Solar System Size & Payback Calculator
What size system do I need, and is it worth it? Sized from your real electricity use with the NREL PVWatts derate chain, and judged on how much of it you actually use yourself rather than sell back.
Your electricity use
It is on your annual statement. Any recent bill will do.
kWh
Filled in from your country — change it to your actual rate
/kWh
Advanced / Custom Input — your bills
Energy portion only. Leave at 0 to use the kWh figure above.
Applied to every year of the 25-year cashflow.
%
Solar does not remove this. Shown so the savings are honest.
/day
Your roof
This changes production more than most people expect
Leave at 0 if there is plenty of space
kW
Advanced / Custom Input — the site
Filled in from your country. The single biggest input there is.
h/day
Filled in from the roof direction above.
%
Part of the PVWatts loss chain.
%
When are you home?
This matters more than the system size — see the result
%
Filled in from your country
/kWh
Advanced / Custom Input — a battery
It is judged on its own merits in the result.
kWh
How much of your generation a battery lets you keep.
%
The system
Leave at 0 and the tool will size it to your usage
kW
Filled in from your country — use your quote if you have one
/W
Taken straight off the cost
Advanced / Custom Input — the PVWatts loss chain
%
%
%
%
%
%
%
%
Modelled separately from the loss chain, as PVWatts does.
%
Over its lifetime
yrs
Panels make a little less every year
%
Advanced / Custom Input — the sums
What money is worth to you over time. Used for NPV and discounted payback.
%
Cleaning, the odd callout.
Inverters rarely last the full 25 years. 0 to skip.
yr
Result
Fill in your usage and your roof, then press “Work out my system”.
Production modelled on the NREL PVWatts method. Estimate only — not a financial recommendation or a performance guarantee. Get quotes and check current tariffs before committing.