Solar System Size Calculator
Calculate the kW solar system size needed for your electricity consumption.
Please note: Estimates for planning only. Actual solar output depends on your latitude, roof orientation, shading, weather and equipment. Get a site-specific assessment and quotes before committing.
What the Solar System Size Calculator does
System size is the capacity needed to generate your target energy given the local solar resource. Because output equals capacity times sun hours times the derate factor, sizing simply rearranges that relationship to solve for capacity.
Formula
Daily consumption = Annual ÷ 365, or Monthly × 12 ÷ 365System kW = Target daily kWh ÷ (Peak sun hours × (1 − Losses))Specific yield = Peak sun hours × (1 − Losses) × 365
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Consumption given as | one of 3 options | Yes | — |
| Electricity consumption | kWh | Yes | Accepts more than 0. |
| Peak sun hours per day | one of 7 options | Yes | Average daily equivalent hours of full-strength sunlight for your location. |
| System losses | % | Yes | Inverter, wiring, soiling, heat and mismatch losses combined. 20% is typical. |
| Share of usage to cover | % | Yes | — |
| Allowance for future use | % | Optional | Extra capacity for an EV or heat pump. |
How to use it
- Choose Consumption given as and Peak sun hours per day.
- Enter Electricity consumption, System losses and Share of usage to cover.
- Optionally add Allowance for future use.
- Select Calculate.
Worked example
900 kWh a month at 4.5 peak sun hours with 20% losses, covering 100%.
- Usage
- 900 kWh monthly
- Sun hours
- 4.5
- Losses
- 20
- Offset
- 100
Daily use 29.59 kWh ÷ (4.5 × 0.8) = 8.22 kW system, generating about 10,800 kWh a year.
Frequently asked questions
What is specific yield?
Annual kWh generated per kW installed. It ranges from around 900 in northern Europe to 1,800 in sunny regions, and is the cleanest way to compare sites.
Should I oversize for an electric car?
If one is likely within a few years, yes — adding capacity at installation is far cheaper than a second job later. Allow 2,000–4,000 kWh a year per vehicle.