You don't really need a calculator 
The advertised TDP of a 6870 is 150 watts. That's 3.6 kWh per day if you run it 24/7. Multiply that with your effective cost of electricity per kWh and you've got an estimated daily additional electricity cost.
Example: If you pay 10 cents per kWh, your daily cost is approximately 3.6 * 0.10$ = $0.36/day.
The advertised TDP of a 6870 is 150 watts. That's 3.6 kWh per day if you run it 24/7. Multiply that with your effective cost of electricity per kWh and you've got an estimated daily additional electricity cost.
Example: If you pay 10 cents per kWh, your daily cost is approximately 3.6 * 0.10$ = $0.36/day.