Solar Size Calculator
Enter your average monthly electricity use from your bill, the peak sun hours where you live and the panel wattage you are considering. You get the system size, the number of panels, yearly production and roof space.
- Sized to your bill
- Panel count
- Roof space
Losses cover wiring, the inverter, heat, dust and shading. 14% is the PVWatts default for a typical install.
Solar systems are sized by how much energy you use and how much sun you get. A home using 900 kWh a month in a place with 4.5 peak sun hours needs about 7.7 kW of panels. The same home in a sunnier place with 6 sun hours needs about 5.8 kW. Peak sun hours are the equivalent number of hours a day of full midday sun.
Peak sun hours by location
Yearly averages for south-facing panels. Search for a city or value.
| Location | Sun hours | kW for 900 kWh/month |
|---|---|---|
| Phoenix, USA | 6.5 | 5.3 |
| Los Angeles, USA | 5.6 | 6.2 |
| Dallas, USA | 5 | 6.9 |
| Atlanta, USA | 4.7 | 7.4 |
| New York, USA | 4 | 8.6 |
| Seattle, USA | 3.4 | 10.2 |
| Sydney, Australia | 4.9 | 7.1 |
| Madrid, Spain | 5 | 6.9 |
| London, UK | 2.7 | 12.8 |
| Berlin, Germany | 2.9 | 11.9 |
| Dubai, UAE | 6 | 5.8 |
| Lahore, Pakistan | 5.3 | 6.5 |
| Mumbai, India | 5 | 6.9 |
Figures are typical annual averages for a tilted, south-facing roof (north-facing in the southern hemisphere). Your exact roof may vary by 10% or more.
Sun hours and system size
A home using 900 kWh a month. Slide the sun hours.
System size falls quickly as sun hours rise. That is why solar pays back fastest in sunny regions, even though panels cost the same everywhere.
How to size a solar system
Start with your bill. Step through the maths.
Find yearly use
Add 12 months of bills, or monthly average × 12. 900 × 12 = 10,800 kWh.
If you plan to buy an electric car or heat pump, add their expected use before sizing. Adding panels later costs more than fitting them at the start.
Grid-tied, hybrid or off-grid
Your goal changes the design. Compare the main system types.
Panels feed the home and export spare power. The simplest and cheapest setup.
Adds a battery to store daytime solar for the evening and power cuts.
No connection to the grid. Must cover the worst winter weeks.
Off-grid systems need three to five days of battery storage and extra panels for winter, so they are often two to three times the size of a grid-tied system for the same use.
Before getting quotes
Know your roof and your use. Tick each item.
Tick each item as you go.
Shading from trees, chimneys or neighbouring buildings can cut output sharply. Panel optimisers or microinverters reduce the loss when only some panels are shaded.
What the result shows
The numbers to compare quotes with. Each tile explains one result.
kW of panels after rounding to whole panels.
At the wattage you chose.
Estimated kWh per year.
How much of your use it supplies.
Space the panels need.
Each 400 to 450 W panel is about 1.7 by 1.1 metres. The roof area figure allows a little extra for gaps and edges.
Frequently Asked Questions
No question matches that search.
How many solar panels do I need for my house?
Divide your daily use by sun hours and allow for 14% losses. A typical 900 kWh a month home needs about 18 panels of 430 W in a 4.5 sun hour area.
What size solar system for 1,000 kWh a month?
About 7.5 to 9 kW in most of the US, or 12 to 14 kW in the UK and northern Europe.
What are peak sun hours?
The number of hours a day the sun would need to shine at 1,000 W per square metre to deliver the day's total energy.
How much roof space do solar panels need?
About 2 m² (21 sq ft) per panel including gaps. 18 panels need about 35 m².
How much electricity does a 6 kW system make?
About 7,500 to 11,000 kWh a year depending on sun hours.
Should I size solar to cover 100% of my use?
It depends on what you are paid for exported power. Where export rates are low, 70% to 90% plus a battery is often better value.
Do solar panels work in cloudy weather?
Yes, at 10% to 25% of their rated output. Yearly averages already include cloudy days.
Size it to your bill and your sky
Use a full year of bills, find your local sun hours and allow for losses. Compare the result with installer quotes, and plan ahead for any electric car or heat pump so the system still fits your needs in ten years.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.