When you collect solar quotes, the total prices are hard to compare because they're for different system sizes. Cost per watt fixes that. It divides the all-in price by the number of watts, giving a single number you can line up side by side — the industry's standard way of judging value.
The formula
It's simple: cost per watt = total installed cost ÷ (system kW × 1000). A 5 kW system priced at 6,000 in your currency is 6,000 ÷ 5,000 = 1.20 per watt. If you'd rather think in kilowatts, divide by kW directly: 6,000 ÷ 5 = 1,200 per kW. Both describe the same value.
Use the same all-in scope
Cost per watt is only fair if every quote covers the same thing. Make sure each price is the complete installed cost — panels, inverter, mounting, cabling, protection, labour and any permits — not just the hardware. A cheap per-watt figure that excludes installation isn't really cheaper.
Why bigger systems cost less per watt
Some costs barely change with system size: the scaffolding, the day's labour, the permits, and often the inverter. Spreading those fixed costs over more watts pulls the per-watt price down, so a 10 kW system usually has a lower cost per watt than a 3 kW one. That's why the chart above slopes down — it's a genuine economy of scale, not a trick.
Cost per watt vs payback
Cost per watt tells you whether a quote is priced well; it doesn't tell you how fast the system pays for itself. For that, combine it with your electricity tariff and production in the ROI calculator and the bill savings calculator. A slightly higher cost per watt for better-quality panels can still pay back faster if it produces more or lasts longer.
Related tools
Work out the right system size with the solar panel calculator and roof area to system size calculator, estimate output with the energy production calculator, and check local pricing and payback on the Solar by Country pages. See a fully worked build in the Real Examples section.
Frequently asked questions
What is cost per watt for solar?
It's the total installed price divided by the system size in watts. A 6,000 system that is 5,000 W works out to 1.20 per watt. It's the fairest way to compare quotes because it removes the effect of size.
How do you calculate cost per watt?
Divide total cost by watts: cost ÷ (kW × 1000). For cost per kW, divide the total by kW. Always use the same all-in price across quotes.
Why is cost per watt better than total price for comparing quotes?
Total price is skewed by size — a bigger system costs more. Cost per watt normalises for size so a 4 kW and a 6 kW quote can be judged fairly. Lower is generally better value, if quality and scope match.
Does cost per watt go down for bigger systems?
Usually yes. Fixed costs (labour, scaffolding, permits, inverter) spread over more watts, so larger systems tend to have a lower per-watt price.