How many solar panels do I need for my home?

Divide your daily electricity consumption (monthly kWh ÷ 30) by your local peak sun hours × 0.80 (system efficiency). That gives you the required system size in kWp. Divide by the panel wattage to get the number of panels.

kWp (kilowatt-peak) is the rated power output of a solar system under standard test conditions (1,000 W/m² irradiance, 25°C). A 400 W panel has a rated output of 0.4 kWp. System size in kWp = number of panels × panel wattage ÷ 1,000.

How much roof space do I need for solar panels?

A typical 400 W panel occupies about 1.7–2.0 m² of roof space. For a 4 kWp system (10 panels), you would need roughly 17–20 m² of unshaded south-facing (northern hemisphere) or north-facing (southern hemisphere) roof.

Solar Panel Calculator

Q: What are peak sun hours?

Peak sun hours (PSH) represent the equivalent hours per day of full sunlight (1,000 W/m²). Sunny regions like southern Europe, the Middle East or Australia typically have 5–6 PSH. Northern Europe may have only 2.5–3.5 PSH. Look up your location's PSH in a solar resource map.

Calculate how many solar panels you need, the system size in kWp and your estimated monthly generation based on your electricity consumption and location.

What this calculates

Calculates the required system size in kWp, number of panels, actual system size, estimated monthly generation, self-sufficiency percentage and roof area needed.

Formula used

Daily consumption = monthly ÷ 30. kWp needed = daily ÷ (PSH × 0.80). Panels = ceil(kWp × 1000 ÷ wattage). Generation = kWp × PSH × 0.80 × 30 days.

Worked example

Monthly use: 350 kWh. PSH: 5 h/day. Panel: 400 W. Daily: 11.67 kWh. kWp needed: 2.92. Panels: 8 (actual: 3.2 kWp). Monthly generation: ≈ 384 kWh. Self-sufficiency: ~100%.

Peak sun hours by region

Sunny climates (Australia, Middle East, SW USA, N. Africa): 5.5–7 h/day. Mediterranean / S. Europe: 4.5–5.5 h/day. Central Europe / UK: 2.5–3.5 h/day. Look up your specific location in a solar resource map (e.g., Global Solar Atlas) for an accurate figure.

When not to use this calculator

This calculator assumes the system is grid-connected and sized to cover average monthly consumption. Off-grid systems, battery storage sizing and shading analysis require a detailed solar design study.

How to use this calculator

Enter your monthly electricity consumption in kWh — this is on your utility bill. Use a 12-month average for accuracy.
Enter the peak sun hours (PSH) for your location — the number of hours per day when solar irradiance equals 1,000 W/m². Check the Global Solar Atlas for your exact location.
Enter the panel wattage — modern residential panels are typically 380–430 W. The calculator works for any value.
Click "Calculate solar panels" to see panel count, total kWp, estimated monthly generation and self-sufficiency percentage.

Peak sun hours by major region

PSH values represent the equivalent daily hours of full sun (1,000 W/m²). Use these as starting estimates:

Australia (Darwin, Alice Springs): 6.0–7.0 h/day. Sydney / Melbourne: 4.5–5.5 h/day.
USA (Phoenix, Las Vegas): 5.5–7.0 h/day. New York / Chicago: 3.5–4.5 h/day. Seattle / Boston: 3.0–4.0 h/day.
Spain / Portugal: 5.0–6.0 h/day. Germany / UK: 2.5–3.5 h/day. France (south): 4.5–5.5 h/day.
Chile (north, Atacama): 6.0–7.5 h/day. Santiago: 5.0–5.5 h/day. Puerto Montt: 3.0–3.5 h/day.
South Africa (Johannesburg): 5.5–6.5 h/day. Nigeria / Kenya: 5.0–6.5 h/day.

Connecting to the grid — regulatory requirements

Before purchasing equipment, verify local grid-connection requirements. Every country has different rules:

Most countries require a licensed installer and utility approval before connecting to the grid.
Net metering / feed-in tariff availability determines whether surplus generation has economic value.
Roof structural capacity must be verified — modern panels weigh 10–15 kg/m² (≈ 1.0–1.5 kPa). Older roofs may need reinforcement.
Some jurisdictions require building permits for rooftop solar above certain system sizes.

Frequently asked questions

How many solar panels do I need?

Daily kWh ÷ (peak sun hours × 0.80) = kWp needed. Divide by panel wattage and round up. The calculator does this automatically.

What are peak sun hours?

Equivalent daily hours of full sun (1,000 W/m²). Typically 2.5–7 depending on location. Find yours on Global Solar Atlas.

What is kWp?

Kilowatt-peak — the rated output of your solar system. 10 panels × 400 W = 4 kWp system.

How much roof space do I need?

About 2 m² per panel. A 10-panel system needs roughly 20 m² of unshaded roof.

Assumptions

System efficiency: 80% (accounts for inverter, wiring and temperature losses).
Roof area: 2.0 m² per panel (standard for modern 400 W panels).
Self-sufficiency capped at 100% — surplus generation not shown.
Does not include battery storage, shading factors or orientation losses.

Technical note

Actual generation varies with roof orientation, shading, local climate and system losses. A professional solar design using satellite irradiance data (e.g., PVsyst) is recommended before purchasing equipment.

Common mistakes

Using average daily sun hours instead of peak sun hours — these are different values.
Not accounting for shading from trees, chimneys or adjacent buildings.
Sizing only for current consumption without considering future EV charging or heat pump loads.

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