The question every new solar buyer asks is: how many kilowatts do I actually need? Installers quote different numbers, online calculators produce different results, and neighbours with similar houses have systems of very different sizes. The confusion usually comes from skipping the fundamentals. This guide gives you the formula, explains every variable, and works through examples you can adapt to your own situation.
The Universal Sizing Formula
The core calculation is straightforward:
Required system size (kWp) = Daily energy consumption (kWh) ÷ Peak sun hours ÷ System efficiency factor
Each variable deserves careful attention.
Daily Energy Consumption
Check your electricity bills for the past 12 months and calculate your average daily consumption in kWh. Annualise it: total annual kWh divided by 365. If you are planning to add an EV or a heat pump, add those loads now — designing for your current consumption and then adding loads later forces an expensive system expansion.
Typical daily household consumption ranges:
- Small apartment (1–2 people): 5–10 kWh
- Medium house (3–4 people, no EV): 12–20 kWh
- Large house or house with EV: 25–40 kWh
- Small commercial premises: 40–150 kWh
Peak Sun Hours
Peak sun hours (PSH) measure the equivalent number of hours per day when sunlight intensity equals 1,000 W/m² — the standard test condition for panel ratings. This is NOT the number of hours of daylight. A location with 10 hours of daylight might have only 4.5 peak sun hours because early morning and late afternoon sun is weaker.
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Approximate annual average PSH by region (from NREL and Fraunhofer ISE datasets):
- Northern Europe (UK, Germany): 2.5–3.5 PSH
- Southern Europe, Middle East: 4.5–6.0 PSH
- Australia: 4.5–6.5 PSH
- Sub-Saharan Africa: 5.0–7.0 PSH
- North America (varies widely): 3.5–6.5 PSH
System Efficiency Factor
No system converts all available solar energy into usable AC power. Losses come from inverter conversion, wiring resistance, panel soiling, temperature derating, and shading. A realistic efficiency factor for a well-designed, unshaded system is 0.75–0.80. Systems with partial shading, sub-optimal orientation, or older inverters may be closer to 0.70.
Worked Examples
Example 1: Family Home in Southern Europe
- Daily consumption: 18 kWh
- Peak sun hours: 5.0
- System efficiency: 0.78
18 ÷ 5.0 ÷ 0.78 = 4.6 kWp
A 5 kWp system (rounding up to a standard package size) would comfortably cover this household's needs.
Example 2: Large Home with EV in Northern Europe
- Daily consumption: 20 kWh (household) + 8 kWh (EV, ~50 km daily driving)
- Peak sun hours: 3.0 (annual average)
- System efficiency: 0.76
28 ÷ 3.0 ÷ 0.76 = 12.3 kWp
This household needs a 12–13 kWp system. At a typical panel rating of 400–430 W, that is 29–32 panels.
Example 3: Small Business in Australia
- Daily consumption: 80 kWh
- Peak sun hours: 5.5
- System efficiency: 0.79
80 ÷ 5.5 ÷ 0.79 = 18.4 kWp
A 20 kWp commercial system (a common package size) would be appropriate, potentially with a 10 kWh battery to shift midday generation into the afternoon peak.
Adjustments for Battery Storage
If you are adding a battery, you have two choices: size the solar system to cover only your self-consumption (offset peak-rate imports) or size it to fill both your daytime loads and the battery. The latter requires a larger system.
Additional kWp needed for battery charging = Battery usable capacity (kWh) ÷ PSH ÷ 0.78
For a 10 kWh usable battery in southern Europe (5 PSH): 10 ÷ 5 ÷ 0.78 = 2.6 kWp extra capacity.
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Comparison: System Sizes for Common Scenarios
| Scenario | PSH | Daily kWh | System Size |
|---|---|---|---|
| Small EU apartment | 3.0 | 8 | 3.4 kWp |
| Medium EU home | 3.5 | 16 | 5.8 kWp |
| Large EU home + EV | 3.0 | 28 | 12.3 kWp |
| Medium AU home | 5.5 | 18 | 4.1 kWp |
| Small AU business | 5.5 | 80 | 18.4 kWp |
| GCC home + AC heavy | 6.0 | 35 | 7.5 kWp |
Roof Space Check
Once you have your target kWp, confirm you have enough roof space. Modern TOPCon panels (400–430 W, ~1.7 m²) require roughly 4–4.5 m² per kWp including inter-panel spacing. A 10 kWp system needs 40–45 m² of suitable roof area, ideally facing within 45 degrees of due south (northern hemisphere) or due north (southern hemisphere).
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