Tony, As far as I know, there is no formula available to calculate the APC UPS runtime. However, the runtime chart of the UPS can be found on APC web site. Click on the below link to view the Smart UPS SUA750XL UPS runtime chart at different load levels. http://www.apc.com/resource/include... More on community.se.com

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

The general guidance is not to have your UPS run as long as possible. Your UPS should do one of two things. One option is to run your systems long enough to shut down. The other is to run them long enough for the generators to kick in. Expectations outside these two use cases are very expensive with diminishing returns. More on reddit.com