How Demand Charges Actually Work

Ask most commercial energy buyers what they pay for electricity, and they'll give you a price per kilowatt-hour. But for most commercial and industrial accounts, kWh consumption is only half the story — sometimes less than half. The other piece is the demand charge, and it's the line item that consistently catches buyers off guard when they see their bill for the first time.

The Difference Between Energy and Power

To understand demand charges, you need to understand the difference between energy and power.

• Energy (kWh) is the total amount of electricity consumed over time. Think of it like gallons of water — you pay for what you used.
• Power (kW) is the rate at which you consume electricity at any given moment. Think of it like the diameter of the pipe — it determines how much can flow at once.

Your utility builds the grid to accommodate the maximum power draw of all its customers at the same time. That infrastructure — the transformers, the cables, the substations — has to be sized for your peak, not your average. Demand charges recover the cost of that infrastructure.

How Demand Charges Are Calculated

Your utility measures your electricity consumption in 15-minute or 30-minute intervals throughout the month. Your demand charge is based on the single highest-interval reading — the peak 15 (or 30) minutes of the entire billing period.

That last point is critical: a single 15-minute period of high demand can cost you thousands of dollars — charged for the entire month. It doesn't matter if you had that peak for 15 minutes at 3pm on a Tuesday and ran at half that level for the other 99.9% of the billing period.

What Causes Peak Demand Spikes

The most common culprits are loads that start up simultaneously — particularly large motor-driven equipment.

• HVAC systems: Chiller startups, compressor staging, and morning warm-up cycles are among the most common drivers of demand peaks in commercial buildings.
• Industrial equipment: CNC machines, compressors, welding equipment, pumps, and conveyor systems all draw high current on startup (called "inrush current").
• EV charging: Level 2 and DC fast chargers are increasingly large contributors to peak demand, particularly in fleet charging scenarios.
• Kitchen equipment: In food service facilities, opening-time or high-service-period simultaneous equipment startup is a frequent demand driver.
• Lighting systems: Older fluorescent and HID lighting systems have high startup demand. LEDs have essentially eliminated this issue for facilities that have converted.

On-Peak vs. Off-Peak Demand

Many utility tariffs for commercial customers distinguish between on-peak demand and off-peak demand, charging significantly more for the former. "On-peak" hours typically cover weekday afternoons, often 2pm–8pm in summer markets and 7am–10am in winter markets. Exact definitions vary by utility and tariff.

If you're on a time-of-use tariff, your peak demand charge is driven only by your highest demand reading during on-peak hours. This creates an opportunity: if your operations can be scheduled to avoid high demand during on-peak windows, you can significantly reduce your demand charge bill — even if your total energy consumption stays constant.

The Ratchet Clause: When One Bad Month Follows You

Some utility tariffs include what's called a "ratchet clause" — a provision that bills you based on the greater of (a) your current month's actual peak demand or (b) some percentage (often 80–100%) of your highest peak in the prior 11 months.

This means a single summer heat wave that drives a demand spike can affect your bills for an entire year. Ratchets are most common in industrial rate classes and in markets where the utility has high fixed infrastructure costs to recover.

Coincident Peak (CP) Charges: The Multi-Year Version

In PJM (the large wholesale electricity market covering the Mid-Atlantic and Midwest), commercial and industrial buyers face an additional demand-related charge based on their load during specific regional peak hours. These are called "coincident peak" or "capacity peak" charges.

PJM identifies its 5 highest-demand hours of each summer (typically hot August afternoons). Your facility's demand during those hours determines your "capacity tag" — a per-kW charge that applies throughout the following year's billing. Missing one of those 5 hours at a high-demand level can cost tens of thousands of dollars over 12 months.

Sophisticated buyers in PJM markets often subscribe to peak alert services that notify them when regional peak hours are likely to occur, allowing them to curtail load and reduce their capacity tag for the following year.

Strategies for Reducing Demand Charges

1. Load staggering

Introduce time delays between the startup of major loads. Starting your HVAC system 15 minutes before your production equipment means those peak draws don't overlap. This costs nothing and often reduces peak demand by 10–20%.

2. Demand response programs

Many utilities and grid operators offer demand response programs that pay commercial customers to curtail load during peak grid periods. The payment effectively offsets your demand charges and can provide additional revenue if your load is flexible enough to participate.

3. Battery energy storage

Behind-the-meter battery storage can discharge during peak demand windows to suppress your metered peak draw. The economics work best in high-demand-charge markets (California, Hawaii, parts of the Northeast) where demand rates are $15–$40/kW or higher. Payback periods are typically 5–8 years depending on incentives.

4. On-site generation

Solar PV, combined heat and power (CHP), and backup generators can all be dispatched or optimized to reduce peak demand. The value of on-site generation is often understated when only the kWh savings are counted — the demand charge reduction can be equally significant.

5. Time-of-use optimization

For facilities on time-of-use tariffs, shifting large loads to off-peak hours (nights and weekends) can both reduce demand charges and lower your energy rate. This works particularly well for battery charging, water heating, pumping, and other loads that aren't time-sensitive.