Commercial Solar Battery Storage: How to Get More from Your Solar Panels

Solar PV is now a fixture on UK commercial rooftops. Thousands of businesses have installed it.

A significant share of those systems are not delivering their full potential. Without battery storage, surplus solar generation at midday goes to the grid at export rates that are a fraction of what the business pays to import. SEG export rates typically run at 3-15p/kWh. Most commercial import contracts are 20-35p/kWh. The gap is significant.

This guide covers the technical options, typical costs, and what a proper sizing analysis involves.

The Limitation of Solar PV on Its Own

Solar generation peaks between roughly 10am and 3pm on a clear day. For many businesses, this period is when consumption is at its lowest, staff are already in, equipment is running, and the midday lull means less activity than the morning start-up or evening wind-down.

The result is a mismatch: the system generates most when demand is low, and produces little during the morning and evening peaks when grid electricity is most expensive. Surplus generation that cannot be consumed immediately is exported, typically earning export payments under the Smart Export Guarantee (SEG) – but SEG rates are generally a fraction of the cost of imported electricity.

The business is effectively selling cheap and buying dear. Battery storage corrects that.

How Battery Storage Complements Commercial Solar

A battery storage system connected to a commercial solar installation acts as a buffer between generation and consumption. When the solar array produces more electricity than the site is using, the battery charges rather than the surplus being exported. When consumption exceeds solar generation, in the morning, evening, or on overcast days, the battery discharges, supplying the site before grid electricity is drawn.

The result is higher self-consumption: a greater proportion of the solar energy generated is actually used on-site, at its full value, rather than being exported at a lower rate. For most commercial solar installations, adding a battery can increase self-consumption from a typical 30-50% to 70-90%, depending on the system size, battery capacity, and consumption profile.

AC-Coupled vs DC-Coupled Systems

When adding battery storage to an existing solar installation, there are two main technical approaches:

AC-coupled systems connect the battery to the AC side of the electrical system, after the solar inverter. This is the most common retrofit approach because it is compatible with virtually any existing solar installation regardless of age or inverter type. The battery has its own inverter and can also charge from the grid if required.

DC-coupled systems connect the battery directly to the DC output of the solar panels, before the inverter. This approach is more efficient because energy is only converted once, but it typically requires a new combined solar inverter and is better suited to new installations designed with battery storage from the outset.

For businesses looking to retrofit a battery to an existing system, AC coupling is usually the practical choice. For new installations, a DC-coupled combined system is worth considering for its efficiency advantages.

Sizing the Battery Correctly

Getting the battery size right is one of the most important decisions in a solar-plus-storage project. An undersized battery fills quickly and limits the amount of solar energy that can be captured. An oversized battery adds cost without proportionate benefit if there is insufficient solar generation to fill it regularly.

The starting point is a consumption and generation analysis:

• How much energy does the solar system generate, and at what times of day?

• What is the site’s hourly consumption profile, when are the morning and evening peaks?

• What is the maximum self-consumption gap, how much surplus solar generation is currently being exported?

For most commercial solar installations, a battery sized at roughly 50-75% of the daily surplus export is a reasonable starting point. A detailed analysis using actual half-hourly metering and generation data will give a more precise recommendation.

The Financial Case for Solar Battery Storage

The return on a commercial solar battery system comes from several sources:

Increased self-consumption value – Every unit of solar electricity consumed on-site rather than exported is worth the difference between the import rate (typically 20-35p/kWh for commercial customers) and the SEG export rate (typically 3-15p/kWh). The battery captures this value by storing surplus generation for later use.

Peak demand reduction – Discharging the battery during morning and evening demand peaks reduces the amount drawn from the grid at the most expensive times, and can reduce capacity charges on the electricity bill.

Grid services revenue – As covered in our guide to battery storage for business, commercial batteries can participate in grid balancing services and earn additional revenue, further strengthening the overall business case.

For a typical 100 kWp commercial solar system with a 100 kWh battery, the combination of increased self-consumption and peak demand reduction can deliver annual savings of £8,000-£20,000, depending on the site’s tariff and consumption profile. Payback periods of five to eight years are common, with a system lifespan of 10 to 15 years.

Retrofit or New Installation?

Businesses with existing solar PV have two options: retrofit a battery to the current system, or replace the inverter and add an integrated solar-plus-storage system.

Retrofitting is generally the lower-cost, lower-disruption option and makes sense where the existing solar installation is in good condition and the inverter is functioning well. An AC-coupled battery can typically be added within one to two days with minimal disruption to operations.

A full replacement with an integrated system may be worth considering if the existing inverter is approaching end of life, if the current system is underperforming, or if the business wants to take advantage of advances in system management software that are better supported by modern combined inverter-storage units.

How Catalyst Can Help

Catalyst works with businesses across the UK on energy procurement and cost management. The starting point for any solar battery assessment is understanding what the site is currently buying, when it’s buying it, and what it’s exporting. That’s the starting point.

If you have existing solar PV and want to understand whether battery storage would improve your return, or if you are planning a new combined installation, get in touch.

Speak to Catalyst about commercial solar battery storage