Solar designer
Design your solar system in about two minutes
Tell us where you live, what you use, and what your roof looks like. We size the panels and the battery, estimate what you would generate and save, and show the working. No sales call needed to see the numbers.
Your home
Your postcode area sets the sunlight we can expect. Roof direction and shading matter almost as much.
A south-facing roof generates the most it can.
36.0 m² of roof — room for about 15 panels once setbacks and vents are allowed for.
Your electricity
Annual kilowatt hours is the accurate one — it is on the last page of your bill or in your online account.
Today 4,800 kWh. We size for 7,400 kWh once your planned additions are in. A 3-person home like yours typically uses about 3,500 kWh.
Your home life
This is the part most calculators skip. Sizing from today's bill alone undersizes the system for a household that is about to add an EV or a heat pump.
We size the system for the home you are about to have, not only the one you have now.
Your system
Start from our recommendation, then move the slider. Everything on the right recalculates as you go.
Above 3.68 kW your network operator has to approve the connection before install. We handle that application.
On a tariff like this the battery also fills up overnight at 7.5p and carries you through the next day.
Your Common Watt plan
10 panels and a 9.5 kWh battery
£1,007
saved a year
44%
of your electricity
Estimates for a typical year, based on the details you entered. A surveyor confirms roof condition, shading and the export connection before we quote.
Does a battery pay?
Same panels, four battery sizes. A battery does not make more electricity — it moves your own solar into the evening instead of exporting it.
| Battery | Your own solar used | Bought at night rate | Saved a year | Installed price | Pays back in |
|---|---|---|---|---|---|
| None | 1,566 kWh (21%) | — | £544 | £6,424–£8,176 | 13.4 yrs |
| 5 kWh | 2,715 kWh (37%) | 329 kWh | £789 | £9,592–£12,208 | 13.8 yrs |
| 9.5 kWh | 3,224 kWh (44%) | 1,151 kWh | £1,007 | £11,493–£14,627 | 13.0 yrs |
| 15 kWhfastest payback | 3,233 kWh (44%) | 2,545 kWh | £1,234 | £13,816–£17,584 | 12.7 yrs |
Three system sizes
Your current battery choice applied to a smaller, recommended and larger array.
| System | Size | Generated a year | Covers | Saved a year | Installed price |
|---|---|---|---|---|---|
| 9 panels — 9 panels | 4.05 kWp | 3,464 kWh | 41% | £961 | £11,097–£14,123 |
| 15 panels — 15 panels | 6.75 kWp | 5,774 kWh | 52% | £1,219 | £13,473–£17,147 |
Through the year
Generation peaks in June, your use peaks in January. That mismatch is why a UK system exports in summer and still draws from the grid in winter.
Get your full home energy plan
A Common Watt specialist checks your roof on satellite imagery, confirms the export connection, and sends a fixed price. No cost, no obligation.
What we send over
- 10 × 450 W panels — 4.50 kWp
- 9.5 kWh home battery, on an overnight tariff
- south facing, 30–35° pitch, light shading
- M28 3QT — about 36.0 m² of roof
- 7,400 kWh a year to cover, including home office, electric car (planned)
- Estimated £11,493–£14,627 installed, saving about £1,007 a year