Solar Panel Calculator — Size, Cost, Savings & Payback

Divide your average monthly electricity use (kWh) by 30, by your local peak sun hours, and by 0.8 (system losses), then divide the resulting kW by your panel wattage and round up. Example: 900 kWh/month with 5 peak sun hours gives 7.5 kW, which needs 19 × 400 W panels (7.6 kW installed). The calculator above does this automatically, including roof-area estimates.

Peak sun hours are not the hours the sun is up — they are the equivalent number of hours per day at a standard 1,000 W/m² irradiance. Five peak sun hours can come from 8–9 hours of weaker daylight. This tool uses climatological (long-term average) peak-sun-hour presets per city, which are stable for decades; you can override the value with your own figure.

It is an estimate built on the same method as NREL PVWatts: your usage, local peak sun hours, and a 20% allowance for inverter, wiring, soiling, and temperature losses. It cannot replace an installer quote, which accounts for roof shading, orientation, local tariffs, net-metering rules, and permits. Use it to sanity-check quotes — not as a quote.

Year-by-year: each year's production (degrading 0.5% annually) multiplied by that year's electricity tariff (growing at your chosen inflation rate), capped at your annual usage and adjusted for self-consumption. The payback year is the first year cumulative savings reach your net system cost. The full 25-year table is shown above. Dividing the net 25-year profit by the net system cost gives a simple solar panel ROI multiple.

In most sunny regions with average tariffs, a simple payback of 6–10 years is typical; the panels keep producing for 25+ years, so the remaining years are net profit. Payback is shortest where electricity is expensive and sun is abundant, and longest where tariffs are low or heavily subsidized. If you are asking how long until solar pays for itself, the honest answer is that it depends on your tariff and sun hours — run your own numbers above instead of trusting a national average.

No — the 30% residential federal Investment Tax Credit ended on December 31, 2025, and we do not claim otherwise. This calculator's incentive field is intentionally generic: enter any local rebate, grant, or tax credit that applies to you as a percentage, and it reduces your net system cost in the payback math. Always verify current incentives with your local authority.

Yes. Most manufacturers warrant no more than about 0.5% annual degradation, which this calculator uses as the default. After 25 years a panel typically produces roughly 88% of its original output. The year-by-year savings table above already includes this degradation.

Multiply your critical load (kW) by the backup hours you want, then divide by depth of discharge (about 90% for lithium) and inverter efficiency (about 95%). Example: 1.5 kW × 8 hours ÷ (0.9 × 0.95) ≈ 14 kWh usable, or three 5 kWh modules. The Battery tab computes this with adjustable assumptions.

Off-grid sizing starts from daily consumption in watt-hours: divide it by (peak sun hours × 0.7) to get the array size, and size the battery from daily watt-hours × days of autonomy ÷ depth of discharge. The RV tab does both, recommends a 100 W / 200 W panel combination, converts battery capacity to amp-hours at 12 V or 24 V, and sizes the charge controller with a 25% safety margin.

As a rule of thumb, set the tilt equal to your latitude for the best annual yield — lower (latitude − 15°) if you want to favor summer, higher (latitude + 15°) to favor winter. The Tilt tab gives the exact monthly optimum from solar declination for your latitude, plus the expected yield loss if your roof tilt or direction differs.

Toward the equator: south in the Northern Hemisphere, north in the Southern Hemisphere (180° and 0° azimuth measured from north). Deviations cost yield — the Tilt tab quantifies the loss for your actual azimuth, so you can judge whether a non-ideal roof is still worth it.

Multiply annual production (kWh) by your grid's emission factor (kg CO₂ per kWh). A 7.6 kW system producing ~11,100 kWh/year offsets about 5.3 tonnes of CO₂ per year at a 0.48 kg/kWh grid factor — roughly the annual emissions of one gasoline car. The CO₂ tab computes this for your system with regional grid-factor presets.

Total solar panel cost = installed watts × your local cost per watt. Residential installed costs vary widely by country — roughly $2–3/W in the US, lower in markets like India and Australia, higher in parts of Europe. The default here is $2.50/W and is fully editable; the payback math always uses your number, never a hardcoded guess, because local labor, permits, and hardware prices differ too much.

Yes — it was built global-first. Costs and tariffs are entered in your own numbers with 20 currency formats, peak-sun-hour presets cover cities on every continent, and no US-only assumptions (state net-metering rules, utility rate plans, or tax credits) are hardcoded. Incentives are a generic percentage you control.

Yes. Balcony and plug-in systems (typically 400–800 W) are just small cases of the same math: enter your monthly kWh and peak sun hours, and the sizing will show how few panels you need. Realistic expectation: a 600 W balcony kit in a 5-sun-hour location produces roughly 870 kWh/year — a meaningful dent in a small bill, not a replacement for a rooftop system.

Installers measure your actual roof (shading, orientation, usable area), apply your utility's exact tariff and net-metering or feed-in rules, and add local labor, permits, and hardware they stock. This tool uses standardized assumptions to give a transparent ballpark. If a quote differs by more than ~25% from this estimate, ask the installer which assumption differs — that conversation usually reveals the reason.