One source of confusion that we see among newcomers to the energy aspect of sustainability is the difference between watts and watt-hours. This confusion can wreak havoc when looking for opportunities to increase efficiency.

A watt is a unit of power. It gives a notion of how strong something is. For example – a 150 watt bulb is stronger (brighter) than a 60 watt bulb. Energy, on the other hand, is the production of a certain amount of power for a certain amount of time. So, while power may be measured in watts (W), energy is measured in watt-hours (Wh). A watt-hour is equivalent to producing a watt of power for one hour.

Consider our two bulbs; although the bulbs have power ratings (60W and 150W), they draw no energy just sitting there. But – for each hour that the 60W bulb remains switched on, it consumes 60 Wh of energy. Similarly, for each hour that the 150W bulb remains switched on, it draws 150 Wh of energy.

Imagine lighting the 60W bulb for five hours but the 150W bulb for one hour. This would require 300 Wh (5 hours x 60 watts) of energy for the 60W bulb and only 150 Wh (1 hour x 150 watts) of energy for the 150W bulb. Although the 150W bulb is more powerful than the 60W bulb, our lighting schedule requires more energy to light the 60W bulb than it does to light the 150W bulb.

One place we see this confusion really misleading people is in the use of power meters to compare the cost and environmental impact of devices. Consider for example, an industrial oven rated at 100 kilowatt (a kilowatt or kW is 1000 watts). Compare to a lighting system rated at 5kW. Naively using a power meter to measure each device’s power might tell the story that the oven is a bigger offender than the lighting system and that efforts should be focused on finding a more efficient oven.

But – what if the oven is only used one hour per week? Then it consumes 100 kilowatt-hours (kWh) per week (1 hour x 100 kW). If the lighting system operates for 10 hours per day, it consumes 350 kWh (70 hours x 5 kW) per week!  The oven may be higher power, but the lighting system is consuming more energy and it’s energy that we pay for and that has environmental impact. In conclusion – we’d really like an energy meter to compare devices rather than a power meter.

Time ties power and energy together. If we can estimate the hours that a device is actually drawing power, we can use a power meter and then multiply measured power by estimated hours of operation to estimate energy. In other cases it may be difficult to estimate how many hours a device is actually drawing power. In such cases, we might need an energy meter (which is basically a power meter configured to integrate measured power over time, resulting in a measure of energy).