Inverter capacity is measured in watts. Units are available from under 200 watts to more than 5,000 watts capacity. The size inverter you will need depends on the wattage demanded by your air conditioner. The air conditioner should have a power supply label that lists the watts or amps required to run the device. If you have amps, you multiply the amps number by 120 volts to find watts. If your air conditioner is a 240-volt unit, use 240 volts instead of 120 volts in your calculations.
You need an inverter with a wattage capacity that matches the maximum wattage demanded by the air conditioner. The figure on the appliance power label is the running wattage. Because of their compressor, air conditioners draw a big but momentary surge of power to start up. A rule of thumb is that air conditioner starting wattage is four to six times higher than the running wattage. For example, a 120-volt single-room window air conditioner drawing five AC amps in steady running would be demanding 600 running watts. Assuming starting wattage demand is five times running wattage, you would need at least a 3,000 watt, 120-volt inverter to handle this small air conditioner.
Battery capacity is measured in ampere-hours, abbreviated amp hr. or AH. The more ampere-hours of capacity your battery has, the more power you can draw from it. A 100 AH battery can support a draw of 1 amp for 100 hours. If the device draws 100 amps, that same battery will only supply one hour of power. Batteries supply DC amps. To convert AC watts to DC amps, divide the AC running watts by 12 volts. In the example of a 120 volt air conditioner requiring 600 AC watts to run, 600 watts divided by 12 volts would equate to 50 DC amps of power draw. Your 100 AH battery would support this air conditioner for about two hours.
The power leads for large inverters, such as the 3,000-watt unit needed for a small room air conditioner, need to be screwed or clamped to the battery’s terminals. Ensure that the positive red lead goes to the plus battery terminal and negative black lead goes to the minus battery terminal. To increase your battery capacity, you can hook up several 12 volt batteries in parallel by connecting all their positive terminals together and then connecting all their negative terminals together before making the connections to your inverter. If you connected two 100 AH batteries in parallel, you would have 200 AH of battery capacity, enough to run a 600-watt AC air conditioner for about four hours.