Harpenden’s households mostly own the roof over their heads, 78.2% across the town’s four wards at the 2021 Census, so many of the arrays already on AL5 roofs were the owner’s own decision. Adding a battery to them is the owner’s decision too, and it is a different job from fitting panels and storage together. This guide covers what changes when the panels are already up: your own generation figures, the inverter, the paperwork, the certificates and the cost. For where a battery goes in Harpenden houses and the power-cut question, see our guide to battery storage in Harpenden.
The short answer is that most existing arrays can take a battery, but the inverter, the grid paperwork and the export arrangements all need checking before anything is ordered.
How many arrays are already up
DESNZ records about 4,118 solar PV installations of every size across St Albans district at the end of 2025, with 18.77 MW of capacity between them, as first published. That is an average of about 4.6 kW per installation (our arithmetic), but the count includes non-domestic sites, so it is not a count of Harpenden homes, and DESNZ says many 2025 installations could not yet be assigned to a council area, so the figure is likely to rise when revised. It does show the scale: a battery added to existing panels is a common job in this district, not an unusual one.
Start with your own generation meter
A household with panels has something a new customer does not: real data. Your generation meter or app shows what your roof actually made, month by month, and that beats any model. For comparison, PVGIS puts a south-facing 4 kWp array at the Harpenden town-centre point at about 136 kWh in December and 156 kWh in January, against 480 kWh in June and 482 kWh in July. On our arithmetic that is about 4.4 kWh a day in December and 16.0 in June. The Met Office station at Rothamsted, in the town, averaged about 56 hours of sunshine in December and 209 in July over 1991–2020.
If your own meter shows a big summer surplus that goes to export while the house is empty, a battery has something to store. If your December figures are tiny, as they will be, the battery’s winter job is storing cheaper overnight electricity, if your tariff offers it. Bring a year of readings and your bills to the survey; our guide to what size home battery you need explains how to use them.
Keep the inverter, or replace it
There are two ways to add storage to an array that is already up. The first keeps your existing solar inverter and adds a battery with its own inverter alongside it, often called AC coupling. The second replaces the existing inverter with a hybrid inverter that runs the panels and the battery together. Which suits your Harpenden house depends on the inverter’s age and model, how much life it has left, and how far the battery would sit from it. Inverters typically carry shorter warranties than panels, so on an older array the inverter may be due for replacement anyway, which tips the choice. The battery systems we fit, from EnSmart and Hanchu, are specified to the house after that check, not before.
The paperwork starts again
Harpenden town centre is in UK Power Networks’ Eastern Power Networks licence area, about 21 km from the nearest other licence area, so there is no doubt about the operator. What changes is the total. Under G98, type-tested equipment up to 16 A per phase (3.68 kW) is notified before commissioning or within 28 days after; above that, a G99 application must be approved before work starts. An array that went in under G98 can cross the line once a battery inverter sits beside it, and a 4 kW inverter on its own, at 17.39 A, is already over. UK Power Networks says it automatically approves applications where the maximum export, after any export limitation, is under 5 kW, and an export limit set in the inverter is one way a retrofit stays inside that. The installer handles the forms.
Certificates, export and VAT
Keep your original MCS certificate and electrical certificate. A Smart Export Guarantee tariff needs an MCS certificate and a meter capable of half-hourly export readings, and suppliers set the rates, which must be above zero under Ofgem’s rules and change at short notice; we never print one. Before the battery goes in, ask your supplier how it treats exports from a system with storage. On tax, HMRC’s VAT Notice 708/6 zero-rates a residential battery installation until 31 March 2027 even when it is fitted on its own to existing panels, with 5% from 1 April 2027.
What the published costs say for a retrofit
The Energy Saving Trust puts a home battery at £1,500 to £10,000, with a 5 kWh system around £4,600, and says fitting storage at the same time as panels saves money; on a retrofit that saving has already gone. DESNZ’s statistics for MCS-registered batteries in existing homes in 2025/26, which leave out any solar fitted alongside, put the average 6 to 10.99 kWh job at about £8,520 (our arithmetic on 9.80 kWh at a mean of £870 per kWh). The same release counts 44,033 such installations in 2025/26, up from 24,242 the year before. The Trust gives a typical battery lifespan of about 10 to 12 years, and our post on how long solar batteries last covers what affects it.
Where it goes in an AL5 house
With 39.7% of households across Harpenden’s four wards in detached houses, a garage or utility room near the consumer unit is usually available. A detached garage some way from the house means a longer cable run, priced as its own line. In the Harpenden conservation area, from The Avenues to West Common, an indoor battery changes nothing on the street, and a listed house needs consent for works affecting its character. Our page on solar panels in Harpenden covers the roof side, and the Hertfordshire solar panels hub covers schemes across the county.
Sources: ONS Census 2021, TS044 and TS054 categories, May 2022 wards Harpenden East, North & Rural, South and West (our sums); DESNZ, Renewable electricity by local authority 2014 to 2025 (29 September 2026); PVGIS 5.3, European Commission Joint Research Centre (PVGIS-SARAH3 2005–2023, run 15 October 2026); Met Office, Rothamsted location-specific long-term averages 1991–2020; UK Power Networks licence boundaries and Solar and storage: cost, time and what’s involved; NESO GB DNO licence areas (May 2024); Energy Networks Association, EREC G98 Issue 2 (10 March 2025); Ofgem, Smart Export Guarantee; HMRC, VAT Notice 708/6; Energy Saving Trust, Battery storage (updated 19 August 2026); DESNZ, MCS-certified domestic battery installation statistics (28 May 2026); St Albans City and District Council, Warm Homes: Local Grant and Conservation Area Character Statement for Harpenden (April 2008).