In most towns, adding a battery to panels you already have is a conversation between you, your installer and the network operator. In Letchworth Garden City there is a fourth party. The Heritage Foundation’s solar design principles, revised in May 2026, say battery storage units and other ancillary equipment need its consent and must not be visible from the public realm. If your array went up before you thought about storage, the battery is a new external-alteration question, even though the roof stays as it is.
This guide covers adding storage to an existing array in SG6: the consent, the two ways to connect, the network paperwork and how to size from the readings you already have. For the battery in general, see our page on battery storage in Letchworth Garden City.
What North Hertfordshire’s solar count shows
DESNZ records 4,214 solar PV installations across North Hertfordshire at the end of 2025, with 43.22 MW of capacity. That averages about 10.3 kW per installation (our arithmetic), well above a typical home array, because the count includes everything from a terrace roof to larger non-domestic sites. It is not a count of homes and not a count of batteries. DESNZ also notes that many 2025 installations could not yet be assigned to a council area when it first published, so the district’s figure is likely to rise. What it does show is that existing arrays are an established part of the district, and each one is a candidate for the question this guide answers.
The Foundation comes first
Under Letchworth’s Scheme of Management, most dwellings need the Heritage Foundation’s prior written consent for external alterations, and leaseholders need consent under their lease too. The Foundation states that this is separate from planning permission. For a retrofit, the practical effect is that the battery, its inverter and any backup equipment are planned out of public view from the start, which usually means indoors: a garage, a utility room or an inside wall near the consumer unit. A rear outbuilding can work if the cable route back to the house is practical. The Foundation’s Heritage and Stewardship team gives pre-application advice at no charge, and that is a sensible first call before a design is fixed.
If the retrofit also changes the panels, the Foundation’s principles on arrays apply again: in the Heritage Character Area panels go on rear elevations not visible from the public realm, and where one half of a pair of semis already has panels, the Foundation expects the arrangement to be mirrored.
Two ways to connect a battery to an existing array
The first route keeps your existing solar inverter and adds a battery with its own inverter beside it, often called AC coupling. The second replaces the old inverter with a hybrid inverter that runs the panels and the battery together. Which suits a Letchworth home depends on the age and model of the inverter already there, how much life it has left and how far the battery will sit from it. Inverters typically carry shorter warranties than panels, so an older one can tip the decision. Whichever route, the Energy Networks Association’s type-test register records compliance per model and firmware version, so each unit going on your wall, old inverter included, is checked before the application goes in.
The paperwork with UK Power Networks
Letchworth’s town centre sits in UK Power Networks’ Eastern Power Networks licence area, about 29 km from the nearest other network. The network operator cares about the total inverter capacity behind your meter, not just the battery. Under G98, type-tested equipment up to 16 A per phase (3.68 kW) is notified before commissioning or within 28 days; 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 is added beside it. UK Power Networks says it approves automatically where the maximum export, after any export limitation, is under 5 kW, so an export limit is one way to keep a retrofit inside the automatic route. It also says there is nothing to pay it unless your supply needs upgrading. Your installer handles all of this.
Certificates and export
Keep the existing array’s MCS certificate and its electrical certificate. A Smart Export Guarantee tariff needs an MCS certificate and a meter capable of half-hourly export readings. Rates are set by each supplier, must be above zero under Ofgem’s rules and change at short notice, so we never print one. Ask your supplier how it treats exports from a system with storage before the battery goes in.
Cost and what a retrofit gives up
The Energy Saving Trust puts home storage between £1,500 and £10,000, a 5 kWh system at around £4,600 and a typical home system at 10 kWh, with a typical lifespan of about 10 to 12 years. It also says fitting storage at the same time as panels costs less than a separate visit later, so a retrofit gives up that saving. A battery fitted on its own still carries 0% VAT until 31 March 2027, then 5% from 1 April 2027. Our guide to how long solar batteries last covers warranties against real-world life.
Size it from your own readings
PVGIS puts a 4 kWp south-facing array in Letchworth at about 477 kWh in June and about 136 kWh in December. A Heritage Character Area home limited to the rear roof may have a smaller array than that, and its own generation meter tells you exactly how much smaller. Bring a year of readings and your electricity bills to the survey: the battery should match what the house uses in the evening and what the array can spare on an ordinary summer day. Our guide on what size home battery you need shows how.
Terraced houses are 33.3% of homes in Letchworth’s built-up area and social renting is 30.1% (Census 2021), so for many households the landlord’s agreement, not the Foundation’s, comes first. We are a NAPIT member (73495), MCS-certified for solar PV and battery storage through NAPIT. The roof side is on our page for solar panels in Letchworth Garden City, and the county on our Hertfordshire solar page.
Sources: DESNZ, Renewable electricity by local authority 2014 to 2025 (published 29 September 2026), North Hertfordshire; Letchworth Garden City Heritage Foundation, Solar Panels in Letchworth Garden City: Design Principles (revised May 2026); Energy Networks Association, EREC G98 Issue 2 (10 March 2025) and type-test register; UK Power Networks, Smart Connect, Solar and storage: cost, time and what’s involved, and Open Data licence boundaries; Ofgem, Smart Export Guarantee; Energy Saving Trust, Battery storage (updated 19 August 2026); HMRC, VAT Notice 708/6; ONS, Towns and cities, characteristics of built-up areas: Census 2021 (tables 5c and 6c); European Commission JRC, PVGIS 5.3, PVGIS-SARAH3 2005–2023 (run 15 October 2026).