Sizing a Battery Energy Storage System (BESS) for commercial and industrial applications requires balancing four variables: load profile, tariff structure, solar generation, and battery economics. Get the sizing right and a BESS pays back in 4–7 years with double-digit IRR. Get it wrong and you've parked capital on an oversized asset that never reaches full cycle count. This guide walks through the four most common C&I BESS applications and shows how to size each.
The four C&I BESS applications
| Application | Primary value driver | Typical capacity ratio |
|---|---|---|
| Peak shaving | Reduce maximum demand charges on utility bill | 1× to 4× of peak excess (kWh per kW) |
| Solar self-consumption | Use more on-site PV during evening hours | 1× to 2× of average evening consumption |
| Time-of-use arbitrage | Charge during off-peak, discharge during peak tariffs | Match peak-tariff window duration (4–6 h typical) |
| Backup power | Critical-load continuity during grid outage | Match critical-load × outage duration target |
Sizing methodology (5 steps)
- Get 12 months of interval data — pull 15-minute interval consumption data from your utility. This is the single most important input. Daily/monthly aggregates aren't sufficient.
- Identify your value driver — peak shaving, self-consumption, arbitrage, or backup? Most C&I projects optimize for 2 of the 4 simultaneously.
- Calculate target power (kW) and energy (kWh) — power determines inverter sizing; energy determines battery capacity. Both matter.
- Apply round-trip efficiency — modern LFP BESS achieves 90–94% round-trip efficiency. Plan for 88% conservatively across the system lifetime.
- Model cycle life — Sungrow's MBL160 is rated 8,000 cycles at 90% DOD. Sungrow ST255CS-2H handles 6,000+ cycles. Plan for 250–365 cycles per year depending on application.
Worked example: 500 kW solar + BESS for a logistics warehouse
Site profile:
- Peak demand: 800 kW between 14:00–17:00 (cold storage compressors)
- Average daytime consumption: 600 kW
- Average night consumption: 200 kW
- Existing PV: 500 kWp rooftop array (estimated 700 MWh/yr in temperate climate)
- Tariff: $0.12/kWh off-peak, $0.18/kWh peak, $25/kW demand charge
Recommendation:
- BESS sizing: 250 kW / 500 kWh — 2 × Sungrow ST255CS-2H PowerStacks deliver 257 kWh × 2 = 514 kWh nominal, 125 kW × 2 = 250 kW AC output.
- Primary use: Peak shaving (cuts demand charge from 800 kW → 550 kW)
- Secondary use: Solar self-consumption (charge from excess PV during 11:00–14:00, discharge 17:00–20:00)
- Estimated savings: $90K/year demand charge reduction + $35K/year energy arbitrage = $125K/year
- Payback: ~5–6 years at $700K installed CAPEX
Round-trip efficiency in practice
The "round-trip efficiency" number on a battery datasheet is at ideal conditions. Real-world C&I systems lose energy at four stages:
- AC → DC conversion (PCS) — 1–2% loss
- DC → battery charging — 2–3% loss
- Battery → DC discharging — 2–3% loss
- DC → AC conversion — 1–2% loss
Total real-world round-trip: 88–93%. The Sungrow ST255CS-2H integrates the PCS into the BESS cabinet, reducing one conversion stage and delivering 92% round-trip in typical conditions.
Cycle economics
Battery cost amortization depends on cycles per year. For self-consumption (1 cycle/day = 365/yr), an 8,000-cycle MBL160 lasts ~22 years — well beyond the 10-year warranty. For peak shaving (1 cycle/day in summer only = 100/yr), the same battery lasts 80 years — way beyond service life. For arbitrage (2 cycles/day = 730/yr), it lasts 11 years.
The lesson: match cycle count to your application. Don't pay for an 8,000-cycle battery if you only need 2,000.