How Many Solar Panels Do I Need?

Find your annual electricity use in kWh on your latest bill, supplier app or annual statement. Use a full year where possible; one winter month or a direct-debit amount is not the same thing.

Then look ahead. An EV, heat pump, electric hot water, home working or other planned electrification can materially raise future use. A system designed only around today’s bill may miss demand you already know is coming.

Energy Saving Trust’s current guidance says an average home solar system is around 4.5kWp, typically around 12 panels and 20–30m² of roof area. That is useful context, not a prescription for your home.

A panel’s wattage is its rated output under standard test conditions. The whole system’s rated capacity is expressed in kilowatt-peak, or kWp.

kWp is rated system capacity. It is not the number of kWh the system will generate in a year.

Annual generation depends on location, orientation, tilt, shading, losses and system design. Two 4.5kWp systems can produce different annual totals.

The exercise shows why bedrooms are weak evidence. Even annual kWh does not finish the job: your roof may generate more or less per kWp, and matching a yearly total does not mean using all that solar at home.

Not automatically.

A system that generates the same number of kWh as you use over a year does not make you self-sufficient. Solar output is concentrated in daylight and summer; your home still imports at night and often through winter, while surplus generation may be exported.

The useful target depends on how much solar you can use directly, what exported electricity is worth, the roof capacity available and what the system costs.

For a good, unshaded roof, we’d usually price the option of filling more of it before deliberately undersizing the system.

If extra panels are sensibly priced and productively oriented, future EV or heat-pump demand can add value, and exported electricity can still earn something when the home cannot use it.

But “maximum panels” is not a universal answer. Poor orientation, shading, disproportionate system costs, inverter limits, DNO export constraints or weak marginal economics can make the last panels poor value.

Ask what each extra panel is expected to generate, where that electricity will go and what it adds to the installed price.

East/west does not rule solar out.

Energy Saving Trust says south-facing and unshaded is ideal, while east- or west-facing roofs still work and tend to generate around 15–20% less than directly south.

Direction also changes timing. East-facing panels favour the morning; west-facing panels favour later in the day. That profile can sometimes align better with when a household uses electricity, even though annual generation is lower.

If an EV or heat pump is reasonably foreseeable, include that future demand when testing the system. Designing only for current electricity use can leave useful roof potential unused.

Timing still matters. An EV may charge overnight on a cheap tariff rather than from daytime solar. A heat pump’s greatest demand is usually in winter, when solar output is lowest.

More annual demand can support more solar, but summer generation cannot be treated as a seasonal supply for winter heating.

Start with your annual kWh. Work out what your usable roof can realistically generate. Then decide whether extra panels earn their place through self-use, export or future demand.

Your installer should be able to show the proposed panel wattage, total kWp, annual generation estimate, roof layout, shading assumptions, inverter choice and any export limitation. Compare those numbers, not just the panel count.

Don’t buy a panel count. Build the right system.