Question

How much does it cost to operate a 25-w soldering iron for 8.0 hours if energy costs 8.0¢/kwh?

Answer 1

The power P is defined as the energy E per unit of time t:

`P= (E)/(t)`
In our problem, the power is P=25 W and the total time is t=8.0h, so we can calculate the total energy used during this time:

`E=Pt=(25 W)(8.0 h)=200 Wh=0.2 kWh`
The energy costs 8€ per each kwh, so we can write the following proportion:

`8€: 1kwh = x:0.2 kWh`
where x is the price corresponding to 0.2 kWh. Solving the proportion we find x:

`x= ((8€)(0.2 kWh))/(1kWh)=1.6`€