UPS Capacity Calculation Formula - Anyone has an easy to understand one to follow for those calculations required every once in a while
If you want detailed information you have to rely on the data given by the manufacturer. Batteries behave differently depending on the load. The only thing that applies pretty much every time is: Try not to go over 80% load. Always leave some headroom, the more the better obviously, but 80% max load is a typical recommendation.
Generally, Watts = Voltage * Ampere. Capacity is often specified in AH, or Ampere Hours (Ampere * Hours). If a battery has a capacity of 9AH, that means: 9 = Ampere * Hours. Fill in ampere, which is what your devices draw, and you'll get to the hours. We assume the battery is designed for usage in 110V scenarios.
If your equipment runs at 110 volts and it has a power rating of 500 watts, that gives you: 500w = 110v * A. Divide 500 by 110 and you get approximately 4.5 amps. If you go back to the battery you'll see it has a capacity of 9AH, which means it can supply 9 ampere for 1 hour. Since you only need 4.5 amps, that would give you a runtime of 2 hours.
Here's the thing though: Batteries are limited in how many amps they can supply. You cannot just draw 1000 amps, batteries have a rated output current as well. This is the number you need to determine how much load you will put on the battery. If the battery is rated for 9 amps output current then drawing 4.5 amps would put it at 50% load. And this is where the charts from the manufacturer come in: The efficiency of a battery depends on the load. The basic math holds up, drawing 4.5 amps from a 9Ah rated battery will give you a runtime of 2 hours. But when you factor in the heat generated by the load, the power factor of the battery/UPS, a bit of loss from the cabling etc. then your results can vary quite a bit. This is why the "half-load runtime" isn't twice as long as the "full-load runtime", even though that should be the case. The UPS you linked to mentions a "half-load runtime" of 9.5 minutes; Doubling the amps drawn (to get to full load) should reduce this to 4.75 minutes; half the time. However, with all other factors in play you get a specified full-load runtime of just 2.5 minutes. That is barely more than a quarter of 9.5 and nowhere near half of 9.5 as you would expect.
Ultimately, even though calculating the runtime is fairly easy if everything was working with an efficiency of a 100%, that unfortunately is never the case. Efficiency varies greatly between manufacturers and the load put on the batteries so the only thing you can do is consult the runtime charts of the manufacturer.
In the end what you should do is: See how much current (ampere) you need for your equipment, look at the output rating of the UPS to determine how much load your equipment will put on the battery, then consult the chart to see what runtime you will get. Always leave some headroom, a) because if you do things like shutting down servers once you know the UPS has kicked in those servers might draw more current during shutdown than they would if they were running regularly. The battery needs to be able to handle that load as well, even if it's just for a minute. Also, b) batteries will deteriorate over time. You will get a minute or two less runtime at some point, you need to accommodate for that as well.
To sum it all up:
Q1: How long can I run in minutes my equipment which needs 500 Watts?
Look at the UPS/batteries to see how much load 500 watts would be (calculate your current draw by dividing Watts / Voltage) by checking your current draw against the power current rating. Then check the charts to see how much runtime a load of X% will give you.
Q2: What is the maximum watt usage I can permit if i want to get X minutes of uptime/runtime?
That is pretty much the same question as above. Check the charts of much runtime you will get with 80% load, then multiply the current at 80% with your voltage and you get watts.
Q3: How can i include in my calculation the presence of an extended battery (for models where it is a possibility)?
Just add up the ampere hours of all batteries included.
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How do I calculate power cost based on UPS load?
You didn't say what voltage, but since it's a SmartUPS, you can connect via USB and install PowerChute. It will calculate this for you after providing electric cost.
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