On a commercial electric bill the demand charge — billed on your single highest 15-minute peak in kilowatts — often rivals the energy charge for total kilowatt- hours. This calculator turns your peak demand and utility rate into a dollar figure and shows how much you save by shaving that peak.
What is a demand charge and why does it exist?
Utilities must build enough generation and grid capacity to serve every customer’s peak load simultaneously, even if that peak only occurs for 15 minutes a month. The demand charge is how utilities recover those infrastructure costs. Rather than spreading the cost into the per-kWh energy rate, they charge directly for the maximum power draw — measured in kilowatts — because that is what drives capacity investment.
For a commercial facility this means a single spike — a bank of air conditioning units starting at once, a large chiller turning on, or a production line beginning its morning cycle — can set the billed demand for the entire billing period, regardless of how efficiently the facility uses energy the rest of the month. Demand charges can represent 30–70% of a commercial electricity bill, making them a major cost management target.
How the calculation works
The demand charge is a simple product, and savings scale exactly with the peak reduction:
demand charge = peak demand (kW) × demand rate ($/kW-month)
shaved peak = peak demand − reduction (kW)
new charge = shaved peak × demand rate
monthly savings = reduction × demand rate
annual savings = monthly savings × 12Because only the highest 15-minute average matters, the bill does not care whether that peak happened once or repeatedly. Reducing it by even a small amount saves that amount times the rate, every month of the year.
Worked example
A retail warehouse peaks at 250 kW during afternoon refrigeration cycling. The utility’s demand rate is $16/kW-month:
- Current demand charge: 250 × $16 = $4,000/month
- A battery providing 40 kW of peak shaving: new peak = 210 kW
- New charge: 210 × $16 = $3,360/month
- Monthly saving: $640
- Annual saving: $7,680
At a battery installed cost, that saving provides a clear payback calculation. Add any energy arbitrage savings (charging at low-rate overnight, discharging at high-rate peak) and the economics improve further.
Peak-shaving strategies
Battery energy storage (BESS). A battery charges during low-demand periods and discharges during the peak interval to reduce the kW the utility measures. The battery does not reduce total energy consumed — it shifts when power is drawn from the grid.
Load shifting. Move flexible loads — EV charging, water heating, pre-cooling a building — out of the peak window. This reduces peak demand without adding any equipment cost.
Staggered startup sequences. Large motors, chillers, and compressors draw high current at startup. Sequencing them so they do not all start simultaneously is often the cheapest first step.
Demand response programs. Some utilities offer incentive payments for agreeing to curtail load during grid stress events. The demand reduction that earns payments also protects against high-demand billing periods.
Reading your utility bill
To use this calculator accurately, find the “demand” or “peak demand” line on your utility bill — it shows the highest 15-minute average kW recorded during the billing period. This is distinct from total kWh consumed. The demand rate ($/kW-month) is on your rate schedule, which may be available from your utility’s website or your account portal. Demand charges and rates vary widely by utility, tariff class, and time-of-use provisions; confirm your specific rate before making investment decisions.
All figures are calculated locally in your browser.