Combining solar and battery storage is increasingly the default recommendation for commercial buildings. But the economics are only compelling when the system is sized correctly — and that requires data most building owners don't have.
Why solar alone often disappoints
A rooftop solar array generates electricity during daylight hours. For many commercial buildings, the mismatch between generation and consumption is significant: offices and warehouses often draw heavy load from 6am and tail off in the afternoon, while retail and hospitality sites may peak in the evening when generation has stopped.
Without storage, any solar generation that exceeds immediate consumption is exported to the grid — typically at 4–8p/kWh under current export tariffs. That same electricity would have saved you 25–35p/kWh if you'd consumed it directly. The economic case weakens significantly when self-consumption is low.
A BESS — Battery Energy Storage System — solves this by capturing excess generation and dispatching it when you need it. But the battery needs to be sized to match both the generation surplus profile and the consumption demand pattern. Getting this wrong in either direction costs money.
The three numbers you need before sizing anything
1. Your daily generation surplus profile
This is the hour-by-hour shape of what your solar array will generate minus what you'll consume at the same time. A building that consumes heavily during daylight has a smaller surplus to store; one that's unoccupied during the day has a large surplus and needs more storage to capture it.
Without real consumption data, installers estimate this from benchmark figures. The error range on benchmarks is typically ±20–35% for commercial buildings — wide enough to make the difference between a 6-year payback and a 10-year one.
2. Your peak demand profile
Maximum demand charges — the element of your electricity bill tied to your highest 30-minute consumption in a billing period — frequently account for 20–30% of a commercial electricity bill. A correctly sized BESS can shave these peaks, adding a significant revenue stream to the storage case that has nothing to do with solar.
If your peaks are short and predictable (a morning startup ramp, a specific piece of equipment), a battery can be sized to cover them. If your peaks are irregular or driven by uncontrollable loads, the economics are harder to model.
3. Your tariff structure
Time-of-use tariffs, demand charges, and export rates all affect the optimisation logic. A building on a flat-rate tariff gets a different answer to one on a half-hourly settled supply contract. Knowing which periods are expensive to import from the grid tells you when the battery should be full, and knowing your export rate tells you what you're giving up when you can't store surplus.
What good sizing looks like
For most commercial buildings in the 50–500kW demand range, we typically see:
- Solar array: 40–80% of peak demand (kWp), constrained by available roof area and grid connection capacity
- Battery storage: 1.5–3 hours of typical demand at the array's output level, sized to capture the daily surplus and shave the peak in the same charge/discharge cycle
- Inverter configuration: matched to allow simultaneous solar input and grid import during peak shave events
The economics at current UK electricity prices (25–35p/kWh import, 4–8p export, demand charges of £3–8/kVA/month) typically deliver paybacks of 5–9 years for solar-only and 6–10 years for solar+BESS, depending on building profile.
The monitoring question
All of the above assumes you have the consumption data to make the modelling credible. If you don't, the first step is getting it.
EKO19's monitoring deployment typically takes 4–6 weeks to establish a reliable consumption baseline. We then use that data to size any subsequent technology proposal — and we guarantee that our modelled savings are benchmarked against your actual numbers, not industry averages.
If you're planning a solar or BESS investment in the next 12 months, start the monitoring phase now.