Is my calculation right?

[Agera One]

CPUID HW Monitor says my laptop battery has 42180 mWh capacity. So, my laptop consumes (or the battery delivers) 42.18 W per hour right?

 

[Someone Somewhere]

No. A watt is a rate of energy usage (joules per second); watts per hour would be a nonsense measurement.

It means that your battery can supply 42.18W for a period of one hour before running out. Or 21.09W for two hours. Or 84.36W for 1/2 hour.

 

[drapacioli]

Watt-hours is the amount of energy the battery can supply for one hour. If memory serves it's not a limit on how much can be pulled at once, at least not strictly, though you'll rarely see your laptop pull that much power in an hour.

 

[Agera One]

It means that your battery can supply 42.18W for a period of one hour before running out.

I think I said the same. The battery delivers 42.18 W per hour (it will be drained to 0% when I use this much power from it for an hour)

 

[Someone Somewhere]

No. Do not use the word 'per' in that case. Per means inverse.

It is watt-hours, not watts per hour.

Watts per hour would mean that your laptop used more power the longer it was left on. Generally the power usage is constant, not continually increasing.

 

[Agera One]

Oh, I used wrong terms, right? my laptop battery (old model battery) is rated for a time of 2-hours of average usage. So, 42.18 watt-hour for 2 hours means 84.36 Watts of energy in 2 hours?

 

[cozmium]

No. A watt is a rate of energy usage (joules per second); watts per hour would be a nonsense measurement.

It means that your battery can supply 42.18W for a period of one hour before running out. Or 21.09W for two hours. Or 84.36W for 1/2 hour.

This is a common misconception. You cannot multiple/divide Wh to give a different time period, especially with batteries which cannot discharge at a constant rate.

 

[Someone Somewhere]

ARGH. A watt is a measure of POWER, not ENERGY. Energy is measured in JOULES (or watt-hours, 1 W·h = 3600J).

Power is defined as a rate of energy consumption, that is energy consumed each second. One watt is one joule each second.

42.18 watt-hours used across two hours would be a continuous draw of 21.09 watts.

 

[cozmium]

You're re-phrasing what you were suggesting. A battery which can supply 42.18W for one hour (as that is what it is rated at) cannot necessarily supply 84.36W for 30 minutes; it may only last 25 minutes, it may last less. I'm not arguing that 41.18 watt-hours used across two hours is 21.09 watts, i'm saying that you cannot divide/multiply what a battery can do over a given time period because they don't discharge perfectly linearly.

 

[Someone Somewhere]

Yeah, that's all theoretical. Typically batteries (at least deep-cycle SLAs) are rated for a 12-hour discharge. The faster you discharge them, the higher the losses.