Every INVERTER is rated in VA and Volt.
VA is the capacity of an INVERTER to run peak load and volt specifies the battery voltage for which the inverter is designed.
The battery which is used in INVERTER application is lead acid battery with voltage in multiples of 12 Volt. Each lead acid battery has 6 cells of 2V each which gives the total voltage as 12V.
Hence, a 12 Volt INVERTER means that it is designed to operate with 1 battery. A 24V inverter means that it is designed to operate with 2 lead acid batteries.