Solar Panel Efficiency Calculator
Enter the panel watts and its length and width to get the efficiency, or work the other way round.
The solar panel efficiency calculator shows what percentage of the sunlight hitting a panel it converts to electricity. You enter the panel’s rated power and its size, and it returns the efficiency, along with power density in watts per square metre and per square foot. It works for a single module or a whole array, and it can also work backwards to find the watts or the area you need for a target efficiency.
The calculator runs in your browser, so nothing you type is uploaded. It is free and needs no sign-up. Use it when you are comparing data sheets, checking a quote, estimating how much roof space a system needs, or writing up a design record.
How to use the Solar Panel Efficiency Calculator
- Open the Find drop-down and choose a mode: Panel efficiency (from watts and size), Panel watts (from efficiency and size), or Area needed (from watts and efficiency).
- Enter Panel power Pmax in W or kW. Use the rated power from the data sheet. This field appears only in modes that need it.
- Enter Efficiency (%) if your mode asks for it. Use the module efficiency at STC, as printed on the data sheet.
- Enter Panel length and Panel width, and pick the unit for each (mm, cm, m, in or ft). Use the outside size of the whole module, including the frame.
- For a whole array, enter the total power and the total area.
- Read the results. Only the rows that match your mode appear: Panel efficiency, Panel power, Area (m² and ft²), Power density (W per m² and W per ft²), and Area per kW (m² and ft²).
- Scroll to “How it was calculated” to see the formula and each step with your own numbers.
To try the tool first, press one of the example buttons above the form, such as 400 W or 1722 × 1134 mm. Copy result copies the result lines with a link. Copy link stores every input in the page address, so the same calculation reopens later. Print / save as PDF produces the inputs, results, steps and date. Reset restores the starting example.
How do you calculate solar panel efficiency?
Divide the panel’s rated power by the sunlight power that falls on its face, then multiply by 100 to get a percentage. First find the full outside area in square metres, multiply that by 1000 W/m², divide the rated watts by the result, and multiply by 100. For the CertainTeed CTM10400HC11-06, that is 1722 × 1134 mm = 1.953 m², then 400 ÷ (1.953 × 1000) × 100 = 20.48 %.
What is the formula for solar panel efficiency?
The formula is: Solar panel efficiency = rated power (W) ÷ (panel area in m² × 1000 W/m²) × 100. The 1000 W/m² figure is the irradiance used in Standard Test Conditions (STC): 1000 W/m², a 25 °C cell temperature and the AM 1.5 spectrum. Because every data sheet uses the same reference, efficiency figures can be compared fairly between panels.
How many watts per square foot does a solar panel produce?
The answer depends on the module. One square foot equals 0.09290304 m² exactly, so W/ft² = W/m² × 0.09290304. The CertainTeed example produces 204.8 W/m², which is about 19.0 W/ft². You can also divide the rated watts by the area in square feet to get the same result.
The efficiency figure on a data sheet and the one from this calculator can differ slightly. Data sheets round the efficiency and measure the area differently, for example with or without the frame, so use the full outside size for a like-for-like comparison between panels.
What is a good solar panel efficiency?
A good efficiency is one that gives the power you need from the roof space you have. Higher efficiency means more power from less area, so a 20.5 % panel needs about 4.88 m² per kW, while a 10 % panel would need about twice as much. The tool flags any result above 30 % as higher than any commercial panel.
Worked examples
| Input | Result |
|---|---|
| 400 W, 1722 × 1134 mm | 20.48 % efficiency, 400 W on 1.953 m² |
| 100 W, 1 × 1 m | 10 % efficiency, 100 W on 1 m² |
| 22 % efficiency, 67.8 × 44.6 in | 22 % efficiency, 429.2 W on 1.951 m² |
| 6 kW at 20.5 % | 20.5 % efficiency, 6,000 W on 29.27 m² |
Calculated by the tool above with these values.
Frequently asked questions
Why is my result different from the efficiency on the data sheet?
The data sheet figure is measured at STC, which is a fixed set of lab conditions. Real output also depends on cell temperature, sun angle and system losses. The solar panel output calculator covers those factors.
Why does the calculator warn that my efficiency is too high?
A result above 30 % is higher than any commercial panel, so it usually means an input is wrong. Check that you entered the full outside size of the panel, including the frame, and that the power is in the correct unit (W or kW).
Can I use the calculator for a whole solar array?
Yes. Enter the total power of the array and its total area. For example, 6 kW at 20.5 % gives 29.27 m² of required area.
Can I work out the roof area for a given system size?
Yes. Choose “Area needed (from watts and efficiency)”, enter the system power and the panel efficiency, and read the area in m² and ft². The area per kW rows show the same figure for 1 kW.
Is a higher efficiency always better?
Higher efficiency gives more power from the same area, which matters when roof space is limited. If space is not a limit, compare the price per watt as well. This tool covers the efficiency side only.
Can I use the results in a quote or report?
Yes. Results, charts and printouts may be used freely in commercial work such as quotes, proposals, reports and service records, and no attribution is needed.
Related tools
- Solar panel output calculator: use it to estimate real output after temperature, sun angle and losses.
- Solar panel calculator: use it as the starting point for sizing a system.
- Solar string calculator: use it to plan how panels connect into strings.
- Solar panel angle calculator: use it to find a suitable tilt for your panels.
- Solar payback calculator: use it to estimate how long a system takes to pay back its cost.
- All solar and battery calculators: use it to browse every calculator on the site.
Sources
- CertainTeed CTM10400HC11-06 data sheet: example 400 W module (Voc 37.04 V, Isc 13.79 A, Vmp 31.01 V, −0.27 %/°C Voc, 1722 × 1134 mm)
- Standard Test Conditions (1000 W/m², 25 °C cell temperature, AM 1.5 spectrum): PVEducation, Module measurement; defined in IEC 60904-3 (the standard text is not reproduced)
- 1 ft = 0.3048 m exactly, so 1 ft² = 0.09290304 m² (NIST SP 811, Appendix B.8 notes)