Most factory owners ask for a battery in kWh. That is the wrong unit to start with. A cabinet is sized by three numbers, in this order: the 15-minute peak demand you want to cut, the length of the window you are cutting it in, and the spare capacity on your transformer. Get those three right and the kWh figure falls out on its own. Get them wrong and you buy either a cabinet that cannot carry the peak or one you cannot connect.
Start With the Demand Charge, Not the Load
In most markets with a demand charge, the bill is set by the highest 15-minute average in the month, not by total consumption. That single number can be 30–40% of the electricity bill in a factory running one or two shifts, because the peak happens when every line starts at once and then never repeats.
So the first number you need is not annual kWh. It is the ratio between your average demand and your peak demand. That ratio tells you how much a battery can actually save.
| Peak / average ratio | What it means | Realistic shaving | Payback signal |
|---|---|---|---|
| 1.1 : 1 | Flat load, steady process | 5–8% | Poor — do not buy storage for demand alone |
| 1.3 : 1 | One shift, moderate start-up surge | 15–20% | Marginal — needs a second revenue stream |
| 1.5 : 1 | Two shifts, motor-heavy | 25–35% | Good |
| 2.0 : 1 or higher | Batch process, big motor starts | 35–50% | Strong — but check the transformer |
A ratio above 1.5:1 is where a battery starts paying for itself on demand charges alone. Below 1.3:1 you are usually better off rescheduling loads than buying hardware.
The Discharge Window Is Set by Your Tariff, Not by Your Equipment
The second number is how long your peak tariff window lasts. This decides whether you need a 1-hour, 2-hour or 4-hour system, and it changes the cost per usable kWh by a factor of two.
| Peak window | Typical market | System you actually need | Trap |
|---|---|---|---|
| 1–2 hours | Some European TOU tariffs | 1C cabinet, small kWh, high power | Cheapest battery, most expensive PCS per kWh |
| 3–4 hours | Most C&I demand-charge markets | 0.5C cabinet, balanced | The sweet spot for 215 kWh class cabinets |
| 5–6 hours | Weak grids with scheduled outages | 0.3C or larger kWh | Battery cost dominates; check cycle life hard |
| 8+ hours | Load shedding, no reliable grid | This is not peak shaving any more | You are buying a generator replacement, size it that way |
A 215 kWh cabinet at 100 kW covers a 2-hour window. The same cabinet at 50 kW covers 4 hours. Same hardware, different inverter setting — which is why the power rating matters as much as the energy rating, and why a supplier who only quotes kWh has not understood your site.
What the Transformer Will Let You Do
This is the constraint that kills more projects than any other, and it is almost always discovered late. A battery that discharges 100 kW adds 100 kW to what your transformer sees. If the transformer is already at 85% during peak, you have 15% of headroom and nothing more.
- Behind-the-meter, no export: the battery can only discharge up to the site load at that moment. It can never push power backwards. This is the easiest configuration to permit and the one most C&I cabinets are built for.
- Zero-export with a CT clamp: the inverter measures the point of common coupling and throttles itself so nothing flows out. Slightly more expensive, and the standard answer where the utility will not allow export.
- Export-capable: requires a grid-code certified inverter and a connection agreement. In most of Europe this means EN 50549-1 and a formal application. Budget three to nine months.
Ask your utility for the transformer rating and the highest recorded 15-minute demand before you ask any supplier for a quote. Without those two numbers every quote you receive is a guess.
Three Configurations That Actually Ship
These are the three shapes we build most often. They are not the only options, but they cover the majority of factory sites in weak-grid and high-tariff markets.
| Site profile | Cabinet | Power / energy | Why this one |
|---|---|---|---|
| Hotel roof, campus car park, small factory yard Peak 80–120 kW, 2-hour window | 215 kWh outdoor cabinet | 100 kW / 215 kWh | Air-cooled, IP54, integrated PCS and fire suppression — no separate plant room |
| Factory with no reliable grid Needs to run islanded through outages | 40 kW off-grid cabinet | 40 kW / 92.3 kWh | Battery, PCS, MPPT and generator input in one enclosure — it can form its own grid |
| Large site, 500 kW+ peak Transformer has headroom | Containerised | 215 kWh cabinets in parallel, or 3.44 MWh container | Parallel cabinets up to MW scale without a custom build |
The Mistake Everyone Makes
Sizing on average load. Average load tells you how much energy you use. It tells you nothing about the 15 minutes that set your bill, and nothing about whether the inverter can follow a motor start.
The second most common mistake is ignoring the motor start. A 90 kW motor draws five to seven times its rated current for a few hundred milliseconds. A battery cabinet rated 100 kW continuous may still trip on that inrush if the inverter is not specified for it. Ask for the overload curve, not just the continuous rating.
A battery sized on average load will be too small every month and too expensive every day.
Frequently Asked Questions
How many kWh do I need to cut a 200 kW peak by 30%?
You need to cut 60 kW. If your peak window is two hours, that is 120 kWh of usable energy. Add 10% for degradation over ten years and 10% for depth-of-discharge limits, and you are specifying roughly 145–150 kWh nameplate. A 215 kWh cabinet covers it with margin and leaves room to shave deeper later.
Can I install a cabinet outdoors in a hot climate?
Yes, if it is rated for it. Look for IP54 or better, active HVAC or air-cooling rated to 50 °C ambient, and a derating curve published in the datasheet. A cabinet rated 100 kW at 25 °C may only deliver 80 kW at 45 °C — that derating is the difference between a working system and an undersized one.
Do I need a separate plant room?
For an outdoor cabinet, no. For a rack-based system, yes — racks need a room with cooling and fire detection. This is the main reason outdoor cabinets cost more per kWh but less per project.
What happens when the battery reaches end of life?
LFP cells typically retain 70–80% of capacity after 6,000 cycles. At one cycle per day that is over 16 years; at two cycles per day, 8 years. Plan the replacement as a module swap, not a cabinet replacement — which is only possible if the supplier still stocks that module. Ask for a written module availability commitment before you buy.
Send Us Your Load Profile
Fifteen-minute interval data for one month is enough to size a cabinet properly. Send it with your country and tariff structure and we will come back with a configuration and a budget price, not a brochure.