UPSs: Converting VA or Watts to Joules?

[Sir Fredrick]

It never occured to me that you could determine the amount of protection that a UPS's surge protection affords based on its VA or watt rating, but that didn't stop the salesperson at Staples from doing it. In fact, he said he could do it in his head.

So what's the secret?

 

[Viper GTS]

Watts and joules are not equivalent terms, a watt is defined as 1 joule/second.

Viper GTS

 

[Jerboy]

<< It never occured to me that you could determine the amount of protection that a UPS's surge protection affords based on its VA or watt rating, but that didn't stop the salesperson at Staples from doing it. In fact, he said he could do it in his head.

So what's the secret? >>




Ask him how much he makes . If he doesn't tell you, I bet it's state minimum wage.


VA or watt has ABSOLUTELY NOTHING to do with UPS's surge protection capability.


VA=Volts times Amp

W=energy consumption, volts times amps times power factor


Comparing surge absorbing capability with output is like comparing a car's horsepower and crumple zone impact absorption capability.


As ViperGTS said, there is a relationship between watts and joules, but in this case we are talking about two different functions.

W denotes number of joules it can provide per second.

J is each joule.

Assume surge protection can take millions of watts for 0.00000001sec or so. Joule rating of surge protection is how many joules it can absorb before the baloon bursts and loses protection capability.

 

[Sir Fredrick]

Well gee, then how did he calculate that a 500VA, 300W power supply could offer 20,000 joules of protection?
Maybe you guys just don't know the secret conversion algorithm!

 

[Jerboy]

<< Well gee, then how did he calculate that a 500VA, 300W power supply could offer 20,000 joules of protection?
Maybe you guys just don't know the secret conversion algorithm! >>



Actually I know the algorithm.

First take the number from the box. Multiply it by the number he pulls out of his ass and voila!

 

[Sir Fredrick]

<<

<< Well gee, then how did he calculate that a 500VA, 300W power supply could offer 20,000 joules of protection?
Maybe you guys just don't know the secret conversion algorithm! >>



Actually I know the algorithm.

First take the number from the box. Multiply it by the number he pulls out of his ass and voila! >>



I never knew it could be so simple! :Q

 

[ElFenix]

joules are going to be an instantaneous unit of energy, and are the surge protection, not the battery power, of the UPS.


and isn't power I^2R, or VI (or V^2/R for that matter) and isn't VI volts times amps, so that would be 500 VA? maybe this doesn't work for batteries. conversion factor would be 1 VA = 3/5W...

a far more useful figure would be Ah, amp hours, so that would could see how long the battery could last... say you're gaming on your 19" and you're actually sucking down 300 watts, with 120 volts thats about 2.5 amps, if you're got a 1 Ah battery thats like 22 minutes... 1 Ah isn't all that much... i could make a 3Ah battery out of 100 NiMH cells pretty easily, that'd get me up to 120 volts if they're in series... probably fry the cells with that much voltage...

 

[Jerboy]

<< and isn't power I^2R, or VI (or V^2/R for that matter) and isn't VI volts times amps, so that would be 500 VA? maybe this doesn't work for batteries. >>



It works beatifully for batteries. Your equations are correct for DC circuits and pure resistive AC circuits.



<< conversion factor would be 1 VA = 3/5W >>



Might not hold true for other equipments. Conversation factor is VA * power factor



<<
a far more useful figure would be Ah, amp hours, so that would could see how long the battery could last... >>



It only tells you about the battery. 1hr rated Ah means how many amps can battery sustain for one hour. 30hr rated Ah means how many amps the battery can sustain for 30hrs at 1/30 the said current. The greater the discharge current, the lesser battery can output as internal loss is increased.



<<
say you're gaming on your 19" and you're actually sucking down 300 watts, with 120 volts thats about 2.5 amps, if you're got a 1 Ah battery thats like 22 minutes... 1 Ah isn't all that much... >>



Completely wrong. Most home UPS uses 12V battery cell and is stepped up to 120V with an inverter. You have a UPS powered by 12V 5Ah cell. The efficiency of UPS inverter is 80%. When you're drawing 300W from the output, you're drawing 375W from the battery(300/0.8=375).
375/12=31.25A drawn from battery.

5Ah/31.25A=0.16hrs=9min 36seconds




<<
i could make a 3Ah battery out of 100 NiMH cells pretty easily, that'd get me up to 120 volts if they're in series... probably fry the cells with that much voltage... >>



You'll have a 120V of PURE DC.


While the computer power supply will probably work at 170V DC, the power on/off is controlled by a relay in the PSU. The relay is not designed to switch off direct current. When you turn off the switch, there will be an arc over at the relay contact and it will melt itself down in a short order. It is MUCH MUCH more difficult to shut down a flow of pure DC current such as that from electrochemical cells than AC or pulsating DC.

120V DC will work perfectly with lightbulbs and heaters.

Not sufficient voltage for switching power supply.

It will instantly fry anything with line frequency transformers in it.