How many solar panels do I need?
Calculate your system size in kWp and panel count from your monthly electricity usage and local peak sun hours.
The number of solar panels you need depends on three things: how much electricity you use each month, how many hours of productive sun your location receives, and the wattage of the panels you choose. The calculation is straightforward once you understand the concept of "peak sun hours" — the figure most people get wrong.
kWp needed = (monthly kWh ÷ 30) ÷ (PSH × 0.80). Panels = ceil(kWp × 1000 ÷ panel wattage).
Step 1 — Find your monthly electricity consumption
Check your electricity bill for monthly kWh usage. Use a 12-month average rather than a single month — summer and winter consumption can differ by 30–50% depending on your climate and whether you heat or cool with electricity. If you plan to add an electric vehicle or heat pump, add their estimated consumption to your baseline.
Step 2 — Find your peak sun hours (PSH)
Peak sun hours (PSH) is the number of daily hours equivalent to receiving 1,000 W/m² of solar irradiance. It is not the same as daylight hours. A cloudy day may have 10 hours of daylight but only 2 PSH of useful solar energy.
Reference values (use Global Solar Atlas for your precise location):
| Region | PSH (h/day) |
|---|---|
| Australia (Darwin, Alice Springs) | 6.0–7.0 |
| Middle East / North Africa | 5.5–7.0 |
| Southwest USA (Phoenix, LA) | 5.5–6.5 |
| Spain / Portugal / Italy | 5.0–6.0 |
| South Africa (Johannesburg) | 5.5–6.5 |
| Eastern USA (New York, Chicago) | 3.5–4.5 |
| France (south), Central Europe | 4.0–5.0 |
| UK / Germany / Benelux | 2.5–3.5 |
| Pacific Northwest USA / Canada | 3.0–4.0 |
| Scandinavia | 2.0–3.5 (seasonal) |
Step 3 — Calculate system size (kWp)
Daily consumption = monthly kWh ÷ 30. System losses (inverter efficiency, wiring, temperature, soiling) typically reduce output by 20%, so we use a system efficiency factor of 0.80.
kWp needed = daily consumption ÷ (PSH × 0.80)
Step 4 — Calculate panel count
Modern residential panels are rated 380–430 W. Divide the required kWp by the panel wattage and round up: Panels = ceil(kWp × 1000 ÷ wattage).
Worked example — 350 kWh/month, 4.5 PSH, 400 W panels
Daily consumption = 350 ÷ 30 = 11.67 kWh/day. kWp needed = 11.67 ÷ (4.5 × 0.80) = 11.67 ÷ 3.6 = 3.24 kWp. Panels = ceil(3.24 × 1000 ÷ 400) = ceil(8.1) = 9 panels. Actual system: 9 × 400 = 3.6 kWp. Monthly generation = 3.6 × 4.5 × 0.80 × 30 = 388 kWh/month. Self-sufficiency ≈ 388 ÷ 350 = ~100%.
Roof space and orientation
Each panel requires approximately 2 m² of roof. A 9-panel system needs ~18 m² of unobstructed roof surface. Ideal orientation: south-facing (in the northern hemisphere) or north-facing (in the southern hemisphere), tilted at 30–35°. East or west orientation reduces output by 10–20%. North-facing (northern hemisphere) loses 30–40% — generally not recommended.
Key limitations of this method
- Shading losses not included. Any shading from trees, chimneys or adjacent buildings — even for part of the day — can reduce output by 10–40% depending on the system design.
- Seasonal variation not modeled. A system sized for annual average may underperform in winter and overproduce in summer.
- Battery storage not included. Without storage, excess daytime generation is exported to the grid (if permitted). With storage, self-consumption increases significantly.
Use the free calculator
Enter your consumption and PSH in the Solar Panel Calculator for an instant panel count, kWp and monthly generation estimate.
Frequently asked questions
How many solar panels does an average home need?
A typical home using 350 kWh/month in a moderate-sun region (4.5 PSH) needs about 9 panels of 400 W (3.6 kWp). In a low-sun region like the UK (2.5 PSH), the same home needs 16 panels. In a high-sun desert region (6.5 PSH), only 7 panels.
What are peak sun hours and why do they matter?
PSH is the daily equivalent of full-sun irradiance (1,000 W/m²). It is not daylight hours. A location with 5 PSH and 14 hours of daylight still only produces the equivalent of 5 hours of peak solar energy. Using daylight hours instead of PSH leads to massively oversized systems.
How much roof space do I need?
About 2 m² per panel. A 10-panel (4 kWp) system needs about 20 m² of unshaded, accessible roof. Modern panels are approximately 1.7 m × 1.1 m each.
Should I add battery storage?
Battery storage increases self-consumption (energy you use from your own panels) from about 30% to 70–80%. It is most worthwhile where feed-in tariffs are very low or negative, or for off-grid properties. The payback period is longer than for panels alone.
What is kWp?
Kilowatt-peak (kWp) is the rated output of a solar system under laboratory conditions. Real-world output (accounting for heat, shade, wiring, inverter) is typically 75–85% of kWp. The calculator uses 80% efficiency to account for these losses.
This guide provides simplified preliminary estimates. Grid-connected solar installations require permits, utility approval and certified installation in most jurisdictions. A professional solar design using satellite irradiance data is recommended before purchasing equipment.