Horsepower ratings of furnaces don't directly translate into power consumption. Technically, 1/3 horsepower is equal to 248 watts, but given inefficiencies and the way motors are rated, the actual power consumption may be 600 or 700 watts. If there is a rating available in watts or amps, you could get a more accurate answer. We don't have the technical specifications for your particular furnace, so we'll use 600 watts, for argument's sake. If this is a high-efficiency motor, the actual number will be less. In 30 days, there are a total of (24x30) 720 hours. If the motor is running continuously, that's (720x600) 432,000 watt-hours, or 432 kWh. To get the cost, you'll need to know how much electricity costs you. If electricity cost you $0.10 per kWh, the total cost is (0.10x432) $43.20 per month, not an insignificant figure.