# Energy from batteries and the mains

When is done on an object, energy is transferred. The at which this energy is transferred is called . So the more powerful a device is, the more energy it will transfer each second. This is important when considering how much an electricity bill might cost.

Work done is the same as energy transferred.

## Calculating electrical power

Power can be calculated using: This is when:

• power is measured in watts (W)
• work done is measured in joules (J)
• time is measured in seconds (s)

The power output of a device increases if:

• more energy is transferred in a given time
• the same amount of energy is transferred in a shorter time

The energy transferred by an electrical device can be investigated using a joulemeter. The power output of a device can be calculated if the length of time the device is switched on is also measured.

### Example

An electric lamp transfers 1,200 J in 2 minutes. Calculate its power.

2 minutes = 2 × 60 = 120 s

power = 1,200 ÷ 120

= 10 W

## Calculating energy transferred

### Joules and seconds

The energy transferred can be calculated using:

energy transferred = power × time

This is when:

• energy transferred is measured in joules (J)
• power is measured in watts (W)
• time is measured in seconds (s)

### Example

A 250 W computer is used for 10 minutes. Calculate the energy transferred.

10 minutes = 10 × 60 = 600 s

energy transferred = 250 × 600

= 150,000 J (150 kJ)

### Kilowatt-hours and hours

The electricity bills from an energy supplier show the energy used in kWh, rather than in J or KJ.

The energy transferred is still calculated using:

energy transferred = power × time

But:

• energy transferred is measured in kilowatt-hours (kWh)
• power is measured in kilowatts (kW)
• time is measured in hours (h)

### Example

A 10 kW shower is used for 12 minutes. Calculate the energy transferred in kWh.

12 minutes = 12 ÷ 60 = 0.2 h

energy transferred = power × time

= 10 × 0.2

2 kWh