How the generation is worked out
System capacity is the number of panels multiplied by the rating of one panel, divided by 1,000. Ten 400 Wp panels make 4 kWp.
Annual generation is the capacity multiplied by the annual yield per installed kilowatt. That yield already bundles up the sunshine hours, the roof pitch, the inverter losses and the shading, which is why it is the figure to check.
A production ratio of 1,400 kWh per kW suits a sunny state; the north east is closer to 1,100 and the south west nearer 1,600.
Worked example
Twenty 400 W panels make an 8 kW system. At 1,400 kWh per installed kilowatt that is 11,200 kWh a year, or 933 kWh a month on average. If 60 per cent is used on site, that is 6,720 kWh, and at your own import price it gives the annual bill saving.
The saving appears only when you enter a price, because the tariff is yours and it changes every year.
Why the yield is the number that matters
Two identical systems a hundred miles apart can differ by a third in annual output. Roof orientation matters more than panel quality: a north-facing roof in the UK produces far less than a south-facing one.
Shading is the other large factor. A chimney that shades one panel can cut the output of a whole string, which is why micro-inverters and optimisers exist.
Panel ratings are quoted under standard test conditions that a real roof rarely reaches. A 400 W panel does not produce 400 W for long, which is already reflected in the yield figure.
Limits of this calculator
It does not know your address, your roof or your shading. The yield is a starting figure you should replace with a local one.
It does not model battery storage, time-of-use tariffs, export payments or degradation, all of which change the economics rather than the generation.
It does not size the inverter, check the roof structure, or tell you whether you need permission. Those are installer and planning questions.
To compare generation with what the house uses for heat, use the US heat loss calculator.
Inputs for the US solar panel calculator
Number of panels
How many panels fit, allowing for shading, access and the roof edges.
Panel rating
Rated output of one panel in watts, 400 W for a current panel.
Annual production ratio
A production ratio of 1,400 kWh per kW suits a sunny state; the north east is closer to 1,100 and the south west nearer 1,600.
Share used in the home
How much generation is used on site rather than exported. A battery raises this.
Your import price
What you pay per kilowatt hour. Enter zero to leave the saving out of the result.
CALCULATOR QUESTIONS
US solar panel calculator questions
How much does a 4 kW system generate?
About 3,800 kWh a year in central England at 950 kWh per kWp, and more in the south west or in sunnier US states.
Does the panel rating mean that output?
No. The rating is under standard test conditions, which a roof rarely reaches. The yield figure already accounts for that.
How much of it will I use?
Typically 40 to 60 per cent without a battery, because generation peaks in the middle of the day when the house is often empty.
Is the saving guaranteed?
No. It depends on your tariff, your usage pattern and any export payments, none of which this page knows.