At this stage, before we delve deeper in to DC set-up, we will introduce the concept of the narrowboat power audit. This is the starting point for all DC system design and should also be forefront in the minds of those of you who are looking to purchase your narrow boat from the used market. By doing this you will also be able to decide whether you need an inverter, solar panels, an engine 230VAC alternator or a gen-set.

Realising how you want to use your narrow boat will add clarity to the process of power specification and is the first step in understanding what is available to you.

Don’t loose sight of the fact that we are primarily talking about narrow boats that are designed to travel the system. There will be periods of time when you will wish to be moored up to take advantage of a beautiful spot or to visit a local feature. Continuous cruisers will usually spend more time moored in one location as they will have the time to explore or relax during their 14 day mooring restriction. Holiday boaters are more likely to travel a distance each day. In each case, mains AC power is not available so you will be dependent on some form of supplementary system to run your AC appliances and recharge your batteries.

In order to specify the system we need we first need to work out what our power requirements are. Everybody’s needs will be slightly different & the best advice here is to simply brainstorm what you’d like to be able to power on your boat. We have found that it’s good to restrict this power audit to proposed DC consumption during the hours outside of normal power generation times. i.e. between the hours of 8pm and 8am. https://www.thefitoutpontoon.co.uk/wp-content/uploads/2017/08/power-audit.pdf

Once your list is compiled, we than need to work out the power draw for each individual appliance, or in the case of lighting the collective maximum you are likely to have on at any one time. As our DC battery system is rated in amps, we need to work out the power draw on each appliance in amps per hour. Simply put, this is the amount of amps an appliance will take out of a battery in one hour.

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 narrowboat 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 narrow 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.