It takes one Btu uf energy to raise one pound of water, one degree F. Water weights about 8.3 lbs per gallon, so 120 gallons weighs about 1000 lbs. Temperare rise is 40F, so it takes 40,000 Btu to heat 120 gallons from 80 to 120F.
Converting Btu to kWh yields 11.7 kWh, so call it 12 kWh.