Whenever I hear someone on the jobsite using the wrong units (they say pounds of air pressure when they mean psi or they say BTUs when they mean BTUs/hour or BTUs/sec), I think two things. One, they don’t really understand what they are talking about. Two, they will be making some mistakes due to that misunderstanding that could be easily avoided. Pet Peeve alert! Learn and use the correct units. It will make you look smarter and save you some aggravation.

So how does energy differ from power? Energy is the ability to do work. 1 BTU heats 1 pound of water 1 degree Fahrenheit. It takes energy to drive your car, heat your house, or charge your battery. Another example: 1 KiloWatt-hour of energy lights a 100 Watt bulb for 10 hours.

I think of power, on the other hand, as capacity. Power is the rate at which energy is used. BTUs/hour, Horsepower and KiloWatts are are units of power. That light bulb has the capacity to use 100 Watts, it’s level of power is 100 Watts. The Formula One race car has the capacity of delivering 1000 HP, while a Ford Pinto powers along at 88 HP. The Formula One race car can use energy faster than the Ford Pinto (unless the Pinto is exploding from a rear collision, then that’s a different calculation).

So let’s do a quick calculation to compare the gasoline usage of the Formula One vehicle and the Pinto. We can easily find that gasoline has an energy value of 125,000 BTUs.

So the energy is in the gasoline (and yes, I know a Formula One doesn’t run on gasoline), but the power is the capacity either vehicle has to use that energy over time. When trying to remember the units, the little graphic above about power (having a W) and watts is helpful.

Since I’m working on some solar energy projects, let’s try an example from that field. The sun’s energy on a given surface is called the irradiance. So that’s the energy portion. The power portion is radiation. The radiation is the irradiance measured over time (just like power is energy measured over time). Solar radiation data is measured in KiloWatt-hours per square meter per day. From solar radiation charts, we find a horizontal plane in Harrisburg PA in March gets an average of 3.3 KiloWatt-hours per square meter per day.

To recap, the most common energy units are BTUs, KiloWatt-hours while the power units are BTUs/hour, Horsepower and KiloWatts. The rest of the conversion units are also available on this site.

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