Deep cycle sizing
How to Calculate the Right Battery Size for Your Boat or Campervan
Choosing the right battery starts with understanding how much power you actually use.
The easiest way to do this is by listing every appliance and accessory that runs from your house battery.
For each appliance, record either its current draw in amps or its power consumption in watts. You'll usually find this information in the owner's manual, on the appliance label or by checking with the manufacturer.
When estimating your power use, be realistic. For example, your boat may have six cabin lights, but if you normally only have three switched on at any one time, calculate the power consumption using three lights rather than all six.
If an appliance is rated in watts rather than amps, you'll need to convert it.
The calculation is simple:
Amps = Watts ÷ System Voltage
For example, a 100-watt spotlight running on a 12-volt system draws approximately 8.3 amps.
Once you know how many amps each appliance uses, multiply that figure by the number of hours you expect to use it each day. This gives you the energy consumed in amp-hours (Ah).
Only include appliances that draw power from your house battery when the engine is not running. Equipment powered directly by the engine while it is operating, such as an engine-driven compressor clutch, does not need to be included in your daily house battery calculation. However, these loads should still be considered when assessing how much charging capacity is available from the alternator.
When you've calculated the daily amp-hour consumption for every appliance, simply add the figures together. This total represents the amount of energy your battery bank needs to supply between charging cycles.
As a general guide, your normal daily power use should represent around one-third of your total battery capacity. In other words, if your appliances consume 100Ah per day, a battery bank of around 250 to 300Ah provides a sensible reserve and helps maximise battery life.
If you plan to stay off-grid for longer between charges, simply multiply your daily power consumption by the number of days you expect to go before recharging. For example, if you intend to recharge every three days instead of every day, size your battery bank to comfortably supply three days of expected power use.
Spending a little time calculating your power requirements before choosing a battery will help ensure your boat or campervan has enough stored energy for the way you travel, while avoiding unnecessary cost and weight from fitting a battery bank larger than you really need.