Calculating Batteries to Power a PC - A Curious Question

[Dougieman1001]

So 8 AA batteries is 1 Watt, I was just wondering how many batteries it would taje to power my PC for an hour, so if I have a 750W PSU you would do 8x750 to work out the amount of batteries, but is this per minute, second or hour?

This is just interest, not actually going to do it!

 

[BvU]

Hello Dougie, welcome to PF !

I would have guessed one AA battery would already be capable of delivering one Watt, but never mind.

The calculations go as follows: Delivering one Ampere of current at a voltage of 1 Volt over the load is one Watt of power.
One Watt of power is one Joule of energy per second. So for Watts given you never have to wonder whether that is per hour or per fortnight: it is one Joule per second.

To answer you rhetorical question, you would need to know for how long your battery can hold up this one Watt. Usually the makers don't tell you on the package. For rechargeable batteries you often find the capacity expressed in mAh -- milliAmps times hours. I have some that say 2500 mAh, meaning that they can deliver 1 Ampere for a period of 2.5 hours (when fully charged and in good condition, at reasonable temperature, etcetera etcetera. I wouldn't bet on it too heavily...). Or one half amp for 5 hours. 2.5 Amp for one hour is asking an awful lot (they heat up internally, a waste of ... yes, energy).

Suppose an AA battery can deliver 2400 mAh at 1.5 Volt. With 80 AA batteries you would get 120 Volt. 750 Watt at 120 Volt means about 6.25 Ampere. So with six sets in parallel of these sets of 80 AA batteries, each set would have to cough up 1.04 Ampere. Still with me ? Each and every one of these 480 batteries can claim (in theory) to be able to keep up this 1.04 Ampere for a time of around 2.4 Ampere hour / 1.04 Ampere = 2 hours and 20 minutes.

Pretty expensive experiment ! In reality you'll be a lot better off, because on average your PC doesn't gobble up these 750 Watts at all. It is boasting 750 W peak but it would be a room heater if it did that continuously.

How about that for a verbose answer to a simple rhetorical question !

--

 

[Dougieman1001]

Hello Dougie, welcome to PF !

I would have guessed one AA battery would already be capable of delivering one Watt, but never mind.

The calculations go as follows: Delivering one Ampere of current at a voltage of 1 Volt over the load is one Watt of power.
One Watt of power is one Joule of energy per second. So for Watts given you never have to wonder whether that is per hour or per fortnight: it is one Joule per second.

To answer you rhetorical question, you would need to know for how long your battery can hold up this one Watt. Usually the makers don't tell you on the package. For rechargeable batteries you often find the capacity expressed in mAh -- milliAmps times hours. I have some that say 2500 mAh, meaning that they can deliver 1 Ampere for a period of 2.5 hours (when fully charged and in good condition, at reasonable temperature, etcetera etcetera. I wouldn't bet on it too heavily...). Or one half amp for 5 hours. 2.5 Amp for one hour is asking an awful lot (they heat up internally, a waste of ... yes, energy).

Suppose an AA battery can deliver 2400 mAh at 1.5 Volt. With 80 AA batteries you would get 120 Volt. 750 Watt at 120 Volt means about 6.25 Ampere. So with six sets in parallel of these sets of 80 AA batteries, each set would have to cough up 1.04 Ampere. Still with me ? Each and every one of these 480 batteries can claim (in theory) to be able to keep up this 1.04 Ampere for a time of around 2.4 Ampere hour / 1.04 Ampere = 2 hours and 20 minutes.

Pretty expensive experiment ! In reality you'll be a lot better off, because on average your PC doesn't gobble up these 750 Watts at all. It is boasting 750 W peak but it would be a room heater if it did that continuously.

How about that for a verbose answer to a simple rhetorical question !

--

Well, I wasn't expecting that response! Still taking it in, but thanks! :D

 

[BvU]

Thanks for the answer :D

1 AA Battery is 1.5v and 1 watt is 12v Right?

AA batteries are 1.5 Volt, yes.

1 Watt is not 12 V. $\quad$ 1 Watt is 1 Volt times 1 Ampere.

So one AA battery powering a 100 mA LED would be 0.15 Watt (1.5 V x 0.1 A)