It all comes down to energy. Energy is stored in batteries, and used up by devices doing work. The way that energy is transferred is by pushing electrons. Current, measured in amps, is how many electrons are flowing through in a given time. Voltage, measured in volts, is how hard the electrons are being pushed. Power, measured in watts, is the amount of energy transferred in a given time. kilowatt-hours (kWh) is a measure of energy, in more convenient units. So to calculate runtime, you start by measuring the power input into all of your devices. You can get that a couple of different ways. measure the number of kWh used over a time period, and then divide by the number of hours in the time period to get kilowatts. measure the current and multiply by the voltage (power = voltage * current) Once you've got that, you know how fast the energy is draining out of the battery. Now you need to know how much energy is in the battery. According to the specs of the UPS you posted, it has "Battery Volt-Amp-Hour Capacity 845". A volt-amp is the same as a watt (sort of, the difference exsts but isn't important here) so that means the battery has a capacity of 0.845 kilowatt-hours. If the load was 0.845 kilowatts, the battery would last exactly one hour from full charge. If the load is 2.535 kilowatts, the battery will last 20 minutes from full charge. Your power draw in that screenshot is 207.5 volts at 8.6 amps, which is 1.7845 kilowatts. So your battery runtime will be 0.47 hours, or about 28 minutes. There are two caveats for this! The first is that the calculation gives you the theoretical maximum runtime. In reality, it will be less - batteries don't like discharging to zero, and there's always losses in the system. The second is that the calculation is crucially based on that current figure, which will vary from second to second. Device power consumption is never constant, it varies with different computing loads, environmental conditions, which devices are in use and other factors. The most accurate way to get runtime is to measure your total energy consumption over a long period, and then divide by that time period to get an average power consumption. I suspect that the 19 minute runtime remaining is because the UPS is averaging the measured power consumption and that figure is higher than the consumption at the time the screenshot was taken. For your new site calculations, is the site up and running already or do you have to calculate consumption theoretically? More on reddit.com

I'd measure the total consumption over a period of time with a meter, or smart plug. Laptop has a battery, consumption will depend on usage and if its charging or not. In case of a long outage it may be acceptable to power down a screen for a bit? More on reddit.com

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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UPS capacity for how many VA/watts it can handle and how long it will run are completely unrelated. A lot of UPS models in the same line will all use exactly the same battery packs. A typical setup is 2 to 4 7Ah sealed lead acid 12v batteries. If you have a 2-cell battery pack, that's 2*12v*8A == ~170 Watt/hours. Assuming the inverter is 90% efficient, and your 380 watts of power need, expect about a 20 minute runtime. More on reddit.com