Why are UPS battery backups all rated in VA instead of AH?

Question

Looking at an online retailer, I see uninterruptible (battery backup) power supplies are marketed as:

• "1500VA"
• "600VA"
• "425VA"
• "425VA/260W"
• "1500VA/900W"

All measurements of power, a rate at which energy can be delivered.

Whereas most actual batteries themselves are sold as:

• "7ah"
• "7.2 ah"
• "20 Ah"
• "12 AH"

i.e. measurements of how much energy they actually store.

Now if I buy a 900 watt UPS and plug a little single board computer in it that draws 5W of power — well, I know I could actually plug 180 such little computers into that UPS I guess. But what I want to know is: if the power goes out on my 900 watt UPS and my computer is drawing 5W, how long will it keep running?

To calculate a runtime I would need to divide the energy available by the power needed to get a duration. So why are all the battery backups marketed only in terms of some "power" rating, instead of disclosing their energy capacity?

Answer 1

There are a number of reasons for this, including -

1. (Especially Lead Acid) battery based UPS's discharge curves are not linear, so there is not always a clear mapping between VA and runtime. Things like VA vs Watts (power factor), power draw (ie C rate), temperature and age of the batteries can substantially affect runtime, and how far you can run a battery down before damaging it.

2. The VA rating tells you about the maximum load you can put on the UPS, and this can also be a very loose proxy for the quality of the components - because higher rated UPS's will typically have componentry and cooling which can support higher currents and are thus more robust.

3. Related to (2) above - very often you can get additional battery packs for a UPS. This won't affect the VA rating - only the runtime.

4. Often, the runtime of a UPS is secondary to its power protection abilities - in many use cases the expectation is it needs enough time for a generator to kick in or for you to close out and save your work. They will also often provide buck and boost so you can continue to work with unstable/out-of-spec power.

I assume you are aware of it but for the sake of completion, battery runtimes are sold in amp hours AT THE RATED VOLTAGE FOR THE BATTERY - so you will need to multiply the AH by the voltage of the battery to get the the watt hours - which is more useful. Most UPS manuals will have runtimes at various loads documented.