Battery Storage in Retrofit: Current State and Future Direction

The retrofit sector is undergoing a fundamental shift as battery storage moves from niche application to standard consideration in energy efficiency projects. As the UK accelerates decarbonisation and grid demands evolve, storage solutions are becoming increasingly critical to maximising the value of heat pump installations and renewable generation.

Why Storage Matters in Retrofit

Battery storage addresses a core challenge in retrofit: temporal mismatch between energy generation and consumption. When a property installs solar photovoltaic (PV) panels or switches to a heat pump, storage enables households to:

• Reduce reliance on grid electricity during peak-rate periods
• Capture excess solar generation for evening use
• Provide resilience during grid outages
• Support demand-side response schemes for financial return

For retrofit coordinators, storage also influences heat pump sizing and operation. Smaller heat pumps paired with thermal storage or battery-backed demand shifting can achieve equivalent performance at lower capital cost.

Current Technology Landscape

Lithium-Ion Batteries

Lithium-ion remains the dominant chemistry for domestic retrofit applications. Contemporary products typically offer:

• Usable capacity: 5–15 kWh for residential settings
• Round-trip efficiency: 85–92%
• Cycle life: 6,000–10,000+ cycles (10–15 year lifespan)
• Integrated inverter/charger systems with software controls

The market has matured significantly. Most manufacturers now offer warranties extending 10 years or more, with degradation curves well understood and predictable.

Thermal Storage

Often overlooked in retrofit discussions, thermal storage is equally relevant. Hot water cylinders with enhanced insulation and buffer tanks can shift heat pump output to off-peak windows, reducing electricity costs without requiring expensive battery hardware. This approach is particularly cost-effective in retrofit where space constraints exist.

Regulatory and Standards Framework

The retrofit industry must navigate an evolving standards landscape:

• BS 7909:2021 – Code of practice for temporary electrical installations, increasingly referenced for retrofit safety
• PAS 37001 – Installation of energy storage systems (published 2022)
• Building Safety Act – New scrutiny of electrical safety in multi-residential retrofit, including battery systems
• Grid connection standards – DNOs require formal application for systems exceeding 3.68 kW export capacity

The upcoming revision of Building Regulations will likely increase requirements for energy storage assessment during retrofit planning, particularly where heat pumps are installed.

Cost and Financial Viability

Battery costs have fallen substantially—from £800/kWh in 2015 to approximately £150–250/kWh installed in 2024. However, retrofit remains different from new build. A typical 10 kWh system with installation now costs £8,000–12,000 before grant support.

For owner-occupiers, payback depends heavily on:

• Electricity price spread (difference between peak and off-peak rates)
• PV generation on-site
• Time-of-use tariff availability
• Grant funding (e.g., Department for Energy Security and Net Zero programmes)

Housing associations often see stronger economics, particularly where storage enables managed heat pump operation across mixed-tenure portfolios.

Integration Challenges

Retrofit presents distinct technical challenges compared to new build:

• Space constraints – Installing 10+ kWh batteries in terraced housing or flats is complex; external enclosures and cabling routes must be carefully planned
• Electrical infrastructure – Older properties often lack capacity in main switchboards; upgrading may be required alongside storage
• Grid connection delays – DNO applications for storage export can take 8–12 weeks, creating programme friction
• Control complexity – Integrating storage with heat pumps, solar and demand-side response requires competent commissioning

Future Directions

Several trends will shape retrofit storage deployment:

Vehicle-to-home (V2H) integration: As electric vehicle ownership rises, bidirectional chargers will allow cars to serve as storage, reducing the case for standalone batteries in some retrofit scenarios.

Grid services markets: The dynamic containment and frequency response markets now explicitly welcome sub-10 kWh systems. Retrofit storage paired with aggregator platforms can generate £200–400 annually in grid services revenue.

Modular and scalable systems: Manufacturers are developing stackable battery modules, allowing retrofit projects to install storage incrementally as budgets allow.

Thermal storage prominence: As awareness grows that batteries are not always necessary, hybrid approaches combining thermal storage with small battery systems (3–5 kWh) will become more common.

Practitioner Guidance

When scoping retrofit storage, practitioners should:

• Establish whether storage solves an energy or resilience problem—or both
• Assess thermal storage options before defaulting to batteries
• Confirm DNO connection requirements early in design
• Plan cabling and mounting during heat pump survey to avoid later complications
• Ensure commissioning includes software optimisation; poor commissioning significantly reduces financial returns

Battery storage in retrofit is no longer speculative. The technology is proven, standards are consolidating, and costs are competitive. The question for retrofit practitioners is not whether storage will be relevant, but how to integrate it effectively within the constraints and opportunities of individual retrofit projects.