The four reasons a commercial buyer adds battery
(1) Self-consumption uplift — shift midday solar excess to evening loads. (2) Time-of-use arbitrage — under MHHS, charge cheap overnight, discharge peak. (3) Demand charge avoidance — peak-shave to stay below capacity thresholds. (4) Resilience — backup against grid outage. Each driver leads to a different battery size.
Sizing approach 1: self-consumption uplift
Look at your HH data. Find the midday hours where solar generation exceeds demand. Sum the excess in kWh. That's your self-consumption opportunity. A battery roughly half that size captures 80% of the value. For example, if a 220 kWp array exports around 170 kWh on a typical day, a battery of roughly 80 kWh would capture most of that surplus.
Sizing approach 2: arbitrage
Under Octopus Agile Business or similar half-hourly tariffs, look at the daily price spread. A typical spread (peak vs trough) is 18–25p/kWh. Multiply by battery capacity × cycles/year (300 typical) = arbitrage revenue. A 50 kWh battery cycled 300 times at 20p spread = £3,000/year arbitrage.
Sizing approach 3: demand charge avoidance
Some commercial sites have Capacity Charges (£/kVA) — your max import in any HH window during winter sets the charge for the year. A 100 kWh battery can shave 50 kW off a winter evening peak; over 12 months that's £15k–£40k in capacity charge savings on top of energy savings.
Sizing approach 4: backup resilience
Backup sizing is critical-load × hours. Cold storage: keep compressors running for 90 min until generator starts. A typical compressor draws 80 kW → 120 kWh battery = 90 min backup. Tier-III data centre cooling: similar maths. Most other commercial sites don't actually need backup — focus on self-consumption + arbitrage.
Cost per kWh delivered (May 2026)
Domestic batteries (GivEnergy AIO, Tesla Powerwall): £550–£750/kWh installed. Commercial batteries (Sigenergy SigenStor, BYD HVS/HVM): £400–£600/kWh installed. Above 100 kWh, prices drop further; we've quoted 215 kWh BYD installs at £380/kWh installed.
A cold-storage worked example
Illustrative, not a named KMM customer. A cold store runs compressors around the clock, so it has real evening demand for stored midday solar. To test a battery, add up three annual values from your half-hourly data: the peak-rate kWh the battery displaces, the extra solar you self-consume instead of exporting, and any capacity or demand charges you avoid. Divide the installed battery cost by that annual value to get a simple payback, then compare it with the battery warranty. KMM runs this on your own data before recommending a battery.
Why most commercial sites don't need a battery
If your demand profile matches solar generation (8am–6pm operations), you'll self-consume 70%+ of generated kWh without a battery. Battery adds complexity, capex and warranty risk for marginal benefit. Battery makes sense when: evening shift, 24/7 operations, capacity charges, MHHS arbitrage potential. Otherwise, just add more panels.
Replacement at year 10–15
Lithium batteries degrade. Plan for 20–30% capacity loss by year 10. Either accept the reduced capacity or replace the cells. BYD and Sigenergy designs are modular — only the cells need replacement, not the inverter or BMS. Budget £80–£120/kWh for cell replacement at year 12–15.
