How many solar panels does your home need?

A solar photovoltaic system is normally described by its peak DC capacity in kilowatts peak (kWp). A 4.4 kWp array could, for example, contain ten 440 W modules. The kWp rating is not the electricity it will produce each year: annual generation also depends on location, roof direction and pitch, shading, system losses and equipment performance.

Start with annual electricity use

Use actual annual kWh from bills or your supplier account when possible. Add new demand you genuinely expect—such as an EV or heat pump—but do not add energy already included in the annual figure. Our electricity usage guide explains how to build a clean demand estimate.

Sizing an array to equal annual demand is a useful first comparison, not a promise of self-sufficiency. Solar production is concentrated in daylight and is much higher in summer, while household demand often peaks on winter evenings. The grid, a battery or load shifting still balances those timing differences.

Convert demand into an initial kWp target

Specific yield expresses estimated annual generation per installed kWp. It is written as kWh/kWp/year. A location and roof-specific yield is more informative than one national average.

For an illustrative 4,000 kWh target and an assumed yield of 900 kWh/kWp/year, the arithmetic gives 4.44 kWp. With 440 W panels, dividing 4,440 W by 440 W gives roughly ten panels. This example is deliberately simple; it does not account for roof constraints or guarantee 4,000 kWh of usable household energy.

Apply roof space, orientation and shading

The theoretical target must be checked against the usable roof. Allow for module dimensions, edges, ridges, valleys, roof windows, access and any exclusion zones required by the final design. A competent survey should assess roof condition, loading and fixing method.

• Orientation: south-facing roofs commonly provide a strong annual yield, but east/west arrays can spread generation across more of the day.
• Pitch: tilt affects annual and seasonal output. Flat-roof systems also need a mounting layout that manages wind and row-to-row shading.
• Shading: trees, chimneys and nearby buildings can change output materially and should not be reduced to a guess.
• Location: UK solar resource varies geographically, so postcode-aware estimates are preferable.

The calculator uses postcode coordinates and roof inputs with the European Commission’s PVGIS service when available. PVGIS is an estimate, not a site survey.

Check the inverter and connection

Array kWp is a DC rating; inverter output is an AC rating. They do not have to be identical, but the DC/AC ratio, string voltage, current, MPPT window and manufacturer limits must be checked together. A hybrid inverter may share an AC ceiling between solar and battery operation, while an AC-coupled battery uses a separate inverter/charger.

The distribution network connection route depends on the complete generating installation and applicable type-tested equipment. Export limitation does not by itself remove the need for the correct DNO process.

Should you size for future demand?

It can be reasonable to model a future EV, heat pump or increased home occupancy, provided the extra kWh is stated separately. Compare today’s demand and a future-demand case rather than silently inflating the baseline. A larger array can increase exports when new demand has not yet arrived, so include the export tariff as its own assumption.

What to do next

1. Find twelve months of actual electricity use.
2. Measure usable roof area or estimate a maximum panel count.
3. Run a postcode and orientation-aware calculation.
4. Compare solar only with storage rather than assuming a battery is necessary.
5. Ask installers to explain generation, self-consumption and export assumptions separately.

Useful sources and related guides

• European Commission PVGIS for location and orientation-aware solar estimates.
• Energy Saving Trust solar panel guidance for UK consumer considerations.
• MCS consumer guidance for certified installation and consumer protection.