
Let’s finish 2013 this way: With a vow to spread the word about the difference between watts and watthours, between power and energy.
“Each bike is expected to generate an average of 75 watts per hour,” said the press release put out last week to publicize a cute little program in Manhattan, in which six stationary Citi Bikes were set up for passersby to jump on and pedal a bit to help offset the energy that will be used lighting the Times Square New Year’s Eve Ball.
Watts per hour? Argh!
A watt is a measure of power – and power by definition is energy flow per unit of time. (Technically, a watt is one joule per second, but you don’t need to know that.) Since a watt is already a rate, it makes no sense to say “watts per hour.” Saying the bikes will “generate an average of 75 watts per hour” is like saying you’ll drive “an average of 65 miles per hour per hour.” And yet it is done all the time. I’ve even seen companies in the energy business do it.
So just to be clear: When you step on a stationary bike and pedal, at any moment your power output can be measured in watts. The rate at which you produce power (watts) and the length of time you produce it will determine how much energy (watthours) your activity adds up to.
It’s just like a light bulb that’s been turned on. A 100watt light bulb uses 100 watts of power every moment that it burns. That is the rate at which it consumes energy.
Now, if that 100watt light bulb burns for an hour, that adds up to a certain amount of energy – 100 watthours. And if that 100watt bulb burns for 10 hours, that adds up to 1,000 watthours of energy – also known as a kilowatthour. This is the basic measure of energy usage that your electricity provider details on your bill each month.
As I said, this distinction between power and energy is lost on many people – countless news outlets repeated the “75 watts per hour” Citi line – but it’s actually very important. For instance, if you want to put solar power on your roof, the size of the system will be described in terms of kilowatts – this is the measure of how much power the system could produce at any moment in time. You’ll want to know this. But you’ll also want to know how much energy it can be expected to produce – how much the power production, over the course of a month or a year, will add up to.