CALCULATION

[MCREMALDI]

Could you explain the proper calulation to find volt-amps and the calculation for watts. I am trying to size a UPS to be capable of suppliying 480 watts for 5 minutes.

 

[Lewish]

Volt-amps are watts unless you are talking inductive loads, such as motors. If you are not running a motor, and I suspect not at 480 watts, and if you have allowed for all possible loading conditions and are sure 480 watts is adequate, then I would look for a 500 VA UPS.
The length of time it will hold up a load is supplied by the UPS manufacturer.

Lewis

 

[buzzp]

Watts and VA's are the same for purely resistive loads, Watts=Volts x Current. For inductive loads(coils-like motors, other non-linear loads) the sizing of the UPS would have to consider, amongst others, the VA rating of the device. VA is volt-amps and is not worth typing in the formula for and trying to explain what everything means(vectors, p.f., etc) but I will if you want to see it.

 

[230842]

To complete Lewish and buzzp answers: Capacitive loads also increase VA values over watts, but in the opposite sense than inductive loads. Julian

 

[dydt]

for that load, i will go for 750VA ..room for motorized load..

the back-up time selection and the corresponding size of the battery are supplied by the manufacturer...at mains failure you have the inverter plus the load running and the battery ampere-hour can only be sized accurately by a particular manufacturer..

good luck
dydt

 

[skenn]

Add one point, to extend the hold-up time, you can open the unit, and connect a larger battery, as long as it is the same voltage. Maybe car/motorcycle.