Around a million UK homes have solar installed under the Feed-in Tariff, between 2010 and 2019. Those owners are now the most heavily marketed group in domestic energy: the inverters are reaching end of life, the original installers have often closed, and every battery company has their postcode. Adding a battery can be worthwhile. It can also be a poor use of several thousand pounds, and it can, if done carelessly, interfere with payments you are entitled to for years to come.
What the FiT actually pays you for
Two things. A generation payment for every kWh your panels produce, measured by the generation meter the installer fitted. And an export payment, which for almost all domestic systems is not measured at all but deemed: you are paid for export as if you exported half of what you generated, whatever you actually did.
That second point is the one that matters for batteries. Because export is deemed, using more of your own generation does not reduce your export payment. A battery that stores your afternoon solar for the evening cuts your import bill and leaves your FiT income untouched. That is the good news, and it is why a battery can make sense on a FiT system in a way it did not when the tariff was first paid.
What a battery must not do
The generation meter has to measure only what the panels produce. If a battery is wired so that electricity drawn from the grid passes through that meter on its way to the house, the meter over-reads, and you are being paid a generation tariff for grid electricity. That is a breach of the scheme rules, and your FiT licensee can claw back payments or remove you from the scheme if it comes to light.
Whether this is a risk depends on how the battery is connected. A DC-coupled battery sits on the panel side of the inverter and generally cannot cause it, but it usually means replacing your inverter with a hybrid one. An AC-coupled battery is a separate unit on the mains side, which is simpler to add, and the installer has to put it on the right side of the generation meter. A competent installer knows this. A battery salesman may not. Ask the question explicitly and get the answer in writing.
You should also tell your FiT licensee, the energy company that pays you, that you have added storage. Most ask for it, and it protects you if a question is ever raised.
Four questions before you sign
1. Will my generation meter still measure only solar? The answer should be a clear yes with an explanation of where the battery connects. If the installer looks puzzled, stop.
2. Does the maths work on my usage, not a typical one? A battery pays for itself by replacing imported electricity in the evening with stored solar, and by charging cheaply overnight on a time-of-use tariff. How much it saves depends on how much you use in the evening, what tariff you are on, and how big your solar is. A quote that assumes a “typical household” is guessing. Give the installer twelve months of bills and ask for the calculation.
3. What happens when my inverter dies? If your inverter is over ten years old, you may be replacing it within a few years anyway. A hybrid inverter that handles both solar and battery may be a better buy than an AC-coupled battery now and a new string inverter later. Ask for both options priced.
4. What exactly is the warranty on the battery? Battery warranties are written in cycles and throughput as well as years, and they often require the battery to be kept online and updated. Ask what would void it.
What to do next
Find your FiT statement and your generation meter reading from a year ago, and work out how much you generate in a year. Then look at a few months of bills for how much you import, and when. Those two numbers decide whether a battery is worth it before anyone quotes. If you would like the calculation done on your actual figures, and the metering question answered properly, that is what the Second Opinion does; if you first want to know whether the solar itself is still performing, start with the Health Check.