Calculating watt hours

 

When calculating what size solar power system you need to install you need to consider how many watts your appliances use and how long you use them for.

What's watt?Your power consumption will determine how much solar is suitable for your caravan or boat

If you have already read our power v energy page you will know that a watt (W) is a measure of how much electrical energy is being used. 1000W equals one kilowatt (kW).

​A Watt hour (Wh) represent the estimated amount of energy an electrical appliance consumes in one hour.

 

Toaster label showing watts

How many watts do appliances use?

An appliance usually has the wattage it uses written on the label.  This label is often at the back or bottom of the appliance. The list below shows some common appliances used in caravans and RVs and the typical watts they use. 

 

 Appliance Device

Min

Max

 

Appliance Device

Min

Max

 Air conditioner

 1000

 1300

 

 Microwave

600

1500

 Alarm Clock

1

2

 

 Mobile Phone charger

2

4

 Blender

300

 

 

 Portable electric fan

10

50

 Coffee Maker

300

1500

 

 Portable electric heater

1500

 

 Electric Blanket (double)

100

200

 

 Radio

70

 

 Electric Kettle

1000

2000

 

 Satellite dish

20

30

 Electric Shaver

15

20

 

 Toaster

800

1800

 Fridge Freezer

80

100

 

 TV

70

100

 Hair dryer

1000

2000

 

 Washing machine

230

500

 Hair curler or straightener

90

100

 

 Water Pressure Pump (shower)

120

180

 Iron

1000

1800

 

 Water Pressure Pump (sink only)

30

40

 Laptop Computer

20

60

 

 LED light

8

 

If you only know the amps rather than watts, you can convert to watts by multiplying the amps x voltage (A x V = W).  If the appliance draws 1.5 amps for example, multiply 1.5 amps by 240 volts to get 360 watts. 

 

How do I calculate watt hours (Wh)?

To work out the Wh take the watts (W) of the appliance and multiply this with the hours used on the average day.   ​This will give you the Wh consumed in your caravan/RV per day.  

Calculation:  Appliance watts x hours used per day = daily watt hours consumption

 

Example 1 - portable fan

If you use a portable fan that uses 20 watts for 4 hours per day then the calculation is

             20W x 4 hours = 80 daily watt hours (Wh) consumption

 

Example 2 - kettle

Some items are used only for a fraction of an hour or minute per day, for example a kettle.  

The calculation for this scenario is:

             Watts × minutes used per day ÷ 60 minutes = daily watt hours (Wh)

 A 1100 watt kettle used for 10 minute per day will therefore only consume 183 watt hours per day or 0.18 kWh:

             ​ 1100W x 10 ÷ 60 = 183.3 Wh

Example 3 - fridge

This is relevant if you have a compressor fridge (rather than a 3way fridge)

Even though a fridge is “turned on" all the time, the actually cycle goes on and off as needed to maintain its temperature. This is dependent on how hot a day it is.  Nevertheless, to estimate the number of hours that a refrigerator actually operates at its maximum wattage you can divide the total time the refrigerator is plugged in by three to give you an average estimate. 

For a typical caravan/boat fridge let’s do the calculation:

               80 watts ÷ 3 = 27 watts

               27 watts x 24 hours = 648 Wh

 

CLICK HERE FOR CALCULATOR