UPS sizing for DC and AC load

[Joe_Boxer]

I need help from you guys please...
I am sizing a UPS but I think I am confusing myself...

I have the following equipment:
R1: 28VDC 649W
R2: 230Vac 298.9W
C1: 24VDC 168W

I calculated the current consumption for all and got 31.54A. I converted to VA and got (230Vac x 31.54A) 7,254.2VA... does it mean I need 5.8kW/7.2kVA UPS??

I think it is too much.... can someone shed some light on where I went wrong?? Thanks!

 

[Itchrelief]

I don't think you will ever be drawing 31.5A of 230V. Your house wiring probably can't handle over 15 or 20A. I don't think you should be adding the amps but the wattage (you're going to be having amps drawn at different V, so you can't add them). Don't 100% trust my post as I'm pre-coffee, but might be a starting point for you.

 

[Joe_Boxer]

Thanks for the reply... much appreciated.

 

[Red Squirrel]

What are the DC loads, like are they strictly DC or do they have an AC adapter that plugs into mains? If they are DC loads you'll probably want a 24v dual conversion UPS so you can hook the loads directly on the battery side. This is typical in telecom, but with 48v. Most of the stuff runs straight off DC. A 24v system will run at about 27v to keep the batteries in float charge so your DC loads will need to be capable of running on a voltage in that range, say 22v to 30v. But if you want to keep things simple just get a regular UPS and a 24vdc power supply for the DC loads.

Always look at watts and not VA. Ex: a 1,000VA UPS is only 600watts. It never hurts to oversize. In your case I would try to find something that is at least 1,200w but probably go even to whatever the next step increment is. Keep in mind the type of plug, as once you start getting into the kw range they sometimes require a twistlock outlet. A typical 15 amp house socket can provide up to 1,800w. (as a rule of thumb I don't like loading one to more than 1,500w)