Two Hertfordshire towns have a rule about home batteries that comes before any brand. In Letchworth Garden City, the Heritage Foundation’s solar design principles, revised in May 2026, say battery storage units need its consent and must not be visible from the public realm. In Welwyn Garden City, the Estate Management Scheme asks homes inside it to get consent before changing the outside. So this comparison of five systems starts from a Hertfordshire question: does it have to go outside? For the system we fit ourselves, see EnSmart battery installation in Hertfordshire.
The county’s roofs are covered on solar panels in Hertfordshire, sizing on our battery storage guide, and town detail on battery storage in Stevenage, St Albans and Letchworth Garden City. Every product figure below is the manufacturer’s own; none of it ranks one brand against another.
EnSmart SolarAll: the system in our own job photos
The battery systems in HCE’s own job photos are EnSmart: the SolarAll-H hybrid inverter, published at 3–6 kW single-phase and IP65, with ELV packs. The wall packs, 5.12 or 10.24 kWh, are IP50 and indoor-only, which in Letchworth and inside Welwyn’s Scheme is less a limit than a starting point, because an indoor layout leaves the outside as it was. In a Stevenage terrace, where 44.9% of households in the built-up area live, the usual indoor spot is an understairs cupboard or utility space; failing that, the 15 or 20 kWh floor packs are IP55 and rated for outdoors away from direct sunlight. EnSmart’s product pages publish a 15-year design life, not a warranty, so the warranty terms are put in writing with the quote. For any of the four brands below, ask us and we will confirm in writing how it would be supplied, who installs and commissions it, and what warranty attaches.
Tesla Powerwall 3: one large unit
A Tesla Powerwall 3 in Hertfordshire is a single 13.5 kWh unit with a built-in solar inverter, 1105 x 609 x 193 mm and 130 kg, floor or wall mounted. Its IP55 enclosure, with IP67 on the battery and power electronics, allows an outside wall, but in Letchworth an outside wall is exactly what the Foundation governs. In Potters Bar, where 46.5% of households live in semis, a garage or side wall is more often available; a loft is not a default position for a 130 kg unit, because joists, temperature and service access come first under Tesla’s installation manual. Tesla’s ten-year warranty, with 80% retained at ten years for UK installations, applies to a Powerwall bought from Tesla or a Tesla certified installer, so check any Hertfordshire company quoting you, us included, in Tesla’s certified-installer finder.
Sigenergy SigenStor: a stack that grows upwards
The Sigenergy SigenStor stacks 6.02 or 9.04 kWh modules beneath a hybrid Energy Controller, up to six per controller, and every module shares one footprint, 767 mm wide and 265 mm deep, at 62 or 78 kg. A stack therefore grows upwards rather than outwards, which can suit a narrow wall in a Hemel Hempstead or Stevenage terrace as well as a Harpenden garage, provided the floor or wall takes the weight. Sigenergy says old and new modules can be mixed through the built-in optimiser. Its 3.0 and 3.6 kW single-phase controllers stay within G98; the 4.0 kW and larger need G99 approval before work. The warranty is 10 years to 60% usable energy or a throughput cap, with an internet-connection condition, and backup needs the Sigen Energy Gateway.
EcoFlow PowerOcean: 5 kWh steps
The EcoFlow PowerOcean builds from 5 kWh LFP packs on a hybrid inverter, up to 20 kWh on one single-phase inverter, in an IP65 enclosure with built-in heating pads. Across Harpenden’s four wards 78.2% of households own their home (our sum of ward figures), and an owner planning an EV or a heat pump later is the household small steps are designed for. Watford is the other end: 28.8% of its 39,628 households live in purpose-built flats and 28.2% rent privately, so the freeholder or landlord comes first. EcoFlow publishes a 15-year warranty, described as daily use until 70% of original capacity. Its UK pages publish no pack weight or dimensions, so the survey has to prove the space.
Fox ESS: small wall units and a cloud condition
Fox ESS in Hertfordshire would usually mean an H1-G2 hybrid inverter, in 3.0, 3.7, 4.6, 5.0 or 6.0 kW sizes, with EP5 batteries of 5.18 kWh, up to four for a Fox-quoted 20.8 kWh. An EP5 is 380 x 625 x 147 mm and about 50.5 kg, and both parts are IP65 for indoor or outdoor use. In Welwyn Garden City 30.8% of households and in Letchworth 30.1% rent from a social landlord (Census 2021), who must agree before anything is fixed. Fox ESS gives EP batteries 10 years to 70% or a throughput minimum; its inverter keeps 10 years only if the system is connected to Fox Cloud within 120 days of installation, otherwise 5. Under Fox ESS’s UK battery policy, only the installer and Fox ESS’s own personnel can make a claim.
The county’s sun, network and backup
PVGIS gives a 4 kWp south-facing array about 133 kWh in December at St Albans to 141 kWh at Potters Bar, and 476 kWh at Borehamwood and Watford to 487 kWh at Bishop’s Stortford in June. On our arithmetic that is about 4.3 to 4.5 kWh on an average December day, under one EnSmart, EcoFlow or Fox ESS pack, against about 16 kWh in June. So each system is sized to your evening use. DESNZ records about 24,567 solar PV installations across the nine Hertfordshire councils our towns sit in (end of 2025, our sum), and DVLA about 35,306 privately kept plug-in vehicles across them, up from about 25,976 a year earlier (our sum).
All 14 town centres we cover are in UK Power Networks’ Eastern Power Networks area; Stevenage’s is about 31 km from the nearest other licence area. Equipment up to 16 A per phase (3.68 kW) is notified under G98, larger units need G99 approval first, and UK Power Networks approves automatically where export after any limitation is under 5 kW. Ofgem’s 2024/25 figures for Eastern Power Networks show about 44.6 supply interruptions per 100 customers, down from 46.2 the year before. None of the five keeps circuits running in a cut unless the installation is designed for backup. Where panels and a battery physically go on the county’s house types is covered in our guide to where panels and a battery go on Hertfordshire homes.
If you live in Letchworth or inside Welwyn Garden City’s Estate Management Scheme, say so when you ask for a Hertfordshire battery survey or call 0204 547 1892, so the consent route is checked first.
Sources: Letchworth Garden City Heritage Foundation, Solar Panels in Letchworth Garden City: Design Principles (revised May 2026); Welwyn Hatfield Borough Council, Estate Management Scheme, and WGC EMS, What is the Estate Management Scheme?; ONS, Towns and cities, characteristics of built-up areas, Census 2021 (tables 1c, 5c and 6c); ONS Census 2021, TS044 and TS054 via Nomis (Watford; Harpenden East, North & Rural, South and West wards); PVGIS 5.3, European Commission Joint Research Centre (PVGIS-SARAH3 2005–2023, 4 kWp, 35°, due south, 14% losses), runs of 15 October 2026; DESNZ, Renewable electricity by local authority 2014 to 2025 (published 29 September 2026); DfT and DVLA, VEH0142, privately kept plug-in vehicles (published 23 September 2026); UK Power Networks licence boundaries (December 2024), NESO GB DNO licence areas (May 2024), and UK Power Networks, Solar and storage: cost, time and what’s involved (read 15 October 2026); ENA Engineering Recommendation G98 Issue 2; Ofgem, RIIO-2 Electricity Distribution annual report 2024 to 2025, supplementary datafile (published 15 January 2026); EnSmart and UK-distributor published SolarAll-H and ELV documents; Tesla Powerwall 3 UK datasheet (2026), installation manual and Tesla Powerwall Warranty, European Warranty Region, Rev. 1.16; Sigenergy Battery and Energy Controller datasheets (read 19 October 2026) and Europe warranty (27 May 2025); EcoFlow UK PowerOcean product pages (read 19 October 2026); Fox ESS EP5 datasheet V1.5, H1-G2 datasheet V2.0, UK battery warranty policy V4.5 and inverter warranty policy V2.3.