Energy Units, Power and Demand

In this course you will learn how to manage energy as a system, using the structure of ISO 50001. Everything in that system depends on numbers: consumption, cost, indicators and savings. If the units are confused, every later decision is built on a mistake. Energy managers regularly receive data in kilowatt-hours, megajoules, therms, litres of fuel and cubic metres of gas, sometimes in the same report. The first skill is to put them on one common basis.

Energy versus power

Energy (kWh) is the total amount used over a period. Power or demand (kW) is the rate of use at a moment. A 10 kW heater running for 3 hours uses 30 kWh. A 30 kW heater running for 1 hour also uses 30 kWh, but it places three times the demand on the supply.

This difference matters because many electricity contracts charge for both. The energy charge pays for the kWh. A maximum demand or capacity charge pays for the highest kW (or kVA) reached in the month, often measured over a half-hour or 15-minute period.

Worked example. A plant starts all five of its 75 kW chillers together at 07:00 each day. The demand peak sets the monthly capacity charge. By starting the chillers at 15-minute intervals, the plant uses almost the same kWh but cuts the peak, and the charge falls. No energy is saved, but cost is.

Common mistakes

• Adding kWh of electricity to kWh of gas and calling it "site energy" without noting that the two have very different costs and carbon factors.
• Comparing a demand figure (kW) with a consumption figure (kWh) as if they were the same thing.
• Mixing gross and net calorific values in one report, which creates an error of about 10% for natural gas.