Retrofit battery storage

A retrofit battery adds storage to a home that already has solar PV panels, or to a home with no generation. The design must fit the existing electrical installation, DNO permission, metering and solar equipment rather than treating the battery as a separate appliance.

The first choice is usually whether to keep the existing solar inverter or replace it.

AC-coupled retrofit

An AC-coupled battery has its own battery inverter connected to the home’s AC distribution. Existing solar keeps its current inverter.

This approach can:

leave the PV strings and solar inverter unchanged
work without solar
keep solar and battery as separate fault domains
simplify a retrofit where the solar inverter has useful life remaining

Solar stored in the battery passes through the solar inverter, then the battery conversion stage on charge and again on discharge. That adds conversion losses, but the practical difference depends on equipment loading and how energy is measured. A generic round-trip percentage is not a reliable design basis.

DC-coupled retrofit

A DC-coupled retrofit normally replaces the existing solar inverter with a hybrid inverter and connects the battery on its DC side.

This can reduce the number of conversion stages and combine control in one unit. It also means the new inverter must be proven against the existing array’s:

cold open-circuit voltage
hot operating voltage
string current
number and length of strings
connector and cable arrangement
isolation and protection

The battery must be approved for the hybrid inverter’s voltage, communications and firmware. A matching plug or nominal voltage is not sufficient.

Changing the solar inverter can affect monitoring, warranties, DNO records and evidence for a Feed-in Tariff installation. Preserve the old and new model details and update the schematic.

Survey the existing installation

Before design, establish:

supply phase and earthing arrangement
service head, main fuse and meter position
consumer-unit condition and spare capacity
existing PV model, registered capacity and connection paperwork
export limitation and meter locations
solar string design and isolators
available cable routes
battery location and environmental conditions
internet or local communications requirements
whether backup is required

Do not assume an old solar installation has complete records. Labels and serial numbers should be checked against the installed equipment.

Where the consumer unit, bonding or intake equipment needs work, allocate responsibility before the battery is installed.

Capacity and power are separate

Battery capacity in kWh determines how much energy can be stored. Inverter power in kW determines how quickly it can be charged or discharged.

A large battery behind a low-power inverter may run modest loads for longer but cannot support a high simultaneous demand. A high-power inverter behind a small battery can cover a larger load briefly but will exhaust it sooner.

Size from measured import, export and load timing where possible. Annual totals alone do not show whether surplus solar and evening demand occur on the same day.

Allow for:

usable rather than nominal capacity
minimum state-of-charge reserve
inverter and battery losses
seasonal solar variation
charge-rate restrictions in cold conditions
battery ageing
backup reserve if island operation is required

DNO connection route

A battery inverter is generation equipment for connection purposes because it can supply power into the installation and potentially the network. The DNO considers the aggregate registered capacity of relevant inverters behind the connection.

As at 22 July 2026, qualifying small-scale generation up to 16 A per phase can use the EREC G98 Issue 2 route. Other connections use the appropriate G99 process and require permission before operation.

An export limit under G100 can cap net export at the connection point. It does not automatically reduce the registered capacity or remove the need to disclose the battery and existing solar equipment.

The grid-connection record includes:

aggregate registered capacity
maximum export limit, if different
G98, G99 or fast-track route proposed
type-test references
who submits the application or notification

Existing Feed-in Tariff installations

Adding a battery does not normally remove the FIT generation tariff simply because storage is present, but the arrangement must preserve correct measurement and comply with the generator’s obligations.

FIT generation payments are based on the eligible generator and its generation meter. Export may be deemed or metered. A battery can change actual export without changing gross solar generation, so the effect on payments depends on the account’s export arrangement.

Before altering the inverter or metering:

check the FIT accreditation details
record the existing inverter and generation-meter readings
tell the FIT licensee about changes where required
preserve evidence that eligible generation remains measured correctly
avoid allowing grid-charged battery energy to pass through a generation meter as if it were renewable generation

A household may be able to retain FIT generation payments while moving export to a SEG (Smart Export Guarantee) contract, subject to the applicable conditions. Confirm the written arrangement with the licensee rather than assuming it from a generic example.

Backup is not automatic

A battery connected to the grid does not necessarily power the home during a power cut. Standard grid-tied inverters stop to prevent islanding.

Backup requires:

an approved islanding arrangement
safe separation from the distribution network
an island-mode earthing design
maintained circuits or whole-home load control
adequate inverter and battery power
tested transfer and reconnection

Selected circuits are often easier to support than every load. Heat pumps, EV chargers, electric showers and cooking appliances need explicit consideration.

Location and fire safety

Battery location follows the current product instructions, electrical design and applicable fire-safety guidance. Consider:

escape routes and occupied rooms
fire-resisting construction where required
combustible materials
impact and vehicle damage
temperature and direct sunlight
water and flooding
ventilation and product clearances
access for isolation and emergency response

An IP rating does not settle fire separation, temperature or escape-route suitability. A garage, loft or outside wall is not automatically acceptable for every system.

Metering and control commissioning

The battery normally uses a meter or CT at the grid connection to distinguish import from export. Test it with known loads and solar states.

Commissioning should prove:

correct phase and direction sensing
self-consumption behaviour
charge and discharge limits
export limitation and failsafe state
response to loss of meter communication
schedules and grid-charge permission
backup transfer where fitted
portal and local control access

A CT error can make a battery import when it should absorb surplus or export when it should cover household load.

Handover

Keep:

updated single-line diagram
battery, inverter and meter model and serial numbers
compatibility and firmware record
electrical test certificate
DNO notification or agreement
G100 settings and test evidence where used
MCS certificate where applicable
FIT and SEG correspondence
backup circuit schedule
commissioned settings and account ownership
warranty, maintenance and emergency information

A financial assessment applies the household’s dated import and export terms to its own interval data and includes losses, ageing and finance cost.