Solar System Size Calculator
Find out what size solar system you need. Enter your electricity use and roof space, and this calculator works out the system size, panel count, and roof area for your location. It also shows whether your use, roof, or grid connection sets the limit.
Your recommended size
A 4.3 kW system with 10 panels could cover about 109% of your yearly use in Los Angeles.
| Option | Size | Panels | Covers |
|---|---|---|---|
| Smaller | 3.44 kW | 8 | 87% |
| Recommended | 4.3 kW | 10 | 109% |
| Larger | 5.59 kW | 13 | 142% |
Your energy use sets this size.
Assumptions used
- Yearly output per kW
- 1,824 kWh
- Roof room for panels
- 85% of the area you enter
- Panel size
- About 210 W per m²
What if?
Save this result, change any setting, and compare.
How this is calculated
Energy you want from solar
We multiply your monthly use by 12, add any expected growth, such as an electric car, and apply the share you want solar to cover.
What 1 kW makes where you live
Long-term NASA climate data gives the yearly output of 1 kW of panels at your location, tilt, and direction.
System size (kW) = yearly energy target ÷ yearly output per kW
Limits
We then fit whole panels to your roof, allowing about 15% for gaps, and apply any grid connection limit. The lowest limit sets your size.
Worked example: 600 kWh a month in Los Angeles
Using 600 kWh a month means 7,200 kWh a year. In Los Angeles, 1 kW of panels makes about 1,824 kWh a year, so full coverage needs about 3.9 kW.
With 430 W panels that rounds up to 10 panels, or 4.3 kW, on about 24 m² of roof.
How accurate is this?
Sizing uses the same model as our generation calculator, which stays within 10% of PVGIS for yearly output at 20 test cities. Roof fit is a quick area check, so an installer's layout may fit fewer panels around vents and edges.
Common mistakes
- Sizing from one month's bill when use changes through the year
- Forgetting planned loads such as an electric car or heat pump
- Covering 100% of use where exports earn very little
- Counting roof area that is shaded or faces away from the sun
Limits of this tool
- Yearly matching does not mean every month is covered, because winter output is lower
- Local rules can limit system size. Check with your network operator
- Roof strength needs a professional assessment
What to do next
- See if this size pays off with the solar savings calculator
- Check your roof properly with the rooftop feasibility calculator
Frequently asked questions
What size solar system do I need?
Divide your yearly use by what 1 kW makes where you live. In Los Angeles, using 600 kWh a month needs about 3.9 kW to match your yearly use.
How many solar panels do I need?
Divide the system size by the panel rating. A 4.3 kW system needs about 10 panels of 430 W.
How much roof space do I need for solar?
Allow about 5 to 6 square metres for each kW of modern panels, plus clear space around edges and obstacles.
Should I size solar to cover 100% of my bill?
Not always. Where export payments are low, a smaller system that you use directly can pay back faster. Compare both in the savings calculator.