# Calculating Watt Hours (Wh)

Author: Solar 4 RVs

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? 1000W equals one kilowatt (kW).

​A Watt hour (Wh) represent the estimated amount of energy an electrical appliance consumes in one hour. ## 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 1 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 / day

### Example 3 - fridge

This is relevant if you have a compressor fridge (rather than a 3-way 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, as a rule of thumb, the average time a fridge runs at maximum will be close to a third, therefore you should divide the maximum wattage by 3 in order to calculate the average wattage the fridge will use per hour.

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

80 watts (maximum) ÷ 3 = 27 watts (average)

27 watts x 24 hours = 648 Wh / day