Spare power, month by month
A battery only helps if the panels make more than the house uses while they are generating. PVGIS gives the monthly output for panels at the best angle in central Wigan (PVGIS 5.3). Ofgem treats 2,500 kWh a year as a typical household's electricity (Ofgem), close to 6.8 kWh on an average day. Against that, a 4 kW system's average day runs like this:
| Month | kWh a day | Against 6.8 kWh use |
|---|---|---|
| Jan | 4.0 | 2.8 short |
| Feb | 7.1 | 0.3 over |
| Mar | 10.6 | 3.7 over |
| Apr | 14.5 | 7.7 over |
| May | 15.4 | 8.5 over |
| Jun | 15.1 | 8.2 over |
| Jul | 14.6 | 7.7 over |
| Aug | 13.2 | 6.3 over |
| Sep | 11.3 | 4.5 over |
| Oct | 7.6 | 0.8 over |
| Nov | 5.3 | 1.5 short |
| Dec | 3.6 | 3.2 short |
In about 9 months of the year the panels make more in an average day than the house uses in total, and much of that surplus arrives in the middle of the day when few people are in. That is the power a battery can hold for the evening. In the darkest months the panels make less than the house uses, so a battery charged by them alone sits mostly empty. These are averages: real days swing with the weather, and a household at home all day will use more of its solar as it is made.
Sizing against the summer surplus
The table gives a simple check on any battery size. In June, July and August a 4 kW system in central Wigan makes on average about 14.3 kWh a day against 6.8 kWh of average use, leaving roughly 7.4 kWh spare on an average summer day (our arithmetic from PVGIS and Ofgem). Not all of that spare power arrives while the house is empty, and not all of the evening's use can wait for it, so a battery much larger than the summer surplus will rarely fill from the panels alone. A smaller battery fills more often. A larger one makes more sense where it is also charged from a cheap overnight tariff, which is a tariff decision rather than a solar one.
Household size moves the sum. Ofgem's typical figure is an average across homes (Ofgem); a larger family in a Wigan semi will use more and have less to spare in summer, a single person in a terrace less and more. Ask the installer to size the battery from your own meter readings and to show the price per kWh, which you can set against the DESNZ medians below. A good quote lists the battery and the panels separately: what a quote should include covers both, and the costs page has the panels' side.
What batteries cost
DESNZ publishes the median price paid for batteries installed in existing homes. For 2025/26 it was £1,300 per kWh under 6 kWh, £890 from 6 to 11 kWh, and £630 from 11 to 30 kWh (DESNZ). Three sizes at those medians, by our arithmetic:
- 5 kWh at £1,300 per kWh: about £6,500.
- 9.5 kWh at £890 per kWh: about £8,450.
- 13.5 kWh at £630 per kWh: about £8,500.
Batteries installed in homes are zero-rated for VAT until 31 March 2027, as solar panels are (HMRC). For lower-income households, GMCA's Warm Homes: Local Grant lists battery storage among its measures, for homes at EPC D to G and households with a qualifying postcode, benefit, proxies or income under £36,000 (GMCA). Compare any battery quote with the solar costs page so the panels and the battery are each judged on their own.
The network: G98 or G99
Electricity North West runs the borough's network. Solar and a battery together count towards the connection limit: up to 16 A per phase (3.68 kW on a single-phase supply), the installer commissions and then notifies the network; above that, it applies first and waits for agreement before connecting (Electricity North West; G99). An installer may instead set export limiting to stay within the smaller route. Either way, the quote should say which applies. How the connection works
Where a battery can go
Placement is governed by PAS 63100, the BSI code of practice for battery storage in homes, which excludes locations like escape routes and bedrooms (BSI). In a typical Wigan semi that often points to a garage, a utility room, an outbuilding or an outside wall rather than the hall or under the stairs. A terrace without a garage may have fewer options, so ask the installer to show the proposed spot on the survey and explain why it meets the code.
What a battery quote should say
- The make, model and usable capacity in kWh, and its warranty in years and cycles.
- Whether it is AC- or DC-coupled, and whether a new inverter is part of the price.
- Where it will be fitted, and how that meets PAS 63100.
- Whether the system now needs a G99 application or export limiting.
- The battery's price on its own line, so the panels can be compared per kW.
See what a quote should include for the rest of the checklist.