3000w/120volts=25amps
keep in mind that as you volts drop you watts go down and amps go up

curently at my house i get 117 volts. a good idea would be to put a volt meter on your plug to see what you volts are at different times of the day and different days.

117v * 25 amps = 2925 watts
115v * 25 amps = 2875 watts

do you have a 25 amp beaker. that seams odd. residential is usealy 15 or 20???