In electrical applications we measure power in watts (W), and energy in watt-hours (Wh), both are derived SI units. It's sometimes hard to wrap your head around the meaning of these units and the correlation between them, and I especially find the word 'hour' in watt-hour very confusing.
A quick overview.
Power is the rate at which energy is generated or consumed, and as such is measured in 'energy per time unit'. The unit for energy is joule (J), which means we can express power in joules/second (J/s).
So what's the relation to this watt unit then?
1 W is defined as 1 J/s
Ok, that's easy. How about watt-hours...
Imagine you have a light-bulb with a power of 50 watt and leave it burning for 1 hour, that light-bulb consumed 50 watt-hours.
And we can express this in joules as well:
1 Wh = 1 W · 1 hour = 1 J/s · 3600 s = 3600 J
An important thing to realize is that watt-hour is a time-independent unit; it expresses an amount of energy. Note the similarity with 'lightyear', which is a distance and has nothing to do with time either.
It gets really confusing when you say device X used 50 watt-hours in 5 minutes, but it's a perfectly valid statement (and you would by now be able to deduct that device X's power consumption must be 600 watt, right?).
It gets really confusing when you say device X used 50 watt-hours in 5 minutes, but it's a perfectly valid statement (and you would by now be able to deduct that device X's power consumption must be 600 watt, right?).
In my head I often use this analogy that watt is like the download speed in bytes/second, and watt-hour is like the amount of bytes you have transferred after a certain amount of time. Perhaps a bit odd comparison and I'm stuck forever with this mental image of the microwave downloading energy.
When this is all clear we're ready to go crazy at power and energy statistics.
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