed

Every formula and constant was put through three rounds of external technical review in August 2026 and is sourced to NEC, IEEE 485, PVWatts, MCS or PVGIS.

Free and unconditional

No account, no email, no trial. The tools are ad-supported and that is the whole business model.

Found A Mistake?

Corrections are genuinely welcome, and they get made. If a number looks wrong, or a standard has moved, say so at hello@solarsizingcalculator.com.

Browse the calculators

Solar Sizing Calculator

Free solar and off-grid sizing calculators

Tools

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