Solar savings and payback calculator

For homeowners

What should solar on your roof save you?

Tell it how big the system is, which way the roof faces and how steep it is, and what you pay for electricity. It works out what the panels should generate, what that is worth over 25 years, and how long the system takes to pay for itself. Every assumption is on the page and you can change all of them.

The system

W

System size 4.40 kWp.

South
°

Drag the dial. South is best; east or west still works.

35°
flat20°35°50°70°
Lat
Lon

Climate band: Britain and Ireland

Your electricity

p/kWh
p/kWh

Installa’s UK defaults, from the Ofgem price cap and Smart Export Guarantee rates. Your bill and your export tariff have the real figures. Leave the export rate blank and exported electricity is not counted.

50%
out all day50%: Installa’s defaultbattery or EV
0%
flat5%10%
for payback
£

If you spread the cost

%
%
yrs

Installa’s finance calculator starts at no deposit, 0% APR and 10 years. Put in the terms you have been offered.

Enter the system cost to see a monthly figure.

Generates a year
–kWh
–
Saves in year one
–
–
Over 25 years
–
output falling 0.5% a year
Pays for itself in
–years
enter the cost to see this
Generation through the year kWh a month
Savings adding up against what you paid
Where the year’s saving comes from

How it is worked out

The money side is the same arithmetic Installa puts on a quote. The generation side is the clear-sky model from Installa’s 3D designer. A UK quote in Installa does not use that model: it takes its kWh from the design’s MCS (SAP 2012) calculation, with a shade factor from modelled irradiance data (Google Solar) for the roof where there is coverage, or from that modelled irradiance itself when the MCS figure cannot be worked out. So a quote’s kWh can differ from this page’s. Nothing is hidden and nothing is tuned to make solar look good.

1. Where the sun is

For every half hour of every day of the year, the sun’s height and bearing are calculated for the latitude and longitude you chose, using the same astronomical formulas as Installa’s 3D designer.

2. How squarely it hits the panels

The angle between the sun and the panel gives the direct light; 22% of the light is treated as diffuse and comes from the whole sky. Low sun passes through more air, so it is weakened by an air-mass factor. Together these give a clear-sky power for that moment, with 15% taken off for the inverter, wiring and heat.

3. Cloud

Britain does not have clear skies. The year’s clear-sky total is multiplied by a clearness factor for the climate: 0.47 for Britain and Ireland, 0.70 for the Mediterranean. That lands a south-facing roof at about 900 kWh per kWp, which is the flat figure Installa falls back on when it knows nothing else about a roof.

4. What the electricity is worth

The share you use at home is valued at the rate you pay; the rest is valued at the export rate. Each year the panels make 0.5% less and, if you set a price rise, electricity costs that much more.

5. Payback

The savings are added up year by year until they pass the cost; the payback is read off to a tenth of a year. If they never pass it inside 25 years, it says so.

What it does not know

Shade from trees and chimneys, the actual weather, your tariff’s time-of-use rates, a battery’s effect on self-use, or inflation. An installer with a proper design and a measured roof will give you a better number. This one tells you whether their number is in the right street.

Questions people ask

It is Installa's UK default, taken from Smart Export Guarantee rates. Some suppliers pay a good deal more, especially if you buy your electricity from them too. Put your own rate in; it makes a real difference to the payback.

A panel facing east sees the morning sun squarely and the afternoon sun at a glancing angle, and the reverse for west. Over a year in Britain that costs roughly a quarter of the output compared with due south. It is often still worth doing, particularly for a household that uses power in the morning or evening.

Not much, in Britain. Anything between about 20 and 45 degrees is within a few per cent of the best. Flat panels lose more to the low winter sun and gather more dirt; very steep ones lose the high summer sun.

It is a projection with the assumptions stated under the charts: output falling 0.5% a year and, unless you change it, no rise in electricity prices. Panels are usually warranted for 25 years or more; inverters are not, and replacing one is a cost this calculator does not include.

Because prices vary with the roof, the kit and the installer, and a made-up figure would only mislead. Use the price from a quote you have been given, then see whether the saving it claims matches what this works out.

Installers: your customers can check your quote against this

Inside Installa the same savings model sits on every quote, worked from the design’s own yield and switched on by you, with the assumptions printed beside the figures. See how quotes work, or try the string sizing calculator.

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