The electric power required to run an appliance is measured in watts.
The wattage of most appliances or devices is readily available printed onto a label attached to the appliance or from the manufacturers website.
Some appliances, such as water pumps will be rated in amps.
Whatever figure you have available, it is the amps we are interested in when it comes to calculating how long our batteries will last.
We can work out how many amps the appliance will draw from our battery by the simple equation of: amps equals watts divided by volts
AMPS = WATTS/VOLTS
Although our 12V system is actually 12.6V when fully charged we will err on the side of caution and use 12 as the divisor. It’s always better to round up and use worse case scenarios when dealing with batteries!
Once we’ve worked out what our power requirements for each individual appliance are, we then consider how long each appliance will be used for before the next full charge of the battery.
With regard to travelling canal boats, most power audits work on the assumption that energy used overnight can be replaced during the following day.
So let's look at a couple of examples:
A Water Pump
A 12v fresh water pump is rated by the manufacturers’ specification as drawing 4.4 amps.
We have thought about how we are going to use the pump and have estimated it will run for about ½ hour per day.
To work out how many amp hours the pump will consume we multiply the running hours, in this case .5, by the rating of the pump.
So our pump will draw 4.4 x .5 or 2.2 amp hours per day from our battery bank.
A 12VDC compressor driven fridge/freezer
We are looking at installing a new compressor fridge. The manufacturer states in the specifications that the fridge will draw between 4 and 6 amps depending upon ambient temperature. Let’s say an average of 5 amps is reasonable. Lower at night maybe but higher in the heat of a summer day
We will be running our fridge for 24 hours per day.
Therefore your 12v fridge will consume 24 x 5 or 120 amp hours per day from our battery bank.
If we are regularly cruising for 6 of those 24 hours then we need only be concerned for the 18 left (theoretically).
So we can look towards an average of 18 x 5 or 90 amp hour consumption.
A good quality leisure market compressor freezer will draw approximately the same.
A 12VDC LED TV
A good quality 12VDC TV will consume around 40 watts or just over 3 amps per hour. 5 hours viewing will need a capacity of 15 amp hours in the battery bank.
We have created a basic sheet for you to download and jot down all your appliances and corresponding ratings.
Click here for a downloadable power audit sheet