If your old microwave had a higher power wattage than this one, then you may be able to use this one at a higher power level or even at full power. Your question has more to do with the amperage load for the circuit you're using for the microwave in your fuse box / breaker box than anything else. If you've got a gazillion things plugged into that same circuit and you're using them all simultaneously, more than likely that's where your issue is.

I have this on a dedicated 20 amp circuit- have had no problems.

I can use this at full power, as I have been able to use other units at full power, also. The thing to check when you trip circuits in an older house is not the wattage but how many amps (amperage) the unit draws. This unit draws up to 12.5 amps. If you use it on a circuit rated for only 15 or 20 amps, and if you have other appliances on the same circuit, and if the total amperage of all those units while in use goes above the rating of your circuit, then it's going to trip.

We run ours at full power with not problems.