Adding a battery to existing solar

Yes, storage can be added to almost any existing solar array, and there are two routes. AC coupling installs a separate battery inverter alongside your existing solar inverter, leaving the original system untouched. The alternative is replacing the existing inverter with a hybrid that handles solar and storage together on the DC side. AC coupling is less disruptive and keeps a healthy inverter in service; a hybrid swap is neater and often makes sense if the existing unit is nearing the end of its life anyway. The right choice depends on the age and condition of what you already have.

AC coupling

The battery and its inverter connect on the AC side of the system, downstream of the existing solar inverter. Solar generation converts to AC as it always did; the battery inverter takes AC from the house supply, converts it to DC to charge, and reverses the process to discharge.

Advantages. Your existing solar system is not touched, so its warranties and configuration stay intact. Product choice is wide. Installation is contained.

Trade-offs. Energy going into the battery from solar is converted DC to AC and back again, which costs a little in round-trip efficiency. There are also now two inverters in the system rather than one, each with its own service life.

Hybrid inverter replacement

The existing solar inverter comes out and a hybrid goes in. The array connects on the DC side, the battery connects on the DC side, and one unit manages both.

Advantages. Solar charges the battery without the extra conversion step. One unit, one monitoring platform, one set of settings. Usually the cleaner installation.

Trade-offs. You are discarding a working inverter if the existing one is healthy. The array’s string configuration has to suit the new inverter’s inputs, which occasionally requires re-stringing. And it is a bigger job.

The rule of thumb: if your existing inverter is well into its service life or has already given trouble, replacing it with a hybrid usually makes sense. If it is comparatively recent and working well, AC coupling avoids throwing away something serviceable.

What a retrofit survey checks

Retrofits carry more unknowns than a new installation, because you are inheriting someone else’s work. We look at:

• The existing array and inverter. Make, age, string configuration, condition, and whether it is still supported.
• The DC and AC wiring. Isolation, protection, labelling and earthing. Older installations frequently fall short of what we would want to leave behind.
• The consumer unit. Whether there is capacity and whether the board itself is fit to take another supply.
• Metering. What is installed, whether export is metered, and how the FIT or SEG arrangement works.
• The proposed battery location. Temperature, ventilation, structural support, cable routes and service access.
• Total generation capacity. Which determines the network route.

The network question

Storage that can export is generation capacity as far as the Distribution Network Operator is concerned. Where total capacity is 16A or less per phase at low voltage — approximately 3.68kW single-phase at 230V, or 11.04kW across three phases at 400V — G98 applies and the installer submits an Installation Commissioning Confirmation within 28 days of commissioning.

Above that, a G99 application must be made and approved before installation. So a house with an existing array close to the G98 threshold can be pushed into G99 by the battery alone. That has to be established before dates are booked, not afterwards.

Existing incentive arrangements

If your array is on a legacy Feed-in Tariff agreement, adding storage can interact with how generation is metered and how export is treated under that agreement. The rules depend on the specific arrangement, so check with your FIT licensee before you commit.

If you are on the Smart Export Guarantee, storage changes what you export rather than your eligibility — you will typically export less, because more generation is being used on site. Since suppliers set their own export rates and terms, it is worth reviewing your export tariff after a retrofit rather than assuming it still suits you.

VAT

Electrical storage batteries were added to the qualifying list of energy-saving materials from 1 February 2024, and installation of qualifying materials in residential accommodation in Great Britain is zero-rated from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027. Your quotation should state the treatment applied.

Getting the sizing right

A retrofit is a chance to size storage against real data rather than assumptions, because you already have generation figures and consumption history. Use them. Look at what you currently export and when, and at your evening and overnight consumption.

Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.