Battery with a new solar system: designing both at once

A combined system usually uses one hybrid inverter for panels and battery. The panels can charge the battery directly, there is one box on the wall instead of two, and the monitoring app shows everything in one place. The inverter is then sized for the panels and the battery together, which a retrofit rarely achieves.

It also means the battery is part of the original design, so the cable routes, the consumer unit and the battery position are decided once. Scaffolding goes up once, the electrician visits once and the network operator is told about the whole system at the same time.

The array size comes from the roof: how much usable area faces which way, and what shading there is. The battery size should then follow from how much of that generation you would not use during the day, in summer and in winter. A battery that fills by lunchtime in June but never fills in December is normal; one that never fills at all is too big for the array.

If you expect to add a heat pump or an electric car within a few years, say so. Both change how much you use and when, and the inverter and consumer unit should be chosen with that in mind even if the extra equipment comes later.

• estimated annual generation and how it was calculated
• expected share used in the house with and without the battery
• inverter rating and whether it allows a second battery later
• usable capacity and power rating of the battery
• whether the design assumes a particular tariff

What matters to the local network operator is the combined output that could flow back into the grid. Panels plus battery can push a small system over the threshold where the installer has to apply before installing rather than simply notifying afterwards. That application takes longer and can come back with an export limit. A good installer checks this at the design stage and tells you whether the timetable depends on it.

Ask for the whole system to be installed under MCS. Suppliers offering Smart Export Guarantee tariffs require an MCS certificate for the solar system, and you apply to the supplier yourself. Some tariffs also pay for battery exports; the terms vary by supplier and change often, so compare them after installation rather than letting the battery size be driven by one offer.

The installation of both panels and battery in a home is currently zero-rated for VAT. The quote should therefore show no VAT on the eligible work, and any items that are standard-rated should be listed separately.

• buying the largest battery offered because the combined price looks close to the panels alone
• choosing an inverter that cannot take more battery capacity later
• placing the battery where the fire safety standard does not allow it, then having to move it
• assuming the system keeps the house running in a power cut when it has no backup function
• comparing quotes on total price without the generation and self-use estimates side by side

Tell us about the roof, your usage and your plans, and we offer the request, without your contact details, to solar and battery installers covering your area. The first to buy it has it exclusively. Before it sees your details, you receive its name, address and phone number from us by email. There is no charge to you, and no guarantee that an installer takes up the request.