📊 Demand Charges, Time-of-Use Rates, and BESS: A Commercial Energy Cost Strategy

Understanding Commercial Electricity Bills

Commercial electricity bills have two primary components that residential customers don't typically see:

Energy Charges — charges for the total kilowatt-hours (kWh) consumed, often in a time-of-use (TOU) structure where energy consumed during on-peak hours (e.g., 2–9 PM on weekdays) costs 2–4× more per kWh than energy consumed during off-peak hours.

Demand Charges — charges based on the highest 15-minute average power demand recorded during the billing period, measured in kilowatts (kW) and multiplied by a $/kW rate. In many US commercial rate tariffs, demand charges account for 30–50% of the total electric bill, yet they are driven by just 15 minutes of peak usage each month.

How Demand Charges Work

A simplified example: a small manufacturer typically operates at 300 kW, but for one 15-minute window each month, an electric furnace and HVAC compressor start simultaneously, pushing demand to 600 kW. At a demand charge rate of $15/kW/month, the bill includes a charge of $9,000/month based solely on that one peak event — regardless of whether the 600 kW level is maintained for 15 minutes or the entire month.

This structure incentivizes "peak shaving" — reducing the highest demand peak to lower the demand charge. A reduction from 600 kW to 500 kW saves $1,500/month in this example.

Ratchet Clauses

Some utility tariffs include ratchet clauses that make demand management even more critical. A ratchet clause sets a minimum demand charge based on the highest demand in the past 11 or 12 months. If your facility hit 1,000 kW last August, you may pay a minimum demand charge based on 80% of 1,000 kW (= 800 kW) for the entire following year — even if your current demand is much lower. Ratchets are common in industrial tariffs and dramatically increase the ROI of peak shaving systems.

Time-of-Use (TOU) Rate Optimization

TOU rates create an opportunity for "energy arbitrage" — charging batteries during low-cost off-peak hours and discharging during high-cost on-peak hours. If off-peak energy costs $0.05/kWh and on-peak energy costs $0.20/kWh, a BESS with 90% round-trip efficiency earns a spread of ($0.20 − $0.05/0.90) = $0.144/kWh for every kWh shifted. For a 500 kWh battery cycling daily on a 250-day on-peak year, annual savings = $0.144 × 500 × 250 = $18,000/year.

BESS Sizing for Demand Charge Reduction

BESS for demand charge reduction must be sized to both supply sufficient power (kW) to shave the peak and store sufficient energy (kWh) for the duration of the peak event. Steps to size a BESS:

1. Analyze 15-minute interval data to identify the timing and duration of peak demand events
2. Determine the target peak limit (e.g., reduce from 600 kW to 450 kW → need 150 kW of BESS power)
3. Determine how long the peak must be sustained (e.g., 2 hours → need 150 kW × 2h = 300 kWh)
4. Add margin for battery efficiency and allowable DoD: 300 kWh / 0.90 DoD = 333 kWh nameplate
5. Calculate annual demand charge savings: target reduction × demand charge rate × 12 months

Combined Solar + BESS Economics

Solar PV and BESS are highly complementary. Solar reduces daytime energy charges; BESS shaves demand peaks and handles TOU optimization. The federal ITC applies to both solar and co-located storage (storage charged predominantly from solar qualifies for the full 30%+ ITC). For commercial buildings in high demand charge territories (California, Hawaii, Northeast states), combined solar + BESS systems commonly achieve paybacks of 5–8 years — often 3–5 years with available incentives.