Solar battery storage installation

Matching a battery to your own usage

The useful question is not how big a battery you can fit. It is how much of your generation you currently export, and how much you import once it goes dark. A house with a modest array and nobody home through the day might push a good chunk of a summer day's output onto the grid and then buy it back in the evening. Storage earns its keep on that overlap. Buy more capacity than the overlap and the extra sits idle most of the year.

Nominal capacity and usable capacity are two different numbers. A battery sold at a headline figure keeps a reserve so the cells are never taken flat, and the round trip in and out through the inverter loses a few percent more, so plan on rather less than the label says. Sizing on nominal figures is how people end up disappointed. A smaller stack that fills and empties every day is worth more than a large one that never properly cycles.

Winter is the honest test. Between November and February a domestic array in Lincolnshire will not fill a battery at all on plenty of days, which is where a cheap overnight tariff comes in. Charging on off peak units and running the house from the battery through the expensive hours works all year round, whatever the weather does, and for a lot of households that turns out to be the larger half of the saving.

Hybrid inverters against AC coupled retrofits

If solar and storage are going in together, a hybrid inverter is usually the neater answer. One unit handles the DC from the panels and the DC from the battery, there is a single point of conversion, and one app shows you the lot. We fit SolaX hybrid inverters, including on a 6.8kW flat roof domestic system where 9kWh of storage sits behind a 6kW hybrid, with shading optimisation across the array and monitoring set up on the day.

Retrofitting to an array somebody already owns is a different job. Usually the existing string inverter stays where it is and the battery is AC coupled through its own inverter, which gives up a little efficiency on the extra conversion but avoids scrapping equipment that works. The alternative is swapping the string inverter for a hybrid, which is worth weighing up if the original is already near the end of its life. We price both ways and explain where the numbers land.

Commercial storage and where it stops paying

At commercial scale storage does a rather different job. It shaves peak import, carries a site through the early evening, and on the right tariff avoids the most expensive units of the day. Alongside a 200kW array at Rolec Services Ltd we installed 200kWh of storage across thirty nine battery modules behind three 30kW SolaX ultra hybrid inverters, and on a poultry farm 30kW of storage supports the evening and early morning loads.

There are cases where the answer is not to bother. A site that consumes everything it generates as it generates it has nothing left over to store. So does a household on a flat tariff with a small array and someone home all day. Storage is a way of shifting energy in time, and if your usage already lines up neatly with your generation, there is nothing much left to shift and the money goes further elsewhere.