How to Size Battery Storage for Commercial Loads
Key takeaways
- Size from a measured load profile, not from the connected load or the catalogue rating.
- Nameplate kWh is not usable kWh. Depth of discharge, heat and end-of-life reserve remove 25–40%.
- Power (kW) and energy (kWh) are separate decisions. Most disappointments come from getting one right and the other wrong.
- The application — peak shaving, backup, or solar self-consumption — changes the sizing entirely. Decide it first.
Start with the load profile, not the catalogue
Buyers usually begin with a number pulled from a supplier's brochure. That is backwards. The only input that produces a correct size is a measured load profile: kWh consumed per 15- or 30-minute interval, for at least two weeks, and ideally a full year of utility bills to capture seasonality.
Three numbers come out of that profile and drive everything else.
- Peak demand (kW) — the highest sustained interval draw. Sets the inverter power rating.
- Daily energy (kWh) — sets the storage capacity.
- Load shape — when the peak occurs, and whether it is sharp and short or broad and flat. This decides whether a small, high-power battery or a large, slow one is the right answer.
Step 1: convert the load into usable kWh
Take the portion of the load you intend to cover. A warehouse that wants four hours of evening coverage draws differently from a cold store that must ride through any outage.
Then derate. Nameplate capacity is measured at 25°C and, in practice, at the cell's full chemical range. On a Gulf site you lose capacity to three factors.
Worked example: a facility wants 400 kWh of evening coverage. At 90% usable depth, 10% thermal derate and 15% end-of-life reserve, the required nameplate is roughly 400 divided by 0.90, 0.90 and 0.85, which gives about 580 kWh. Quote 600 kWh.
Step 2: size the power, separately
Capacity answers how long. Power answers how fast — and it is the number that most often gets missed. A 600 kWh battery paired with an undersized power conversion system cannot discharge fast enough to shave a sharp peak, and the savings case collapses.
The C-rate relates the two. A 0.5C system discharges fully in two hours; a 0.25C system in four. Most commercial lithium storage in the region is specified between 0.25C and 0.5C.
Ask for the discharge curve, not the headline
Two batteries with the same kWh rating can deliver very different energy at a 1-hour discharge rate. Request the usable energy at your actual discharge power, at 40°C ambient. That is the only figure that belongs in a financial model. The GBP lithium storage system datasheet publishes both.
Five mistakes that cost money
- Sizing from connected load. The sum of your breakers is not your demand. It overstates capacity by a factor of two or more.
- Ignoring the cooling load of the battery itself. An outdoor enclosure with air conditioning consumes real energy; include it in the profile.
- Mixing essential and non-essential loads on backup. Decide what must run in an outage and size only for that.
- Forgetting the inverter's own limits. Check continuous and surge ratings separately, especially with motor and compressor loads.
- Sizing for year one. Add the end-of-life reserve now, or buy twice.
Information to send your supplier
A complete enquiry gets a realistic quotation on the first pass. Send these six items.
Short answers
Lithium iron phosphate, almost always. It tolerates higher ambient temperatures, accepts partial state of charge without damage, and occupies a fraction of the footprint. Lead-acid remains viable only where upfront cost dominates and the room is actively cooled.
At 90% depth of discharge and controlled temperature, 6,000 cycles or more is a reasonable planning figure for LFP. Warranty terms vary — check whether the guarantee is on cycles or years, and which comes first.
Only if the architecture allows it. Decide at purchase: modular racks with a common bus expand cleanly, while mixed-age banks do not.
