[kalee20]

As Radio Wrangler says, it will give true power. Volts x Amps, averaged.

You can indeed use a current transformer to alter scaling, but design of a CT depends on accuracy needed and the load imposed by the wattmeter's current coils. It probably won't be very much: you could get a cheap toroidal transformer from RS of about 30VA, wind 100 turns for connection to the wattmeter; wind 10 turns for connection to your own circuit, and be reasonably confident of a 10x scaling achievement.

If you wanted to be a bity more certain, I'd get a 1kW radiant electric fire which will be virtually pure resistance, connect AC voltmeter and ammeter, multiply the readings, and compare this against what the wattmeter says.