Solar panel, battery and inverter sizing

Annual consumption on your electricity bills gives the scale of your use. Half-hourly smart-meter readings, if available from your supplier, show its timing: evening cooking, overnight charging and winter heating can create very different needs. A complete year is useful because summer and winter rarely look alike. A short summer sample can give a misleading picture of winter demand.

Grid import is not always total household demand. If you already have solar or a battery, some electricity may be supplied without passing through the import meter. Generation and battery records can help explain the difference.

If you only know your annual kWh, you can still start planning. Consider how many people are at home during the day, evening appliance use and future heating or EV needs. Estimates based on a typical daily pattern are less personal than measured readings, and an annual total cannot establish a measured peak.

An unshaded south-facing roof generally gives strong annual generation; east and west roofs can also be useful. Direction, slope and shade change both the amount and timing of output. Annual generation is not a promise that solar will cover the same amount of household demand: generation and consumption must coincide, or energy must be stored.

A satellite map can help identify roof faces and approximate area. Check direction, slope, chimneys, roof windows and trees, then ask for a generation estimate for the proposed layout. A survey must confirm panel dimensions, roof edges, structure and access. A map outline alone does not establish how many panels can be safely installed.

Consider planned heat-pump or EV use, the export tariff and whether extra panels would earn enough to justify their cost. A home battery moves energy between hours; it cannot practically move a summer surplus into winter.

• Energy Saving Trust: roof suitability and solar generation

Look at the energy used outside low-price periods and the solar surplus available to store. A battery can serve evening demand, shift cheaper grid electricity to expensive hours, or do both. A larger battery only adds value when there is enough energy to charge it and enough worthwhile demand or export opportunity to discharge it.

Use the manufacturer's usable capacity when comparing products. Nominal capacity can include energy that normal operating limits keep unavailable. A further reserve for backup reduces what is available for daily bill savings. Check which reserves are already included so you do not subtract them twice.

• MCS guidance: usable battery capacity (section 4.5, PDF)

Capacity tells you how much can be stored; charging and discharging power tell you how quickly. The battery, inverter and controls can each impose a limit. Check continuous ratings, not just a short-duration peak figure.

Charging 10 kWh at 5 kW takes at least two hours in an ideal calculation. Losses, charging taper and competing household loads can extend that time. A large battery with a short cheap window may never fill as expected. A low discharge rating can also leave the grid supplying part of a busy evening.

Use every tariff window, the standing charge and export rate in the comparison. Stored solar has an opportunity cost: electricity kept in the battery cannot also be sold for export.

• Energy Saving Trust: using battery storage and time-of-use tariffs

For a shared hybrid inverter, solar and battery output can share an AC power limit. AC is the electricity supplied to household appliances; panels produce DC electricity, which the inverter converts. An existing solar system with a separate battery inverter has a different arrangement. Tell your installer what is already installed so compatibility and any replacement work are included.

While connected to the grid, an inverter does not have to cover every household peak: the grid can supply the remainder. Half-hourly data shows average demand, so an installer must still check short appliance surges and any backup loads.

Some designs use a solar array rated above the inverter's AC output. This can increase generation at lower sunlight levels, while limiting output during high-power periods, known as clipping. It must stay within the manufacturer's DC voltage, current and other limits; there is no single ratio suitable for every product.

• Sandia National Laboratories: inverter clipping and input limits

A larger system can save more per year while taking longer to recover its extra cost. Compare the additional installed price with the additional annual benefit. An online planning allowance or advertised equipment price is not a complete installation quote.

Identify equipment that will be retained, replaced or added, so existing equipment is not priced as a new installation. Keep tariff-switch savings separate from equipment savings: a cheaper tariff alone may reduce the bill without buying a larger system.

• Exact panel count and rating, roof layout, generation estimate and shade assumptions.
• Battery nominal and usable kWh, continuous charge/discharge kW, reserve and inverter compatibility.
• Inverter AC rating, solar input limits, export controls and any included backup circuits.
• Equipment, labour, scaffolding, electrical upgrades, commissioning and applicable tax, with exclusions stated.
• Warranty duration, retained capacity, throughput or cycle limits, and likely replacement costs.
• Savings assumptions: usage coverage, tariff dates, export rate, storage losses and future heating or EV use.