The Official Magazine of the Bluewater Cruising Association

How Much Battery Power Is Enough?

Melissa “Missy” Gervais

As You Wish
Bayliner Avanti 34
March 15th, 2021

Growing up in my house, it was quite common to hear “turn the lights off when you leave the room,” or “who left the lights on? Do you think we are made of money?”  Now that I have my own house, I get it.  But what about on the boat?  Many marinas have replaced the flat cost per day or cost per month with metered power, so it makes sense to know how much your boat draws when you are not there.  Are you using an efficient fan or a heater with a thermostat that turns off when the weather warms up?  These are easy things to do and don’t affect the way you use your boat; however, what about when you are at anchor or tied up at an outstation without power?  How much power is enough?

Many evenings I have been visiting friends on their boat and find myself chatting by the light of a single candle or a small, yellowish light bulb in an older bulkhead fixture. It’s not flattering, and it’s not right.  Since working with Jeff at Pacific Yacht Systems, I have learned a few things about electrical systems.  I never really understood amps (A), amp-hours (Ah) or volts (V), and I couldn’t figure out the battery monitor to save my life.  After sitting through many presentations, editing articles and listening to Jeff speak with hundreds of clients, there are three simple things I would tell my younger boater self.

How Many Amps Do You Use

The first thing is to decide what is “electrically” important to you and your crew.  How much energy or amp-hours do you use per day?  The biggest draw on my boat is an older Nova Kool fridge; no way around it – that beast sucks up 40 – 50 Ah per day.  I try to reduce the number of times I open and close the fridge so that the compressor does not have to come on, which saves energy.  A great tip is to keep all your beverages in a cooler and use the fridge for food.  I make ice blocks in Tupperware containers while the boat is plugged into shore power and then drop them in the cooler for the weekend.

I have an older Bayliner, and the galley was dark and dingy before I installed Lunasea LED lights, so I want to use them.  I have a tablet but prefer a keyboard, so I want to use my laptop whenever I have the urge to google.  I try to charge it completely when the generator is running and use the laptop battery for the rest of the day.  I was surprised to learn that my older Xantrex Freedom 2500W inverter/charger onboard still draws 2 – 3 A even when I am not using it, so I have made a conscious effort to turn it off.  Ironically, now I hear, “who left the inverter on?”  Turning the inverter off at night will save 8 hours X 3 A = 24 Ah for doing nothing!  However, the cell phones have to stay charged, so I installed a cigarette lighter / USB charger on the dash – this draws directly from the battery, and I can keep the main inverter off.  The dinner hour on my boat is much later in the summer, so I installed a dimmer LED light on the back deck.  I also like to play my music for a few hours a day and during the evening.  Add this to the 50 Ah my fridge draws, and my total usage is 80 Ah per day.

I enjoy boating and openly admit that I am not a camper.  It is nice to wake up in the morning to a good cup of coffee.  On a rainy night, it is fun to watch a movie on a real TV, with popcorn.  And yes, I dry my hair.  Add in a warm crab dip with toasted pita at happy hour and my daily usage can be as high as 200 Ah.

How Much Battery Power Do You Have?

That brings us to the second thing I would tell my younger self: figure out how much battery power you have on board.  My house bank has six flooded lead-acid golf cart batteries that store 660 Ah of battery power at 12 VDC.  Remember, to extend the life of your lead-acid batteries, you should only deplete them to 50%, which means I have 330 usable amp-hours.  Therefore, 330 Ah divided by 200Ah = 1.65 days of power.  This means that I have to run my generator every two days.  I don’t have room to add more batteries, so I decided to add a 100 W solar panel.  On a sunny day in Howe Sound, the panel starts working at 0600h and is still putting in power until the sun goes down at 2100h – that’s 15 hours of sunlight.  The panel I installed averages 3 amps per hour X 10 hours = 30 Ah/day.  This helps offset my fridge, which uses 40 – 50 Ah/day, and I get an extra day without running the generator — game changer.

Battery Monitors

Now that you know how much power you use and how much power you have available, the third thing is to install a battery monitor, or learn how to use the one you have installed.  It truly is a gas gauge for your batteries.  Stay with me here and don’t get overwhelmed by all of the details.  There are typically four screens, and #2 and #3 are the ones to look at:

  1. V = Volts.  Voltage is used to assess the approximate state-of-charge and to check for proper charging.  For example, an at-rest, fully charged 12 V battery bank will show about 12.6 V to 12.8 V.  A 12 V battery is 100% discharged when it reaches 10.5 V under load.  A 12 V battery that is being charged with an appropriately sized charger or alternator will read above 13 VDC  to 14.6 VDC.
  2. A = Amps.  Amps is the flow of current in or out of the battery.  Current is analogous to speed: it’s the rate at which electricity is flowing. While driving a boat, you would say I’m doing 10 knots, and you would convert 10 NM if you traveled for one hour. Amps are similar to boat speed.  For example, I am drawing ten amps and, if the current was constant for one hour, you would say I used ten amp-hours (more information on amp-hours below). Your fridge may draw six amps of current and this is displayed as -6.0 A.  Discharge is shown as a negative number and shows the number of amps that are being consumed.  This is an important function to teach your crew as it serves as an excellent reminder to turn off unused lights, navigation equipment, etc. If you had no loads on your batteries, any charge going into the batteries (e.g. solar, alternator, charger, etc.) would show up as a positive number.
  3. Ah = Amp-Hours.  This shows the amount of energy stored or removed from the battery.  If you run a 10 A load for one hour, then 10 Ah are consumed.  The battery monitor will show -10 in the Ah display.  During charging, the battery monitor will compensate for charging efficiency and count back up toward zero. A full battery is displayed as zero amp-hours or 0 Ah.  Any draw from the battery is reflected as negative amp-hours, e.g. – 47.9Ah, and recharging will bring the number back to 0Ah.
  4. H or t = Time.  Don’t worry about this one as “hours” left is calculated on the last four minutes of use, which doesn’t give you any practical information.  If this screen reads CCC, then you know that the batteries are charging (e.g., the generator is running).

[Editor’s note: data may display differently depending on the models of battery monitors.]

Another note to remember, your hot water tank should not run off your batteries.  On my boat, the hot water tank stays warm enough for two days of showers and dishes, then I have to run the generator.  If you plan on being in a busy anchorage and don’t want to run the generator, one option is to buy a solar shower bag for quick rinses on the back deck.  Also, I have a small electric kettle in the galley that I use to boil water for a single sink of dishes.

I did not understand the electricity use on my boat when I started boating, and I didn’t know where to start or what questions to ask.  When I did start asking questions, the answers were way over my head, and, in many cases, the people I was asking did not have a complete understanding either.  Don’t be afraid to raise your hand.  Knowledge is 12 V or 110 V power.


  1. John bae says:

    Great article! I like the tip on keeping beverages in a cooler and shutting off the inverter!

  2. Rod Morris says:

    Thank you for the wonderful explanations of all this electrical stuff. I especially liked the boat speed to distance covered analogy for understanding amps to amp hrs. That is going to get a lot of use as I explain energy use on Oh! to friends and clients. A tip in return regarding hot water and showers – drill 4-6 1/8” holes in the lid of several 64 oz juice bottles ( like a bottle of cranberry juice). You can then fill them and leave them in the sun to warm for rinsing off, or showering. On cloudy days, fill 2/3 with cool water and 1/3 from a kettle for a warm shower. They are easy to hold and can be used on deck, or in your boats shower. On a sunny day it is free hot water, no amps required..

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