Solar Panel Electricity Output Estimate Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

The UK industry baseline is around 850 kWh per year for every kWp installed, for an unshaded south-facing roof in an average location, so a typical 4 kWp system generates roughly 3400 to 3800 kWh annually, around the total electricity use of an average home. Generation is strongly seasonal, with a summer month producing four to five times a December one, which is why annual figures rather than daily ones are the honest way to size expectations.

System size

Solar systems are sized in kilowatts peak (kWp), the output of the panels under standard test conditions. A 4 kWp system of around 10 modern panels is the most common choice for a UK three bedroom home, while larger households, or homes with an electric vehicle or heat pump, often go to 6 kWp or more. Bigger systems cost less per kWp because fixed costs such as scaffolding, the inverter and installer overheads are spread across more panels, and the calculator reflects that economy of scale.

Roof orientation

South-facing roofs at a 30 to 40 degree pitch are the UK optimum. South-east and south-west give up only around 5 percent, and straight east or west roofs around 15 percent, often with the useful side effect of spreading generation into morning or evening when you are more likely to be home. North-facing roofs produce roughly 35 to 40 percent less than south and are usually only worth panelling when no better face exists.

Shading

Shade is the biggest silent output killer, because on a conventional string inverter one shaded panel drags down its whole string. Light intermittent shade from a chimney or aerial costs a few percent; trees or neighbouring buildings shading panels for hours cost far more. Optimisers or microinverters recover much of the loss on shaded roofs by letting each panel perform independently, at extra hardware cost.

Region (solar resource)

The same panels generate more in Cornwall than in Caithness. The industry baseline is around 850 kWh per year for every kWp installed on an unshaded south-facing roof in an average UK location, with southern England running around 5 percent above that and Scotland 10 to 15 percent below. The calculator scales its output and savings figures accordingly.

What else affects the figures

Real output depends on the local solar resource, orientation, pitch, shading, panel temperature and system losses in cables and the inverter, which together explain why two neighbouring homes can differ by hundreds of kWh. Panels also degrade slowly, losing around 0.3 to 0.5 percent of output per year, so a quality system still produces roughly 85 to 90 percent of its original output after 25 years. An MCS-certified installer must give you a location-specific generation estimate using the standard MCS methodology before you sign, and comparing that figure across quotes is a good check that nobody is overpromising.

Certification, planning and grid connection

If a calculation here prompts a project, the practical next step is an MCS-certified installer’s survey: MCS installers must provide a location-specific generation estimate using the standard industry methodology, which turns the broad ranges on this page into a figure for your actual roof. Most domestic solar work falls under permitted development, with the usual exceptions for listed buildings, flats and conservation areas, and installations must be notified to the district network operator under the G98 or G99 procedures, which a competent installer handles as part of the job.