How many solar panels do I need?

There is no panel count that follows from the size of your house. It follows from two things: how much electricity you use and when you use it, and how much genuinely usable roof you have once setbacks, obstructions and rafter positions are accounted for. Most domestic roofs take fewer panels than their footprint suggests. The honest answer for any specific property comes out of a survey, but the reasoning behind it is straightforward enough to follow before anyone visits.

Start with consumption, not roof area

Ask for your annual electricity consumption in kWh — it is on your bill or in your supplier’s app — and then ask a harder question: how much of that is used during daylight hours?

A household where nobody is home between eight and five uses very little of what the array makes at the moment it is made. The surplus goes to export, and export is worth considerably less than the import you avoid. So a system sized to match annual consumption on paper can behave quite differently in practice from one sized to match daytime consumption.

This is why the sizing conversation usually turns into a storage conversation. A battery moves midday generation into the evening, which raises the proportion you use yourself and changes the array size that makes sense.

What the roof actually allows

Panels do not tile a roof edge to edge. The layout has to keep clear of:

• Ridges, hips and valleys
• Verges and gable edges
• Vent tiles, soil pipes, aerials and satellite dishes
• Chimneys and the shade they cast
• Dormers, rooflights and any change of pitch

On top of that, the mounting rails are fixed to rafters, so the array has to line up with rafter centres underneath. A layout that looks tidy on a drawing sometimes has to shift by half a panel once the roof is measured properly.

The practical result is that the usable area is smaller than the roof area, often noticeably so on cut-up roofs, older properties with multiple additions, and anything with a lot of roof furniture.

Panel wattage and system size

System size is quoted in kWp — kilowatts peak — which is simply the panel wattage multiplied by the number of panels. Current domestic panels are commonly rated in the region of 400–450W each, so the same kWp can be reached with different panel counts depending on the product.

That matters when roof space is tight. A higher-wattage panel gets more capacity onto a constrained roof for the same physical footprint. It matters much less when roof space is plentiful, where the sensible approach is usually the panel that suits the roof and the budget rather than the highest figure on the datasheet.

The threshold that shapes many designs

Where total generation capacity is 16A or less per phase at low voltage — roughly 3.68kW on a standard single-phase 230V supply, or 11.04kW across three phases at 400V — the installation falls under G98. The Distribution Network Operator does not have to be contacted in advance, and the installer submits an Installation Commissioning Confirmation within 28 days of commissioning.

Above that, a G99 application is required and must be approved before installation goes ahead. That approval takes time and the outcome is not automatic. It is not a reason to under-size a system that genuinely warrants being larger, but it is a reason to know early which side of the line your design sits on.

Sizing when there is a battery, an EV or a heat pump

Each of these raises the amount of generation you can use rather than export, and each pushes sensible array sizes upward.

An EV charged at home during the day is a large, flexible daytime load. A heat pump shifts consumption into the colder months, when solar output is lowest, so it raises annual consumption more than it raises self-consumption. A battery does not create demand but redistributes supply. The three interact, and a design that ignores plans you already have in mind tends to be the one you regret.

What happens at survey

We measure the roof rather than estimate it, check rafter centres and covering condition, assess shading across the year rather than on the day, and look at your consumption data alongside it. The output is a panel count and a layout for your roof, not a national average applied to your postcode.

Any range you see quoted publicly for a “typical” home — including from independent bodies such as the Energy Saving Trust — is an industry figure for illustration, not a Solar Fx figure for your property. Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.