BESS sizing guide.

The two numbers

Every BESS is described by two numbers. Get these right and you get the right unit. Get them wrong and you'll either pay too much for an oversized unit or run out of power mid-shift.

1. Output power (kW)

This is the maximum instantaneous power the battery can deliver. Think of it as the "size of the pipe" — how much power can flow out at once. If your equipment draws 30 kW peak, you need a battery rated for at least 30 kW output (with some headroom).

2. Energy capacity (kWh)

This is how much total energy the battery stores. Think of it as the "size of the tank" — how long you can run before recharging. Capacity ÷ average load = runtime in hours. A 100 kWh battery powering a 10 kW load runs for 10 hours.

How to calculate your needs

Step 1: List your equipment

Make a list of everything that will be plugged in. For each, note:

• Rated power (in kW or W) — usually on a label or in the manual
• Whether it runs continuously or intermittently
• Hours per day in use

Step 2: Calculate peak power (kW needed)

Add up the rated power of everything that could be running simultaneously at the same moment. This is your peak. Add 25% headroom for surge currents (motors, lighting fixtures starting up).

Step 3: Calculate energy use (kWh needed)

For each piece of equipment, multiply rated power × hours of use per day. Add them all up. That's your daily kWh need. Round up by 20% for safety margin.

Worked examples

Example 1: Wedding marquee, 200 guests

• Stage lighting: 8 kW × 6 hours = 48 kWh
• Sound system: 3 kW × 6 hours = 18 kWh
• AV / projector: 2 kW × 4 hours = 8 kWh
• Catering hot boxes: 6 kW × 3 hours = 18 kWh
• Decorative lighting: 4 kW × 6 hours = 24 kWh
• Peak power: 8+3+2+6+4 = 23 kW (+25% headroom = 29 kW)
• Daily energy: 48+18+8+18+24 = 116 kWh (+20% margin = 139 kWh)

Recommended: trailer 100–150 kWh unit with 30+ kW output.

Example 2: Indoor mall fit-out, overnight shift

• Power tools (rotating use): 5 kW peak × 6 hours = 30 kWh
• Welder: 8 kW × 2 hours = 16 kWh
• Site lighting: 3 kW × 8 hours = 24 kWh
• Peak: 5+8+3 = 16 kW (assume welder + lights coincide; +25% = 20 kW)
• Daily: 70 kWh (+20% = 84 kWh)

Recommended: mid-size 50 kWh unit, possibly with daytime swap or mid-shift recharge.

Example 3: Tower crane site, 24/7 operation

• Tower crane lift peaks: 250 kW for 30 seconds, 30 lifts/day
• Site office: 3 kW × 24 hours = 72 kWh
• Site lighting: 5 kW × 12 hours = 60 kWh
• Tools / welders: 8 kW × 8 hours = 64 kWh
• Peak: 250 kW (lift)
• Daily: 72+60+64+lift energy ≈ 250 kWh

Recommended: hybrid setup — 200 kWh battery (60–100 kW continuous) + 100 kW diesel generator. Battery handles lift peaks; generator handles average load and recharges battery.

Common sizing mistakes

• Forgetting peak surge — motors, AC compressors, and lighting fixtures draw 2–7× their rated power at startup. Without headroom, the battery trips out
• Confusing kW with kWh — a unit can have huge capacity but limited output (or vice versa). Both numbers matter
• Ignoring recharge time — if you need to recharge during the project, factor in charge time and the source (grid, generator, solar)
• Sizing for 100% rather than 80% — batteries deliver best life when not regularly drained to 0%. Size for 80% usable capacity

When in doubt — call us

You don't need to figure this out alone. Send us your equipment list. We size for free in under an hour. We'd rather size correctly the first time than have you stuck on-site with the wrong unit.