When actually using it the load on the tool will not be constant so the current draw will change. This is something best estimated or measured. Unless you know the precise working load and either actual current draw or moto efficiency it will be difficult to estimate precisely. I'll use made up specs to keep things simple. A 1 Ah 1 volt battery should provide 2 amp for half hour, 1 amps for 1 hour, half an amp for 2 hours. The power out put will be amps X volts so 2 watts at 2 amps, 1 Watt at one Amp and so on. However without a known load you will only be estimating the draw. This is fine for comparing batteries if they are not lying about their specifications. To turn that into actual work time on a tool you need to know not just the current rating on the tool but the current draw while actually using it. Then apply that load to the battery run time formula. The load will be dynamic, a weed wacker (assume these are all the same model weed wacker) running with no string will draw less current than one running with a string. Both of those will draw less current than another that's activity cutting down brush with a heavy metal blade on it. More on reddit.com

The quickest way to do a back of the envelope calculation is to calculate the battery capacity in Wh = Ah * V = 7.5 * 56 = 420Wh. Being conservative, you might get 80% of that from inverter and other losses = 336Wh, then divide by how many watts your sump pump consumes to get a guess at run-time. More on reddit.com

Use Watt-Hours. A 3.6V, 4.2Ah battery has 3.6 * 4.2 = 15.12Wh of energy. A 12V, 0.1A fan will consume 12 * 0.1 = 1.2W of power, which 15.12Wh would be able to power for 15.12 / 1.2 = 12.6 hours. It won't work because the voltages are different, but you get the idea. And then always keep in mind that nothing is 100% efficient. Putting energy into a battery loses some, extracting energy from it does so as well. Converting from one voltage to another loses his energy as heat, and so does using a fan speed controller. In general think of anything electronic being around 80%-ish efficient with all the inefficiencies stacked together. This acts as a 0.8 multiplier to any number you end up with. But of course all of these are just estimations. The only way to get actual numbers is to run the thing and see how long it actually lasts. More on reddit.com

That's the math. The battery is rated for some number of "watts times hours", and the load is some number of watts, so you can divide the watt-hours by the watts to get hours. This only gets complicated if you start to draw energy much faster than is specified in the measurement of the battery's capacity. This isn't a big factor for lithium batteries, but some chemistries, like lead-acid, produce many fewer watt-hours when they're connected to higher-watt loads. More on reddit.com