# power and energy

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Hello,
There is something I don’t get about power and energy.
If I got it right, power is expressed in Watts or Joules ans energy is in Watts.hours.
So if I buy à power supply rated at let’s say 700W. Does that mean (assuming, for the sake of the argument 100% efficiency and 100% load) it also consumes 700Wh ? And also that 700W is it’s maximum power output ?

In: 2 If it’s rated for both 700W and 700Wh it will discharge 700W for one hour before running out. It could also discharge 350W for two hours, or 175W for four hours, etc. Just one small correction, Joules is a measurement for energy not power. Power is just the amount of energy used over time and 1 Watt is equal to one Joule per second If you think of the power supply like a pump, and say it can deliver 1 gallon of water a minute – assuming the hose connected to it is wide enough – then after an hour it will have pumped 60 gallons of water right.

Well your PSU can pump 700w of power, and if it’s connected to a wide enough pipe it can have delivered 700wh in an hour >power is expressed in Watts or Joules ans energy is in Watts.hours.

Watts is power, that’s correct.

Energy is Joules.

1 Joule = 1 Watt – second.

Energy can also be expressed in Watt-hours as you mentioned, or as kilojoules.

>So if I buy a power supply rated at let’s say 700W. Does that mean (assuming, for the sake of the argument 100% efficiency and 100% load) it also consumes 700Wh ? And also that 700W is it’s maximum power output ?

The supply doesn’t consume the 700Wh, the 100% load does. 700W is the maximum power output (that’s correct).

HTH 1 watt = 1 joule per second. This is a measure of power, it’s how fast you can spend energy.

1 watt hour = 1 watt * 1 hour = 3600 joules, This is a measure of energy. Your electric meter likely reads in kilowatt hours. Power supplies generally don’t list this unless they’re integrated with batteries.

A power supply is generally rated for power. Or more specifically, it will be rated for a current and a voltage it can provide to its load.

Batteries are a little more complicated, their power ratings are often given in relation to their capacity(C rating), rather than absolute current draw.