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
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 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.
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.
Energy is Power multiplied by Time.
Power just describes how fast energy is beeing used – the rate of energy usage.
Unit of power is 1W that is the same as 1 joule per second (J/s).
Unit of energy is W*s, same as 1 Joule.
In electricity consumption we use Wh because of convince, so that numbers are smaller.
1 Watt-hour = 3600 Watt-seconds = 3600 Joules.
Because there’s 3600 seconds in a hour.
If you have a device that uses constantly 100W of power, then after one hour it will consume 100Wh, after 2 hours 200Wh, after 24h – 100W*24h = 2400Wh.
Regarding power supplies – that is the maximum power they can draw from wall socket and pass on to whatever device it supplies. Connected device will use the amount of power it needs, no more. If device at this moment needs just 100W, it will use just 100W through power supply, regardless of rated maximum power. But if device will need 800W at some moment, that will exceed what power supply can deliver and it will shut off it’s output to protect itself.
Watts is a unit of energy flow. Joules is just a unit of energy. Watt-hours is also a unit of energy, just in a different unit that makes sense for electrical applications that use Watts (like Celsius vs Fahrenheit are both temperature units).
So for your 700W power supply, it is rated to supply 700W continuously to its downstream user. At max, consuming 700W continuously. If you run it for an hour, it will have consumed 700Wh.
But usually, you select your power supply with some error margin, so say your downstream user of the PSU only consumes 350W. The PSU will only consume 350W from the wall, even though it could handle more. If it ran like that for 4 hours, it would have consumed 1400Wh (350W x 4h). So if you had a 1000Wh backup battery, it could run that PSU for a little under 3 hours (1000Wh / 350 W = 2.86 h)
The supply rate is how much energy per second the device can draw from the grid. That being 700 joules per second(watts).
If you were to leave that at 700watts for 1 hour. You would have used 700 watthours, or 0.7 kwh.
Alternatively, you would have used 700joules/second x 3600 seconds = 2.56 million joules.
A kilowatthour is equal to 1000 joules/second*3600 seconds = 3.6 million joules.
It is a bit confusing that we used “kilowatthours” as a unit of energy, since watt is power(joules/second), but when you times it with a time you get just joules.
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