The short answer
A 4 kWp south roof in Congleton makes about 15 kWh a day in June but about 3 in December, so a battery fills easily in summer and rarely from panels in midwinter. Size it on spring and summer surplus. Panels plus a battery over 3.68 kW on a single-phase supply need network approval first.
What a battery does
A home battery charges from surplus solar output that would otherwise go to the grid, then discharges when the house needs more than the panels are making. In practice that usually means charging from late morning to mid-afternoon and discharging through the evening. Some batteries can also charge from the grid at times when electricity is cheaper, on tariffs that vary by time of day. The battery itself does not make electricity; it changes when the electricity is used.
The value of that shift depends on the gap between what you would pay to import a unit in the evening and what you would be paid to export it at midday. Where that gap is wide, storage earns its place more readily. Export and self-use explains the split between units used and units sold.
How much there is to store, month by month
The table converts the PVGIS figures for 4 kWp on a south roof at 35° in Congleton into an average day in each month (PVGIS 5.3).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 3.5 | 6.6 | 10.2 | 14.5 | 14.9 | 15.2 | 14.4 | 13.0 | 11.2 | 7.3 | 5.0 | 3.4 |
From April to August a 4 kWp roof averages well over ten kWh a day, much of it around midday. Even after the background load of the house, there is often surplus to fill a battery. In March the average is about 10 kWh and in October about 7. In December it is about 3, which a typical household will use as it is made. A battery charged only from panels sits nearly idle for part of the winter.
The practical rule is to size storage on the months when it will actually fill: a battery much larger than a typical spring or summer surplus costs more without being used. An east and west array spreads output across the day and leaves less midday surplus, which can point to a smaller battery: see east and west roofs.
Batteries and the grid rules
A battery’s inverter is a source of generation as far as the electricity network is concerned. On a normal single-phase supply, the G98 procedure covers equipment up to 16 A per phase, or 3.68 kW, which the installer connects and then notifies to the network operator within 28 days. If panels and battery together can export more than that, the G99 procedure applies, and the network operator must approve the design before installation. An export limit set in the equipment can sometimes keep a combined system inside G98.
Find the operator for your address with the ENA postcode search. The installer normally makes the application, but it is the household’s connection, so keep copies of what is sent and the reply. Grid connection sets out both procedures.
VAT on batteries
Batteries installed in homes are zero-rated for VAT until 31 March 2027, alongside solar panels, and the reduced rate of 5% applies from 1 April 2027 (HMRC notice). The relief applies to the installed job, so a battery bought as a box and fitted separately may be treated differently. Ask for VAT to be shown on its own line on every quote. Costs in Congleton covers the other parts of the price.
The council’s group-buy
Cheshire East’s Switch Together scheme has offered batteries alongside panels to registered households. The 2026 registration window opened on 3 August 2026 and closed on 25 September 2026, and that round has closed (council page). Any later round will be announced on the council’s page. Grants and schemes covers the other routes that may help with cost.
Charging from the grid
Many batteries can be set to charge from the grid overnight on a tariff with cheaper night-time rates, then supply the house in the morning and evening. In winter, when the panels have little surplus, that can keep the battery working through the months it would otherwise sit idle. The value depends on the gap between the night rate and the day rate, and on the tariff rules, which change. It also changes how the battery is treated for export: some export tariffs pay only for units that came from the panels, so the metering and the tariff terms need to fit together.
Ask how the battery decides when to charge and discharge, whether the household can change those settings, and whether the controls depend on an online service from the maker that might change or end.
How long a battery lasts
Battery capacity fades with use and age. Makers usually state a warranty in years and in cycles or throughput, with a minimum share of the original capacity promised at the end. Read the warranty terms rather than the headline number: what counts as a cycle, what conditions void it, and whether it passes to a new owner if the house is sold. A battery is likely to need replacing before the panels it serves, so think about access for that day when choosing a position.
When a battery is not the first step
For some households the order of spending matters. A hot-water diverter costs much less than a battery and can absorb summer surplus in homes with a hot water cylinder. Moving washing and dishwashing into daylight hours costs nothing. A household that does both and still exports a large share of its summer output has a clearer case for storage than one that has done neither. Batteries can be added later, though doing so changes the grid paperwork, so it is worth choosing an inverter that can take one if storage might follow.
Where it goes and what it needs
A battery needs a dry, ventilated space within the temperature range set by its maker, close enough to the consumer unit for a short cable run, and with room for servicing. Garages, utility rooms and weatherproof enclosures on outside walls are common choices. Current guidance discourages lofts, bedrooms and escape routes. In older town-centre houses with small rooms and no garage, finding a good position can be the hardest part of the job, so raise it at the first visit.
In a flat, a battery inside the flat is the occupier’s decision subject to the lease; anything in shared areas needs the building owner’s agreement. Mill conversions and flats covers the order of decisions.
Questions to put to any firm
- What size of battery do you suggest, and on which months’ surplus is that based?
- Will the battery charge from the grid as well, and how is that controlled?
- Does the system keep any circuits running in a power cut?
- Does the combination need G99 approval, or will output be limited to stay within G98?
- Where will it be installed, which guidance does that follow, and what warranty covers capacity over time?
- Is the battery work covered by the same certification and consumer code as the panels?
Choosing an installer adds the checks on the firm itself. If you are not sure whether storage suits your home, the question form reaches the publisher by email, and the FAQs page gathers short answers on the grid, costs and schemes.