If the electric oven uses power worth $500 in a month, twice what Joseph pays for the power the water heater uses, Joseph pays 1/2*$500=$<<500/2=250>>250 for power used by the water heater.
The total amount of money that Joseph pays for the power used by the water heater and the electric oven is $250+$500 = $<<250+500=750>>750
The total amount of money that Joseph pays for the energy used by the refrigerator is three times the amount he pays for the power used by the water heater, a total of 3*$250=$<<3*250=750>>750
For the three gadgets, Joseph pays $750+$750 = $<<750+750=1500>>1500 for the power they use.
#### 1500