Solar Panel Calculator
Work out how many solar panels you need to cover your electricity use.
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 Panel Calculator does
The number of panels follows from how much energy you need and how much each panel actually delivers. A panel rated 400 W does not produce 400 W all day — it produces its rating only at full sun, which is why output is calculated from peak sun hours and then reduced by real-world system losses.
Formula
Daily output per panel = (Panel watts ÷ 1,000) × Peak sun hours × (1 − Losses)Target daily energy = Monthly kWh × 12 ÷ 365 × Offset sharePanels = ⌈Target daily energy ÷ Output per panel⌉
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Start from | one of 2 options | Yes | — |
| Monthly electricity use | kWh | In some modes | Accepts more than 0. Shown Start from is My electricity consumption. |
| Target system size | kW | In some modes | Accepts more than 0. Shown Start from is A target system size. |
| Panel rating | W | Yes | Modern residential panels are typically 350–450 W. Accepts 1 or more. |
| 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 | Below 100% if you only want to offset part of your bill. |
How to use it
- Choose Start from and Peak sun hours per day.
- Enter Panel rating, System losses and Share of usage to cover.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
900 kWh a month, 400 W panels, 4.5 peak sun hours, 20% losses, covering 100%.
- Monthly
- 900 kWh
- Panel
- 400 W
- Sun hours
- 4.5
- Losses
- 20
Target 29.59 kWh/day. Each panel gives 1.44 kWh/day → 21 panels, an 8.4 kW system.
Frequently asked questions
Why do I need more panels than the simple wattage suggests?
Because panels only hit their rating in ideal conditions, and 20% of what they do produce is lost to the inverter, heat, wiring and dirt before it reaches your meter.
What are peak sun hours?
The equivalent hours of full 1,000 W/m² sunlight per day. A location with 4.5 peak sun hours might get 10 hours of daylight, but the weaker early and late sun adds up to 4.5 full-strength hours.
Should I aim to cover 100% of my usage?
Not always. Where export rates are much lower than import rates, sizing to cover 70–90% often gives a better return than overbuilding to sell surplus cheaply.