Tens of thousands of commercial solar PV systems installed across the UK in the 2010s are now operating without battery storage — generating electricity that is exported to the grid at low rates while their owners still pay peak prices for evening energy. Retrofitting a BESS changes that equation entirely.

Why Businesses With Existing Solar Are Missing Out

When commercial solar PV was first widely adopted, battery storage was expensive and the technology was immature. Most systems were designed purely for daytime generation — export what you don’t use, import from the grid the rest of the time.

Today, that model is far from optimal. Battery storage costs have fallen by over 80% in the last decade, export tariff rates remain low, and grid electricity prices have been persistently high. A business with a 100 kWp rooftop solar array installed in 2015 may be exporting 40–60% of its generation at rates of 5–15p/kWh whilst paying 25–35p/kWh to import electricity in the evenings.

What Is a Retrofit BESS?

A retrofit BESS is a battery energy storage system added to an existing solar PV installation — one that was originally commissioned without storage. The battery system is integrated into your existing electrical infrastructure to capture surplus generation, store it, and deploy it when it’s most valuable to your business.

Unlike a new-build installation where solar and battery are designed together from the outset, a retrofit requires careful assessment of your existing inverters, switchgear, protection systems and DNO connection agreements before the right technical approach can be determined.

AC-Coupled vs DC-Coupled Retrofit: What’s the Difference?

AC-Coupled Retrofit BESS

In an AC-coupled system, the battery and its own dedicated inverter/charger are connected on the AC side of your existing installation — after your solar inverters. This is the most common retrofit approach because it works with virtually any existing solar inverter brand and requires minimal changes to your existing system. AC coupling is flexible, scalable and typically lower cost to install as a retrofit.

DC-Coupled Retrofit BESS

A DC-coupled retrofit connects the battery directly to the DC side of your solar array, before the inverter. This approach requires replacing your existing inverters with a hybrid inverter system capable of managing both the solar input and battery simultaneously. DC coupling is more efficient but more disruptive to install on an existing system. Novalux Solar will assess whether the additional efficiency gain justifies the cost and downtime for your specific installation.

What Does the Retrofit Process Look Like?

Step 1: Site Assessment and Data Analysis

Our engineers review your existing solar PV system documentation, inverter specifications, half-hourly meter data and DNO G98/G99 connection agreement. This gives us a precise picture of how much energy you currently export, when your demand peaks and how a battery would perform in your specific scenario.

Step 2: Financial Modelling

We model the projected performance of several battery size options against your actual consumption and generation data. This produces a detailed 25-year cashflow showing payback period, net present value and annual savings — so you can make an informed investment decision.

Step 3: DNO Notification or Application

Adding a battery storage system to an existing grid connection almost always requires notification to your Distribution Network Operator (DNO) — and in some cases a formal G99 application. Novalux Solar handles all DNO interaction on your behalf, including any required protection relay upgrades.

Step 4: Installation

Our NICEIC-accredited installation team fits the battery modules, inverter/charger, protection devices and monitoring system with minimal disruption to your operation. Most commercial retrofit installations are completed within one to three days depending on system size.

Step 5: Commissioning and Monitoring

Once commissioned, your BESS is configured with a charge/discharge strategy optimised for your tariff and demand profile. Our remote monitoring platform gives you real-time visibility of your system’s performance, and our team monitors for any alerts or optimisation opportunities.

What Systems Are Compatible With Retrofit BESS?

The great majority of commercial solar PV systems installed in the UK are compatible with AC-coupled battery retrofit. This includes systems using inverters from SolarEdge, SMA, Fronius, Huawei, ABB, Sungrow and most other mainstream commercial inverter manufacturers.

Novalux Solar has particular experience retrofitting BESS to older string inverter systems common on rooftop installations across agricultural, manufacturing and distribution sites in Wales and the West Midlands — many installed in the Feed-in Tariff era and ideal candidates for battery upgrades.

How Much Does Retrofit BESS Cost?

Commercial retrofit BESS pricing depends on battery capacity, inverter topology, site-specific installation requirements and any DNO upgrade work needed. As a general guide:

• 50–100 kWh AC-coupled retrofit: typically £30,000–£70,000 installed
• 150–300 kWh commercial system: typically £80,000–£180,000 installed
• 500 kWh+ containerised BESS: project-specific pricing

Novalux Solar provides detailed fixed-price proposals following our site assessment — so there are no hidden costs.

Retrofit BESS Across Wales and the UK

Novalux Solar is based in Skenfrith, Monmouthshire and has installed commercial solar and battery systems across South Wales, Mid Wales, Herefordshire, Gloucestershire, Worcestershire, Shropshire, the West Midlands and beyond. Whether you’re a food producer in Pembrokeshire, a manufacturer in the West Midlands or a hotel group in the Wye Valley, our team can assess whether your existing solar system is ready for a BESS retrofit.