I?m no expert so maybe Cardinal2k can help out here but here?s what I understand:
To covert from VA to Watts in an AC system (like a PC), you need to calculate the power factor (pf).
Power Factor = Watts/VA, We want to calculate the VA so
VA=Watts/pf
PC power supplies generally have a power factor of .6 to .7
However, an incandescent light bulb has a power factor very close to 1, thus we get the following calculations
250 Watt PC Power supply example:
PC Power Supply (VA) = 250/.7 = 357VA
Generally you would like to have 20 to 30% margin so for a 250 Watt PC power supply you would want a UPS that provided at least 357VA/.7 = 510VA
40 Watt Incandescent light bulb example:
OK this seems like a duh but,
Light bulb VA = 40W/1 pf = 40VA
Using the typical run time table at Sutton Designs website
http://www.suttondesigns.com/net700cht.htm
You can see that a 75VA load will run for approximately 113 minutes on a 700VA UPS.
MJ99,
If you plot the data you will recognize the exponential relationship between run-time versus load. Now I know this is not a mathematical calculation but at least you get the idea.
It may appear that a 40 watt light bulb on a 700VA UPS would run significantly longer then 113 minutes (maybe 3 to 3.5 hours). I don?t think this is true since there are operating losses of the UPS itself. I suspect Cardinal2k is right, 2 to 2½ hours is close to the most you would probably get.
But why speculate, buy one and do an experiment. Nothing like real life testing to find out. In fact I have a 700VA UPS at home, maybe I'll play with it this weekend.
Hope that helps.
Please note I am not an electrical designer and haven?t done real electrical engineering work since I was in school in the early 80?s.
Pigseye
To covert from VA to Watts in an AC system (like a PC), you need to calculate the power factor (pf).
Power Factor = Watts/VA, We want to calculate the VA so
VA=Watts/pf
PC power supplies generally have a power factor of .6 to .7
However, an incandescent light bulb has a power factor very close to 1, thus we get the following calculations
250 Watt PC Power supply example:
PC Power Supply (VA) = 250/.7 = 357VA
Generally you would like to have 20 to 30% margin so for a 250 Watt PC power supply you would want a UPS that provided at least 357VA/.7 = 510VA
40 Watt Incandescent light bulb example:
OK this seems like a duh but,
Light bulb VA = 40W/1 pf = 40VA
Using the typical run time table at Sutton Designs website
http://www.suttondesigns.com/net700cht.htm
You can see that a 75VA load will run for approximately 113 minutes on a 700VA UPS.
MJ99,
If you plot the data you will recognize the exponential relationship between run-time versus load. Now I know this is not a mathematical calculation but at least you get the idea.
It may appear that a 40 watt light bulb on a 700VA UPS would run significantly longer then 113 minutes (maybe 3 to 3.5 hours). I don?t think this is true since there are operating losses of the UPS itself. I suspect Cardinal2k is right, 2 to 2½ hours is close to the most you would probably get.
But why speculate, buy one and do an experiment. Nothing like real life testing to find out. In fact I have a 700VA UPS at home, maybe I'll play with it this weekend.
Hope that helps.
Please note I am not an electrical designer and haven?t done real electrical engineering work since I was in school in the early 80?s.
Pigseye
