Solar Panel System Size Calculator
Work out what size solar system you need from your electricity usage, sun hours and panel wattage.
What this tool does
A solar system size calculator works out how large a solar panel system you need to cover your electricity usage. Enter your monthly electricity consumption in kilowatt-hours, the peak sun hours for your location, and the system losses (inverter efficiency, wiring, temperature). The calculator divides your daily energy needs by peak sun hours and applies a derating factor to find the required system size in kilowatts, then shows how many panels that represents at a standard wattage.
Takes your monthly electricity consumption in kWh and converts it to a daily energy requirement.
Divides by peak sun hours and applies a derating factor for inverter efficiency, wiring losses, temperature and soiling.
Shows the required system size in kilowatts and the estimated number of panels at a standard wattage.
Lets you adjust panel wattage to see how panel efficiency changes the count.
How to use it
- Enter your average monthly electricity consumption in kWh.
- Enter the peak sun hours for your location.
- Adjust the system loss percentage (the default covers typical inverter and wiring losses).
- Enter the wattage of the panel you are considering.
- Read the required system size in kW and the estimated panel count.
Why use this calculator
Accounts for the derating factor every real system has - inverter efficiency, wiring losses, temperature and soiling - rather than dividing by peak sun hours alone. A system that looks big enough on paper often underperforms in the real world because of these losses. This calculator builds them in from the start, so the size it recommends has a realistic chance of delivering the energy you need.
Why we built it
We built the solar panel system size calculator to show both kilowatts and panel count, because the two numbers answer different questions: kW tells you the size of the system, and panel count tells you whether it will fit on your roof. The solar generation calculator works the other way - it tells you how much a given system will produce. The solar savings calculator then shows what that production is worth in money saved.
If your task goes further, try the electricity bill calculator to work out your current costs, and the energy consumption calculator to break down where your electricity goes.
Who this tool is for
- Homeowners considering solar who want a realistic system size
- Renters or buyers comparing properties with different sun exposure
- Students learning about solar energy and system design
- Anyone who has been quoted a system size and wants to sanity-check it
When to use it
- Before getting solar quotes to know what size system to expect.
- When moving to a new home with different electricity usage.
- When considering a battery and wanting to size the solar array to match.
- When checking whether a quote for a given number of panels makes sense for your usage.
How it is calculated
Daily energy need = monthly kWh ÷ 30. Required system size in kW = daily energy need ÷ peak sun hours ÷ derating factor. Panel count = system size in watts ÷ panel wattage, rounded up.
Worked example: a household using 900 kWh per month in a location with 4.5 peak sun hours and a derating factor of 0.8 (20% losses). Daily need = 900 ÷ 30 = 30 kWh. System size = 30 ÷ 4.5 ÷ 0.8 = 8.33 kW. With 400-watt panels, that is 8,333 ÷ 400 = 20.8, rounded up to 21 panels.
A derating factor of 0.8 is typical for a well-designed system with a good inverter. Older systems or those with long cable runs may need a lower factor.
What you enter, and what you get back
| Item | Details |
|---|---|
| Monthly consumption | Your average electricity use in kWh per month. |
| Peak sun hours | The equivalent hours of full-power sunlight per day for your location. |
| System losses | The derating factor for inverter, wiring, temperature and soiling losses. |
| Panel wattage | The wattage of the panel you are considering. |
| Result | Required system size in kW and estimated panel count. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| System size seems too large | Peak sun hours are low for your location. | Check a reliable solar irradiance source for your area. |
| Panel count is not a whole number | The calculation always rounds up to the next panel. | Round up or accept slightly more coverage. |
| Quote does not match | The installer may use a different derating factor or panel wattage. | Ask the installer what assumptions they used. |
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Monthly consumption | 900 kWh |
| Peak sun hours | 4.5 hours |
| System losses | 20% (derating 0.80) |
| Panel wattage | 400 W |
With those inputs, the calculator returns:
- Daily energy need: 30.0 kWh
- Required system size: 8.33 kW
- Estimated panel count: 21 panels at 400 W
Assumptions and limitations
- Uses average monthly consumption; seasonal variation is not modelled.
- Peak sun hours must be sourced from a solar irradiance database or local knowledge.
- Does not account for shading from trees, chimneys or neighbouring buildings.
- Roof orientation, tilt and available area are not checked - use a site survey.
- A planning tool, not a substitute for a professional design.
Frequently asked questions
How do I calculate what size solar system I need?
Divide your daily electricity consumption in kWh by the peak sun hours for your location, then divide by a derating factor (typically 0.75 to 0.85) to account for system losses. This gives the required system size in kilowatts.
What are peak sun hours?
Peak sun hours is the number of hours per day when sunlight averages 1,000 watts per square metre. It is a measure of total solar energy, not just daylight hours. A location might get 12 hours of daylight but only 4 to 5 peak sun hours.
How much do system losses affect the size?
System losses from the inverter, wiring, temperature and dirt typically reduce output by 15 to 25%. Ignoring them means undersizing the system by that amount, so a 6 kW system might only deliver 4.8 kW in real conditions.
How many panels will I need?
Divide the required system size in watts by the wattage of your chosen panels. For example, an 8 kW system with 400-watt panels needs 20 panels. Higher-wattage panels mean fewer panels and less roof space.
What is a kW of solar?
A kilowatt (kW) is a thousand watts and measures the maximum power a solar system can produce under ideal conditions. A 6 kW system can produce up to 6 kW at midday on a clear day, but its total daily energy depends on how many hours it runs near that peak.
Does shading affect the calculation?
Yes - shading from trees, chimneys or neighbouring buildings can dramatically reduce output, especially if panels are not optimised. The calculator cannot account for shading, so if your roof is partially shaded, the real output will be lower than the estimate.