Auditing Power On-board Your Boat
Understanding the power consumption that your boat is likely to use daily can help you prepare for a long passage. By having a clear idea of your boat's energy needs, you can ensure that you have an adequate size battery bank on-board as well as sufficient power generation sources.
Listing out all the items (“consumers”) on your boat with their power consumption and the time they are likely to be used for in each 24-hour window will give you your daily demand. Power for a consumer is often published in an items user manual specification table or printed on the back of the device. This will be given in Watts or Amps. List your audit in Watts. Watts can simply be calculated by multiplying the current (amps) by the relevant operating voltage equalling the power. For example, a 12V light operating at 1Amp is 12 Watts.
Calculate what each DC consumer uses when operating (this is Amps – Watts divided by volts)
multiply this by the number of hours it operates in a typical 24 hour period (this is Amp hours)
add up the Amp hours
This tells you how many Amp hours you will take out of your energy store (the batteries) during a fixed period of time.
Next you should look at your sources for recharging the batteries (“Producers”). Engine alternator, Generators, hydro, solar and wind will all contribute to varying amounts at maintaining positive charge to your batteries. List these as well, with their power capabilities. Be aware that sources have varying efficiencies: alternators and solar often only give about 60% of their stated capabilities and only the continuous rated outputs of generators should be considered. Battery chargers can also give lower outputs in warmer environments. The producers should be divided into continuous (hydro, solar, wind) which produce power all the time, and top- up (generator, engine alternator) which can be turned on at intervals to top up the batteries.
Calculate what each continuous DC producer (hydro, solar, wind) produces when operating (Watts)
multiply this by the number of hours it produces in a typical 24-hour period (this is Watt- hours) and calculate the Amp hours (divide by charging voltage, e.g. 13.8V)
add up the Amp hours
The difference between the consumer Amp hours and the continuous producer Amp hours is the gap that you’ll need to fill using your top-up producer(s). Be aware that the continuous producers may only work for some of the time (while the boat is moving, the sun is shining, or the wind is blowing).
With your daily consumption calculated and charging capabilities, you will now be able to work out how long your batteries will last and take to recharge. For example, a 200AmpHour depletion would take your 80Amp battery charger 2.5hours to fully recharge the battery bank. Ideally, you would not want to be charging you battery bank more than twice daily so an increase in your battery bank size may be required in order to achieve this goal.
Boats have multiple battery banks, but your domestic house bank will be the largest and the one in constant drain. Batteries sizes are given in AH (amp hours). This means that a 100AH battery with a 1Amp load would last 100 hours from 100% SOC (State of charge) to 0% (flat), although discharging by that much will destroy a battery. In practice we look to never discharge lead acid or AGM batteries below around 50%. Batteries have a lifetime, which is defined as the number of full cycles they can be used for. This varies between types of battery technology. A full cycle of a battery is theoretically from 100% SOC to flat so discharging to 50% of the capacity will mean that the battery can be used for twice the number of cycles. Different battery technologies have different discharge capacities: a rule of thumb for Lead Acid/ AGM is a minimum 60% SOC, so assume you only have 40% of your battery capacity as daily usable power. A good battery monitor will give you an accurate battery SOC so can be used as your “fuel gauge” for your battery bank.
Feel free to contact us if you have any questions or need help auditing the power on your boat.
