Battery retrofit on existing commercial solar — when it works and when it doesn't

Most commercial solar systems installed since 2018 support retrofit battery integration without major modification. As battery economics have improved (capex down ~30% since 2024, capacity-market revenue increasingly accessible), more existing solar operators are evaluating retrofit. We're seeing about 1 in 4 retrofit enquiries convert to actual deployment — the others either don't pencil economically or face DNO-consent barriers we couldn't resolve cost-effectively.

When retrofit makes economic sense

The scenarios where retrofit pencils are the same four as new-build battery deployment, but the economics need to clear a slightly higher hurdle because retrofit doesn't share fixed installation costs with new solar capex:

• Export-constrained sites with curtailment. Where DNO has reduced or withdrawn export consent since the original solar commissioning, generation that would otherwise be exported is now curtailed. Retrofit battery captures the curtailed energy. Strongest economics.
• Recent move to time-of-use tariff structure. Where the supply contract has changed to half-hourly tariffs with peak/off-peak differentials above 5p/kWh since the original solar deployment.
• Capacity-market eligibility opening up. Battery systems above 1 MWh able to access T-1 and T-4 capacity market auctions through aggregator partnership. Retrofit-only economics work where storage scale clears the threshold.
• Resilience requirement emerging. Where business operations have moved toward resilience-critical (cold-storage expansion, IT-load growth, customer SLA commitments), the resilience case shifts the financial threshold.

Inverter compatibility — the controlling factor

The technical question that determines retrofit viability: does your existing inverter architecture support battery integration?

• Hybrid inverters (2020+): Native support for battery integration. Plug-and-play retrofit. Typical install timeline 1–2 weeks.
• Modern string inverters (2018+): Most major brands support DC-coupled or AC-coupled battery integration via additional battery inverter. Typical install timeline 2–3 weeks.
• Older string inverters (pre-2018): May require AC-coupled battery integration with separate battery inverter. Sometimes simpler to replace inverter at same time as adding battery (especially if approaching end-of-life year-12 replacement). Adds £80–£120/kWp to project cost but combines two interventions.
• Central inverters (large commercial): Support depends on specific manufacturer. Typically support battery via grid-side integration rather than DC-coupled. Some require firmware updates from manufacturer.

DNO consent for retrofit

Battery retrofit is treated as a modification to the existing G99 connection rather than a new generation connection in most cases. Process:

• DNO notification of battery addition (typically 4–6 weeks for response).
• Possibly a connection variation if battery materially changes the import/export profile.
• Modification commissioning (G99-style witness inspection or self-certification depending on size).

DNOs vary in their attitude to retrofit batteries — some welcome them as load-balancing infrastructure; others see them as additional flexibility services that may complicate broader network management. Engage the DNO early in the retrofit planning process.

Worked example — 250 kWp existing system, 100 kWh battery retrofit

FYA on the £59,500 capex: £7,438 year-one tax saving (50% × 25% main rate) for profitable trading companies before 31 March 2026 deadline. Effective net cost £52,000.

25-year cumulative cash benefit on this retrofit: approximately £155,000. Project IRR on retrofit-only capex: approximately 9% — modest but reasonable for a single-purpose intervention.

When retrofit doesn't pencil

Three scenarios where retrofit typically doesn't make economic sense:

• Daytime-heavy demand profiles with good export consent. Site already self-consumes 80%+ at solar; remaining export earns reasonable SEG tariff. Retrofit battery captures only 5–10% additional self-consumption — IRR typically below hurdle.
• Sub-100 kWh battery on small commercial sites. Below 100 kWh, fixed installation costs (DNO, inverter, commissioning) dominate the per-kWh capex. Per-kWh economics improve substantially above 200 kWh.
• Pre-2018 string inverters approaching end-of-life. If existing inverter is within 2 years of replacement, the financial case typically benefits from waiting until year-12 inverter replacement and combining with new hybrid inverter rated for battery integration. The combined intervention is more cost-effective.

Where the retrofit market is in 2026

Battery storage retrofit is a growing UK market — we estimate 8-12% of existing commercial solar deployments will retrofit batteries by end-2027 driven by export-curtailment, time-of-use pressure, and capacity-market accessibility. Retrofit lender financing (green-loan or asset-finance) is standard at typical commercial rates, and the FYA capture on retrofit capex remains valuable for the next 11 months.