Retrofit Battery Storage to Existing Solar
Already have solar panels? Here's everything you need to know about adding battery storage — the technical options, compatible systems, real costs, and honest payback numbers.
If you already have solar panels but no battery storage, you are likely exporting a significant proportion of your generation to the grid at a rate of around 10–15p per kWh — while buying electricity back in the evening at 25–30p. Adding a battery changes that dynamic substantially, storing your surplus for use when you actually need it.
The important caveat: a retrofit battery attracts 20% VAT unless it is installed alongside new panels (where 0% VAT applies). This adds cost and extends payback. We always give an honest assessment of whether the numbers stack up for your specific situation before recommending a retrofit battery.
AC-Coupled vs DC-Coupled: Which Do You Need?
AC-Coupled
Battery connects to your home circuit via its own inverter. Works independently of your existing solar inverter.
- Compatible with virtually any existing solar system
- Works even if original inverter fails
- Simpler installation in most cases
- Two conversion steps means ~95% round-trip efficiency
- Requires compatible battery with built-in inverter
DC-Coupled
Battery connects directly to the DC bus, sharing the solar inverter. Requires a hybrid inverter.
- Slightly higher efficiency (~97%)
- Simpler single-inverter setup
- Often lower installed cost with compatible inverters
- Requires replacing your inverter if it is not hybrid-ready
- Must be compatible with your panel configuration
Compatible Battery Systems
| Brand | Capacity | Coupling | Notes |
|---|---|---|---|
| Sigenergy | 5–19.2 kWh | AC or DC | Excellent app, strong UK support |
| Sigenergy | 9.7–48 kWh | DC (SE hybrid) | DC-coupled if you have SE inverter |
| Sonnen | 5–15 kWh | AC | Premium option with virtual power plant |
| Tesla Powerwall 3 | 13.5 kWh | AC | Integrated inverter, grid backup capable |
| Fox ESS | 5–10.2 kWh | AC or DC | Good value, strong warranty |
Retrofit Battery Cost and Payback
A standalone retrofit battery attracts 20% VAT in the UK — unlike batteries installed alongside new solar panels, which benefit from 0% VAT until March 2027. This means costs are typically:
- 5 kWh battery: £3,000–£4,200 installed (with 20% VAT)
- 10 kWh battery: £5,400–£7,200 installed (with 20% VAT)
At these costs and a typical annual value gain of £400–£700 (depending on usage profile and tariff), payback periods for retrofit batteries typically run 8–12 years. Whether this is worthwhile depends heavily on your specific situation — your existing system's export rate, your evening consumption, and your tariff structure.
For households on time-of-use tariffs (e.g. Octopus Go at ~7.5p/kWh overnight), retrofit batteries become substantially more attractive because you can charge from cheap overnight grid electricity, not just from surplus solar. In this scenario, payback can fall to 5–7 years.
Our battery ROI analysis and 2026 battery cost guide cover this in full detail. See also our battery storage installation page.
Retrofit Battery Facts
Battery & EV
Storage and smart charging to maximise every unit you generate
Battery Retrofit FAQs
Battery storage is worth it if you use most of your electricity in the evening (when solar panels are not generating) or if you want protection from power cuts. A battery typically increases self-consumption from 30-50% to 80-90%, making more of your solar investment pay for itself.
Modern lithium-ion batteries typically last 10-15 years, with most manufacturers offering 10-year warranties guaranteeing at least 70% capacity retention after 6,000+ charge cycles. Battery technology is improving rapidly.
Yes. Lumos Energy regularly retrofits battery storage to existing solar panel systems. We assess your current setup, recommend a compatible battery, and install it alongside your existing inverter or with a new hybrid inverter if needed.
Many modern battery systems offer backup power functionality, but not all. If power cut protection is important to you, we ensure your system specification includes this capability. Some systems switch to backup within milliseconds.
Find Out If a Retrofit Battery Is Worth It
Free, no-obligation assessment from an MCS certified installer. We'll tell you honestly whether the numbers make sense for your home.
Free survey · No obligation · Broughton Gifford, Melksham · Open Mon–Fri 8am–6pm, Sat 9am–2pm