How to size a solar system from your electricity bill
· 7 min read
Everything an installer's quote rests on is one number on your bill: the kWh. From it, seven short sums give the system size, the panel count, the roof area, what it will make in year one and when it pays back. Worked below for the average US home (899 kWh a month, EIA) in Texas, which comes out at 17 panels, 6.8 kW and $17,680, and for a medium UK household (2,500 kWh a year, Ofgem) in London: 6 panels, 2.4 kW and £4,056. The three calculators do the same sums for your own bill and place.
Solar panel calculatorSolar output calculatorSolar payback calculator
Step 1: find the kWh on the bill
You want kWh a month. A US bill prints the month’s kWh; take a year of bills if you can, add them and divide by 12, because a Texas August is not a Texas April. A UK bill prints an estimated annual usage in kWh; divide by 12. Ofgem’s medium band for a single-rate meter is 2,500 kWh a year, 208 a month; the EIA average US home buys 899 kWh a month. If you are about to add a heat pump or an electric car, add their kWh now, not after the panels are on.
Step 2: turn it into kWh a day
Daily kWh = monthly kWh × 12 / 365. The Texas home: 899 × 12 / 365 = 29.6 kWh a day. The London home: 6.8 kWh a day. Everything from here on is per day, because that is how sun hours are published; the 30 kWh a day guide starts from this step if you already think in daily figures.
Step 3: look up the peak sun hours
Peak sun hours are the kWh/m² a day that reach a south-facing array at a good tilt, averaged over the year: 5.3 in Texas (NREL data), 3.48 in London (PVGIS, 2005-2020). The panel calculator fills them in from your state or city; for an east or west roof the Energy Saving Trust says to expect 15-20% less, which you can enter as extra losses.
Step 4: the system size
System kW = daily kWh / (sun hours × 0.86), the 0.86 being what is left after the NREL PVWatts default of 14% losses for soiling, wiring, mismatch and the rest. Texas: 29.6 kWh / (5.3 × 0.86) = 6.48 kW. London: 6.8 kWh / (3.48 × 0.86) = 2.28 kW. This is the number to compare quotes on: an installer who offers 8 kW to a home that needs 6.5 kW should be able to say why.
Step 5: panels and roof area
Panels = kW × 1,000 / 400, rounded up; a 400 W panel is the middle of the Trina Vertex range the calculator uses and takes 1.922 m² (20.7 sq ft). Texas: 6.48 kW becomes 17 panels, 6.8 kW installed, on 352 sq ft (32.7 m²) of panel alone; gaps, edges and the roof’s shape need more. London: 6 panels, 2.4 kW, on 11.5 m². If the roof will not take it, go back to step 4 with a smaller share of the use (the calculator has a coverage field) or ask for higher-wattage panels.
Step 6: check what it will make
Year-one kWh = installed kW × sun hours × 0.86 × 365. The 17 Texas panels make 11,313 kWh a year against 10,788 kWh used, so they cover 105% because the count rounded up. The London system makes 2,622 kWh against 2,500 kWh, but not evenly: 315 kWh in June and 95 kWh in December (PVGIS monthly profile), which is what the output calculator shows for any UK city and the 4 kW UK guide tabulates for all 14 of them.
Step 7: price it and find the payback
Cost = installed kW × 1,000 × the price per watt: $2.60 in the US (EnergySage, June 2026), £1,690 per kWp in the UK (Energy Saving Trust, August 2026). Savings a year = year-one kWh × your rate if you use it all; payback = cost / savings. The table runs both homes through all seven steps.
| Step | Texas, US average home | London, Ofgem medium |
|---|---|---|
| Use a month | 899 kWh | 208 kWh |
| Use a day | 29.6 kWh | 6.8 kWh |
| Peak sun hours | 5.3 | 3.48 |
| Size needed | 6.48 kW | 2.28 kW |
| Panels (400 W) | 17 | 6 |
| Installed size | 6.8 kW | 2.4 kW |
| Roof area | 352 sq ft | 11.5 m² |
| Year-one output | 11,313 kWh | 2,622 kWh |
| Share of use covered | 105% | 105% |
| Installed cost | $17,680 | £4,056 |
| Savings a year | $1,803 at 15.94 ¢/kWh | £690 at 26.32 p/kWh |
| Simple payback | 9.8 years | 5.9 years |
Two things to notice. The Texas cost prices the installed 6.8 kW, the way the payback calculator does when the panel calculator hands it the result; the exact 6.5 kW would be $16,860, the figure the power-a-house guide quotes. And the London payback is shorter because the Ofgem cap is 26.32 p/kWh against Texas’s 15.94 ¢/kWh and £1,690 per kWp is a lower price per kW than $2.60 per watt; the UK cost guide and the Texas cost guide take the price and payback further, city by city and state by state.
When to size for less than all of it
The coverage share in step 4 is a choice. Size for less when the roof is short, when your export rate is poor (a grid-tied system at 100% exports its noon and summer surplus), or when the budget is fixed. Size for more when a car or a heat pump is coming. The table runs the Texas home at four shares; the payback stays at 9.8 years because cost and output both scale with the kW, so the share decides how much you spend, not how fast it returns.
| Share of use | Size needed | Panels | Installed | Roof area | Cost | Payback |
|---|---|---|---|---|---|---|
| 50% | 3.24 kW | 9 | 3.6 kW | 186 sq ft | $9,360 | 9.8 years |
| 75% | 4.86 kW | 13 | 5.2 kW | 269 sq ft | $13,520 | 9.8 years |
| 100% | 6.48 kW | 17 | 6.8 kW | 352 sq ft | $17,680 | 9.8 years |
| 125% | 8.11 kW | 21 | 8.4 kW | 434 sq ft | $21,840 | 9.8 years |
Take the numbers to the quote
Arrive with the kW you need, the panel count and the year-one kWh, and every quote becomes three questions: what is the price per watt, what is the installed kW, and what does the installer say it will make. The panel calculator carries its result into the payback calculator, which adds the 25-year table with the 0.5% a year output decline and any price escalation you expect; the Pro report puts all of it, month by month, in one printable page and an Excel file whose formulas recalculate when you change a cell.
Frequently asked questions
- Which number on the bill do I use?
- The kWh, never the money. A US bill shows the month's kWh next to the meter readings; add twelve months if you have them, because summer and winter differ, and divide by 12. A UK bill or annual statement shows the estimated annual usage in kWh; divide by 12 for the monthly figure the calculator asks for (the Ofgem medium band, 2,500 kWh, is 208 a month).
- What are peak sun hours and where do I find mine?
- The kWh per square meter a day that reach a south-facing panel at a good tilt, averaged over the year: 5.3 in Texas, 3.48 in London. The panel calculator fills them in for every US state (NREL data) and 14 UK cities (PVGIS), and lets you type your own from PVGIS or PVWatts for anywhere else.
- Why is the installed size bigger than the size I need?
- Because panels come whole. The Texas home needs 6.48 kW; 6.48 kW / 0.4 kW is 16.2 panels, so you install 17, which is 6.8 kW and covers 105% of the year's use rather than 100%.
- Should I size for 100% of my use?
- Only if the roof has room and your utility or tariff pays a fair rate for what you export, because a 100% system exports a lot at noon and in summer. Sizing for 75% of the Texas home's use drops it to 13 panels and $13,520; the coverage table shows 50% to 125%. In the UK the Smart Export Guarantee pays around 12p a unit against a 26.32 p/kWh cap, so a kWh you use is worth about twice one you export.
- Does the payback change with the size?
- Not at one price per watt and one electricity rate: 9.8 years in Texas for every row of the coverage table, because cost and output both scale with kW. What moves it is your quote per watt, your rate, the share you export and your place: the Texas cost guide runs the same system in every state and the UK cost guide in every UK city.
Estimates from published averages (NREL, EIA, Ofgem, industry cost surveys), each with its source and date: an installer's site survey and shading analysis replace them. Not financial advice.