24,000BTU may well be equal to 7kw in terms of heat output, but you can't use this type of conversion to work out the power consumption of an air conditioning unit.
Unlike, for example, a fan heater, an A/C heat pump heats or cools by moving heat from one location to another. In doing so, it can quite happily give a cooling capacity of say 7kW while requiring only 3kW of electrical power. That's a very broad approximation, and the actual ratio of heat output to power input depends on the COP (Coefficient Of Performance) of the air conditioning unit in question.
Nobody can give you a definitive answer without more information, but I expect a 20A radial will more than suffice.