Estimate the solar system size and number of panels you need from your electricity use and local sun hours — plus rough cost, annual savings and payback.
Number of panels = system size (W) ÷ panel wattage, rounded up
Annual production = kW × sun hours × 365 × efficiency
Payback = years until cumulative savings ≥ net system cost
How to use the solar panel calculator
Enter your average monthly electricity use in kWh (add up the last 12 bills and divide by 12 for the best figure) and your location's peak sun hours — the number of hours per day that, on average, deliver the equivalent of full-strength sunlight (1,000 W/m²). Choose how much of your bill you want to cover, the panel wattage you are considering, and your local installed cost, incentives and electricity price. The calculator sizes the system, counts the panels, estimates the roof area, and projects savings and payback over 25 years.
The solar sizing formula
Number of panels = system size (W) ÷ panel wattage, rounded up
Annual production = kW × sun hours × 365 × efficiency
Payback = years until cumulative savings ≥ net system cost
The system efficiency (also called derate or performance ratio) accounts for real-world losses: inverter conversion, wiring, panel heating, dust, mismatch and minor shading. 75–85% is typical for a well-designed rooftop system. Panels also lose roughly 0.5% of their output each year, which the payback projection includes.
Worked example
A home using 900 kWh a month needs about 29.6 kWh per day. With 4.5 peak sun hours and 80% efficiency, the system must be 29.6 ÷ (4.5 × 0.8) = 8.2 kW. Using 400 W panels that is 20.5, so you would install 21 panels (8.4 kW), producing about 11,000 kWh a year and covering roughly 40 m² (430 sq ft) of roof. At $2.80 per watt the system costs about $23,500. With electricity at $0.17/kWh rising 3% a year, first-year savings are around $1,880 and the system pays for itself in roughly 11 years — then keeps producing for another decade or more.
Typical peak sun hours
| Region | Annual average peak sun hours |
|---|---|
| UK, Ireland, northern Germany | 2.5–3 |
| Northern US, Canada, central Europe | 3–4 |
| Southern US, Spain, Italy | 4.5–5.5 |
| India, Australia, South Africa | 4.5–6 |
| US Southwest, Middle East, North Africa | 5.5–6.5 |
For an accurate figure, free tools such as NREL's PVWatts or the EU's PVGIS give location-specific solar data, including the effect of roof angle and orientation.
What affects cost and payback
- Installed price per watt varies widely: around $2.50–$3.50/W in the US in 2026, and often well under half that in Australia, India and parts of Europe.
- Incentives change frequently. In the US, the 30% federal residential clean energy credit no longer applies to home systems placed in service after 2025, but state, utility and local programmes may still help. Enter whatever applies to you as a percentage.
- Net metering rules matter enormously. This calculator assumes every kWh you generate is worth your full retail rate. Where exported power is paid much less (as under many newer tariffs), self-consumption — or adding a battery — becomes important, and payback lengthens.
- Roof orientation and shade: south-facing roofs (north-facing in the southern hemisphere) at 20–40° tilt produce most; east/west roofs produce 10–20% less.
This is a rough planning estimate, not a quote or financial advice. Get site assessments from accredited installers, who will account for shading, roof structure, local tariffs and incentives.
Cite or link to this page
Using this solar panel calculator in an article, report or homework? Copy a citation or a ready-made link:
Calcivia. (2026). Solar Panel Calculator. Retrieved October 31, 2026, from https://www.calcivia.com/solar-panel-calculator/"Solar Panel Calculator." Calcivia, 2026, https://www.calcivia.com/solar-panel-calculator/. Accessed 31 Oct. 2026.<a href="https://www.calcivia.com/solar-panel-calculator/">Solar Panel Calculator</a> by Calcivia