Energy Storage Systems
This domain tests battery and storage metrics, applications, codes, and economics. Questions often ask you to separate nameplate from usable capacity, calculate round-trip efficiency effects, or evaluate demand-charge savings against arbitrage.
Metrics and applications
- Energy capacity is kWh; power capacity is kW. C-rate = power ÷ energy. Depth of discharge (DoD) limits usable energy.
- Round-trip efficiency (RTE) accounts for charging and discharging losses. A 90% RTE battery needs more input energy than it delivers.
- Degradation depends on chemistry, temperature, state of charge, and cycle depth. Warranties specify throughput or cycles.
- Lithium-ion LFP and NMC dominate new stationary projects; lead-acid is cheaper but shorter-lived; flow batteries offer long duration.
- Applications include peak shaving, demand-charge management, energy arbitrage, resiliency, frequency response, and demand response.
- Codes and standards include NFPA 855, UL 9540/9540A, and local interconnection rules.
Worked example
A 400 kWh/200 kW battery has 90% usable energy and 90% RTE. It discharges at 200 kW for 2 hours, delivering 400 kWh. Monthly demand savings at $15/kW = 200 × $15 = $3,000. For arbitrage, discharging 400 kWh at $0.30/kWh yields $120 of avoided energy. To deliver that energy, the battery draws about 400 ÷ 0.90 = 444 kWh at $0.15/kWh, costing $66.70. Gross arbitrage margin = $120 − $66.70 = $53.30 per cycle. The reasoning: RTE applies to the charging energy, and demand savings are separate from energy arbitrage.
Traps that cost marks
- Using nameplate kWh as if all of it is usable.
- Ignoring RTE on both charge and discharge.
- Forgetting demand ratchets, standby charges, and exit fees.
- Assuming net metering treats storage exports the same as solar exports.
- Ignoring fire clearance, ventilation, and UL 9540A test requirements.
- Double counting demand and energy savings from the same dispatch.