How many solar panels do I need?

How this calculator works

“How many solar panels do I need?” has a precise answer once you know three numbers: how much electricity you use, how much one kilowatt-peak (kWp) of panels produces on your roof, and the wattage of the panel you plan to buy. Most calculators guess the middle number from a national average of “peak sun hours”. This one calculates it.

Pick a location and the calculator downloads NASA POWER’s long-term monthly climate for that point: global irradiance, clearness index, air temperature and wind. For each month it builds an average day hour by hour, splits sunlight into direct and diffuse parts, projects it onto your roof pitch and orientation with the Hay–Davies sky model, reduces output for hot cells using the temperature coefficient of the chosen technology, and applies your loss budget and inverter efficiency. Out comes a specific yield in kWh per kWp per year, plus the monthly shape of production. We checked this monthly method against our full 8,760-hour solar panel calculator on six sites from Helsinki to Phoenix; the two agree within about ±3%.

Already have a reliable yield figure from an installer quote or PVGIS? Switch Sun data to manual and type it in.

The formula

The 4% allowance covers clamp gaps between modules. The optional 25-year average toggle divides by an extra 0.95 so the system still covers your target after two decades of 0.4%/yr degradation.

Worked example: 4,500 kWh in Bucharest

Take the defaults: 4,500 kWh a year, 430 W TOPCon panels, a south-facing roof pitched at 32° and 14% losses. NASA climate gives a specific yield of about 1,185 kWh/kWp. Covering 100% of use therefore needs 4,500 ÷ 1,185 = 3.80 kWp, or 3,800 ÷ 430 = 8.8, rounded up to 9 panels (3.87 kWp). They produce about 4,580 kWh in the first year and occupy roughly 18 m² of the 40 m² usable roof, which could hold 19 panels.

The monthly chart shows the catch: July produces about three times as much as December. From April to September the panels exceed demand by 25–50%; in December they cover only about 40%. Over the year the balance is 102%, but without a battery or net metering much of the summer surplus is exported. At €1.10/Wp the system costs about €4,260 and, with half the output self-consumed at €0.25/kWh and the rest exported at €0.06/kWh, saves roughly €710 in year one. That’s a simple payback of about six years.

How to read the results

• Panels needed is always rounded up, so coverage usually lands slightly above your target.
• Specific yield matters more than any other number. Compare it with the table below. If it looks low, check the roof azimuth (180° = south in the northern hemisphere, 0° = north in the southern).
• Roof area needed vs. max system on your roof tells you whether the target is physically possible. If not, higher-efficiency panels or a second roof face help more than higher-wattage (bigger) panels.
• Months short of demand reveals seasonal mismatch. A household with electric heating can produce 100% of its annual use and still import heavily in winter.

Typical values

East- or west-facing roofs typically produce 15–20% less than south-facing ones at the same pitch, so they need correspondingly more panels.

Tips for sizing correctly

• Use 12 months of real data. One winter bill overstates consumption; one summer bill understates it.
• Add the loads you are planning. An EV driven 12,000 km a year adds about 2,000–2,500 kWh; an air-to-water heat pump for a typical house adds 3,000–6,000 kWh, almost all in winter.
• Match the size to your export tariff. Generous net metering rewards 100% offset. Low feed-in prices favour a smaller array or a battery. The net metering calculator compares the schemes.
• Check the inverter limits. Many grids cap single-phase systems at 3.68–6 kW; see the inverter sizing calculator.
• Plan the layout. Setbacks from roof edges and ridge often eat 20–30% of the raw roof area. The solar roof area calculator draws the grid for you.

Limitations

The model uses long-term average weather, so any single year can be 5–8% better or worse. Shading from trees, chimneys or neighbouring buildings isn’t modelled, so raise the loss percentage if parts of the roof are shaded for several hours a day. Coverage is an annual energy balance, not a measure of independence from the grid. How much of the output you use directly depends on when you consume electricity, which the solar battery calculator simulates hour by hour. Cost and payback are rough estimates that ignore financing, taxes and subsidies.

Method & assumptions

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