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Message: <blockquote data-quote="kamalboost" data-source="post: 26099763" data-attributes="member: 570168">However, to determine how long it might continue doing this you need to know how much energy is in the battery. This is measured in watt-hours. My computer is plugged into a 700 VA UPS right now, so let’s assume it is typical. It contains a lead-acid battery rated at 12 V and 7 Ah. The voltage drops from about 13.6 V to 10 V as the battery discharges, so the total watt-hours that can be extracted is about 80. However, this battery rating is measured over a long-term discharge of 10 or 20 hours, so the total energy than can be obtained over a shorter period is less.Meanwhile, your laptop power supply is probably rated at somewhere between 65 and 90 watts of output (unless it is a particularly powerful laptop). Since it will just be running the laptop and not charging the battery at the same time, it will not need to provide full output. Let’s say that the laptop may draw 40 watts. So, if you have 80 watt-hours of energy in the UPS battery, and the laptop draws 40 watts, you will use up all of the energy in the battery in 2 hours.However, we ignored several factors. The UPS is probably nowhere near 100% efficient in converting 12 V to 120 V. The UPS circuitry consumes some of the battery power itself when running. The laptop AC adapter isn’t 100% efficient. And the battery doesn’t really have 80 Wh when discharged this fast. When all of these things are added up, your laptop might get only 50% of the battery output, and in that case you’d get an hour of operation maybe.A UPS rated at 700 VA is really designed to power a largish desktop and PC with a monitor or two, a total load of up to about 400 W. With a load of that size, the battery has only enough energy to power the system for a few minutes. That’s OK, because the average power outage ranges from a fraction of a second to a few minutes. A UPS isn’t designed to provide power to a small load (probably less than 50 W) for a long period, so it isn’t likely to be very efficient doing it.</blockquote>

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