Home battery for solar panels: whether it pays, what size

A home battery stores the electricity your panels make at midday and would otherwise send to the grid, and gives it back in the evening. Whether that is worth paying for comes down to one gap: what you pay to import a kWh versus what your supplier pays you to export one. In both the UK and Ireland you are paid per metered exported kWh, so the gap is real, but its size is your tariff’s business, not the battery’s. And the figure that tells you how big a battery could ever be useful is one you already own: your own metered export.

What a home battery does with your midday surplus

A battery does not make the roof produce more. It moves kWh that were leaving the house at 1pm to the kettle at 7pm, turning an exported kWh, paid at your export rate, into an imported kWh you no longer buy.

Some of the energy is lost on the way. Enphase gives the IQ Battery 5P an AC round-trip efficiency of 90% (96% DC), measured at half power and 25°C, and says that in a real house the efficiency “will always be lower”. The battery also uses a little energy for its own upkeep when it is idle. Treat 90% as a best case in any sums you do.

The rule that decides whether it pays: you are paid per metered exported kWh

Under the Smart Export Guarantee, every supplier with at least 150,000 domestic customers must offer at least one SEG tariff, and smaller ones may. It pays only for metered export: you need an export meter that takes half-hourly readings and has an export MPAN. The rate is the supplier’s to set, and the spread is wide: in April 2026 Which? found SEG rates from “a paltry 1p to 25p per kWh”.

If your export rate is near the bottom of that spread, every kWh you stop exporting and start using yourself is worth almost your full import price. If you are on one of the tariffs near the top, the same kWh moves much less money.

Two rules to know before you sign anything:

• A battery can complicate what you are paid. If your export meter also records electricity from a source that is not SEG-eligible, such as a battery charged from the grid, the licensee does not have to pay SEG on it: it can pay for all of the export, none of it, or ask you to fit extra meters. So whether you can charge cheaply at night and export at a better rate is your licensee’s call. Ask them, in writing, before the battery goes in.
• You are paid on the meter, not on the inverter. A SEG licensee does not have to pay until it has your actual export meter reading, and under the feed-in tariff Octopus states plainly that “we can’t make FiT payments based on estimated or inverter readings”.

Ireland: the Clean Export Guarantee, and the form a retrofit battery needs

Ireland works the same way: your supplier pays for every metered kWh you export over the billing cycle, and a smart meter records it every half hour. Power from your panels that is stored in a battery and used or exported later still counts as renewable, the CRU confirms.

If your house has no smart meter, ask your supplier in writing how your export is established before you size anything against it.

A retrofit needs one piece of paperwork. ESB Networks needs an NC6 form before the installation for all microgeneration under 6 kW/25 A single-phase (11 kW/16 A three-phase), and counts the battery as a generator in its own right: an AC-connected battery goes on the NC6 with its rated output in kW, as an additional generator. It must meet EN 62109 and EN 50549 with Irish protection settings, and the AC isolator must be labelled “Microgeneration / Battery” or equivalent. Ask your installer to confirm all of that in the quote; none of it is a job for you.

Subsidies and VAT: the relief that exists today, and the date on it

United Kingdom. Since 1 February 2024, a battery installed in a home is zero-rated for VAT, whether it is retrofitted to existing solar panels, stands alone and charges from the grid, or does both (HMRC Notice 708/6). The relief has an end date: the zero rate runs from 1 May 2023 to 31 March 2027, and from 1 April 2027 it reverts to the reduced rate of 5%.

Ireland. The rule is narrower. Revenue has zero-rated solar panels supplied and installed on private homes since 1 May 2023, and the batteries with them when they are part of the same supply-and-install contract. A battery bought on its own, years later, is not what that covers, so ask your installer to state the VAT treatment on the quote.

What size battery, in kWh: sizing it from your own export figures, not from a brochure

The only number that limits what a battery can earn is the surplus that is currently leaving your house. You have two records of it:

1. Daily production, from the inverter. Your manufacturer app (SolarEdge, Enphase, Growatt) shows kWh per day, and some will export a month or a year to a spreadsheet, so check yours. If you are not sure how to read it, how to tell whether your panels are working walks through where the figure lives.
2. Metered export, from the meter or the bill. In the UK this is the SEG statement against your export MPAN; in Ireland it is the metered export on your bill.

Take a run of ordinary good days (not the single best day of the year) and read the export. The battery can only ever recycle what is in that column, and only up to its usable capacity. Enphase quotes 5.0 kWh usable for the IQ Battery 5P; use the usable figure, not a nameplate one, and check which figure any quote you are given refers to. Batteries are quoted in both kW and kWh, and the two are not interchangeable: kW is the power the battery can deliver at once, kWh is the energy it holds, and only the kWh figure tells you how much of your surplus it can take. When you compare prices between quotes, compare them per usable kWh.

Your own production history is the input, and you already have it

Every number above is a multiplication where one input is yours: how many kWh the roof made, day by day, over a real year. The manufacturer app or a monthly report gives you that. Check first whether that record is any good: a year in which the system underperformed makes a battery look smaller than it should, and a year with a dead string makes it look pointless.

So before you size anything, compare the days and weeks in that history against what the weather allowed. The method is in check your solar production against the weather. If a month comes out well below what the light that month should have produced, sort that out first, because the battery decision changes once the roof is back to full output. And if the production record and the export record disagree in a way you cannot explain, remember Ofgem’s rule above: the export meter and the inverter are two different records, and you are paid on the meter.

What a battery does not fix

A silent inverter stores nothing. The battery charges from production. If the inverter stops, the battery has nothing to store, the export stops, and, because payment follows the meter reading, nothing about the bill announces it. Which?’s April 2024 member survey (n>2,000) found 15% of owners had experienced inverter problems, while 22% said they monitor day-to-day performance (Which?). Neither of those is something a battery addresses: it charges from whatever the inverter produces, and it adds a second device to the same unwatched system.

The roof still needs watching, and a fault does not have to announce itself. A string can be dead while the inverter shows no error at all:

Since installation the generation figures have been slightly lower than before but I put this down to cloudier weather and deterioration of the solar panels. However when I checked the inverter I noticed that whilst string 2 was working fine, string 1 had the correct voltage (360v) … but the ampage was zero and hence not generating any output. The inverter appeared to be working fine and had no error messages.

A battery in that house would have kept cycling on what was left, and the bill would have looked slightly worse rather than obviously wrong. If your production has been drifting down, producing less than expected covers putting a number on it.

Backup is a separate purchase. On Enphase, backup needs the IQ System Controller 3 INT as well as the battery. What happens to a plain grid-tied system while the power is off, and what is worth checking once it is back, is in a power cut and your solar panels.

Retrofitting a battery to existing solar panels

The IQ Battery 5P is an all-in-one AC-coupled battery: 5.0 kWh usable, with six built-in IQ8D-BAT microinverters giving up to 3.2 kVA continuous. Because it is AC-coupled, Enphase lists it as compatible with “third-party PV string inverters” as well as its own microinverters, which is the retrofit route for a house whose panels run on someone else’s inverter. It runs in self-consumption or full backup mode, and the warranty is “>60% capacity, up to 15 years or 6,000 cycles”.

For SolarEdge and Growatt, ask the installer to put the specific inverter model, the battery model, whether the coupling is AC or DC, and the usable capacity in writing on the quote, and to reference the manufacturer document those figures come from. If you are shopping without your original installer (because they are no longer trading, or because you inherited the system), what to do when your installer has gone bust covers getting back control of the monitoring account first, which is where the production history you need for all of the above lives.

What a real roof’s year says about battery size

On a 6.09 kWp Enphase roof in Almere with no battery, the brightest month (May 2025) made 965 kWh, nineteen times the darkest (December 2024, 51 kWh). The best single day was 42.2 kWh, in June 2024, while the best day of December 2025 was 4.8 kWh, not enough to fill a 5.0 kWh battery even if the house used none of it.

Whole years differ too: 5,645 kWh in 2024 (927 kWh per kWp) against 6,251 kWh in 2025 (1,026 kWh per kWp). A battery sized against either year starts from a different number, which is why the size has to come from your own record.