Power and Energy Primer: Introduction
How many homes can the Hoover Dam supply with electricity?
We hear this type of question often in the context of our electricity system and the potential for new energy technologies, for example, the debate over offshore wind farms in the Great Lakes. The trick isn’t about how much of the electricity gets to the home, although that is an issue. It’s a question of how much electricity a home “needs.” The answer lies in the difference between power and energy.
There are different ways of looking at how much electricity a home consumes. The first is what we are most familiar with: the amount shown on our electricity bill. A utility charges residential customers based on the ENERGY they consume, which the utility measures in units of kilowatt-hours (written as kWh). A kilowatt-hour costs on the order of 8¢ in Minnesota, where the average home uses just over 800 per month.
If electricity consumption was a road trip, ENERGY would be a measure of how many miles the car drove in a month.
A second approach is to ask how much electricity a home needs at any given second of the day. When all the lights are on, the TV is blaring, the refrigerator and laundry are running, and electronic devices are charging - how much electricity is needed at that instant? What about in the middle of the night? This is a measure of POWER.
On your road trip, POWER is analogous to the speed of the car: how many miles per hour you are driving.
Just like on the road trip, all you need to turn POWER into ENERGY is a measure of time. How long did you leave all the lights on and the TV blaring? Thats what will show up on the bill.
So how many homes can Hoover Dam supply? Well, it depends if we are asking how much POWER or ENERGY the home needs. To be fair, we should look at both.
At any given instant in time, Hoover Dam can POWER about 800,000 homes. Hoover Dam has a capacity of 2 gigawatts of POWER, and this assumes that homes on average require 2.5 kilowatts of power during peak periods. Conversely, in one year, Hoover Dam produces enough ENERGY to supply 350,000 homes. Those are two different numbers to consider, and there are many factors at work (for starters, the Hoover Dam never runs at full capacity).
Why does this matter? Well, a number of different decisions about how to compare future electricity alternatives depend on this distinction, and quoting apples-to-apples numbers is important. For power plants, this explains the difference between quoting capacity (a measure of power) and generation (a measure of energy). These two snapshots of Minnesota’s electricity system are both true, but tell a different story: