More Sizing UPS challenges

Here is another case of nameplate sizing of equipment.  
I got a request for a UPS to power a fridge that uses 7.5amps that will last 4 hours.

7.5amps times 120volt is 900volt amps or va.   But thats not the whole story.....

Step one is to understand the amperage value and its meaning for the size of the UPS.   There are 4 "types" of amperages

• FLA or full load amps or nameplate amperage  (this is usually on the equipment by the manufacturer)  It is used to size the circuitry.   That amperage never happens in real life.
• SLA or starting load amps or the peak load that occurs just briefly during startup (depending on the motor, this can be 2 to 10 times the FLA)
• RLA or running load amps  (this is the average amperage during operation)
• RMS Amps or Observed amps  (this is when someone actually measures the amperage during the equipment's operation)

So,  7.5amps  is probably the FLA and the SLA on a new high efficiency equipment is about 15amps. (15a x 120v = 1800va).  So any 3000va UPS will work 

Step two is to understand the actual load over the 4 hour outage period.  Fridges cycle on and off, so getting an observed amperage is critical.  Factoring in the duty cycle of on and off over a 4hour period suggests the RLA is about 4.5amps.  Converting to wattage, suggests this fridge uses 430 watts.

Bottomline, left to his devices he might of gotten a 900va UPS with a trunk of batteries, but by understanding is real load meant he got what he needed for about half the price.