Solar Energy Calculator

This tool provides estimates for fuel bill saving and financial payments you may receive by installing a solar photovoltaic (i.e. solar PV) system. These figures are based on information you provide, using a number of assumptions to indicate potential benefits. This is to help you decide whether a solar PV system is suitable for you.

Use of the tool and its outputs is not a guarantee that you will see the exact same fuel bill savings and financial payments from the installation of your planned system.

This tool estimates potential payments from the Smart Export Guarantee , which was introduced on 1st January 2020 and requires energy suppliers to pay customers for the electricity they export to the grid. There are a range of tariffs available for this and in some cases participation requires metering. Deals may also be time limited or have other conditions which you should consider carefully in order to make the best choice for you.

The Solar Trade Association website lists current deals available.



Property details

The amount of sunlight a roof is exposed to depends on the location of a property, the amount of shading the roof receives, the slope of the roof and the direction of the slope. These factors can be specified below.

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Roof slope

The slider below will help you specify the slope angle of the most suitable roofspace. It's worth noting that the typical roof slope for the average UK home is about 30 degrees.


0° is a flat roof and 90° means that you want to install PV panels on a vertical surface such as a wall.

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Use the slider to indicate how much shading you expect to have on your roof where the solar PV system will be installed.


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Roof direction

The slider below will help you specify the direction of the most suitable roofspace.

Adjust the map to find your property and then adjust the compass to align it with the desired direction.

Please adjust slide bar to correct direction
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Installation size

Please choose the size of the system you’re interested in installing.

Our standard assumption is Medium (4kWp) as this is around the average size of solar PV panels that have been installed on domestic properties. Typical solar electricity systems usually require 10-20m2 of unshaded space. Smaller systems are possible, but less likely to be cost effective.

OR, If you know it, what is the specific peak generation of the system you plan to install in kilowatts?

When you are at home

Please tell us when you are typically at home during the week to help us estimate how much of the electricity you will use

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Your results

For more information, please read the calculator assumptions.

 kg / year
 / year


* A negative value here means that your PV system is estimated to cost this amount over its lifetime. For the non-financial benefits of a solar PV system please see the Energy Saving Trust's Solar PV web page.

kWh / year

Next steps

  • We recommend you get three quotes to compare costs and services and be sure to consider planning permission.
  • Explore the Smart Export Guarantee tariffs available to you on the Solar Trade Association website.
  • Sign up to Energywire, our monthly e-newsletter, for energy saving tips, news and more.

Solar Energy Calculator - Roof slope

If you have a flat roof, panels can be angled when installed. The optimum angle for solar power collection in the UK is 35°.

One way of determining the slope of your roof is to look at the rafters on your attic. Use the default of 30° if you are unsure as this is the typical for a UK roof.

Solar Energy Calculator - Overshading

Shading from nearby obstacles, such as trees and buildings, will reduce the solar energy that reaches your PV panels, and therefore reduce the electricity generated.

Ideally you would only install panels on roofs with no nearby obstructions, but that isn’t always possible.

Solar Energy Calculator - Roof direction

You can use the map to see which way the best side of your roof is facing. The closer you can be the better but it doesn't have to be exact. The best direction for solar power collection in the UK is south facing.

If you have a flat roof your panels can face any direction.

Solar Energy Calculator - Installation size

Solar electricity systems are given a rating in kilowatts peak (kWp). This is the rate at which they generate electricity at peak performance, for example at noon on a sunny day. The capacity (kWp) of a domestic system will vary depending on how much a customer wants to spend and the roof area available to accommodate the panels.

Solar Energy Calculator - Assumptions

This calculator estimates the performance of domestic solar photovoltaic (PV) systems. Its calculations account for the local effects of incident solar radiation (sunlight), ambient air temperature, roof direction and roof slope.

This calculator is designed to give estimates for those who have not yet installed a PV system and the outputs should not be considered a guarantee of performance.

Your actual fuel bill savings will depend on a number of factors including the size, efficiency and location of your PV system and how much electricity you use when the system is generating.

For the avoidance of doubt, the results in this calculator are not exactly the same as those estimated by the Standard Estimation Method which is used by MCS certified installers in solar PV quotations. The methodology used in this calculator is based on the Government's Standard Assessment Procedure (SAP) 2012 which uses different assumptions for the average level of solar radiation in an area.

This calculator is regularly reviewed and updated to reflect changes in electricity prices, as well as any new data relating to equipment performance and electricity use in the home.

Annual calculation assumptions

  • No annual maintenance costs have been included in our calculations (e.g. replacing the inverter or inverter components).
  • Average domestic electricity prices are used and updated regularly - the price used currently is XX p/kWh.
  • Assumed Smart Export Guarantee (SEG) tariff is XX pence / kWh.
  • Carbon emissions are estimated using the latest national grid carbon intensity published by the UK Government.
  • The amount of generated electricity which is consumed in the property is estimated following Loughborough's self-consumption model which accounts for different occupancy patterns and annual electricity generation.

Lifetime calculation assumptions (additional to the above)

  • We assume a system lifetime of 25 years with a XX% decrease in efficiency every year.
  • We assume static electricity prices and SEG tariffs throughout the lifetime of the system.
  • We estimate lifetime maintenance costs of two instances of XXX (e.g. replacing the inverter or inverter components twice during the lifetime of the system at year 10 and year 20).