Energy calculation on board for cruisers and longer trips

I regularly see questions in the various ‘cruisers forums’ (Facebook, forums, whatsapp etc) such as:

-: How do I know how much power my boat uses?
-: How much battery storage capacity do I need?
-: How much power generation do I need ?

In the past I have already written a lot about this but never ‘really’ made a step-by-step plan of attack, I will try to do that now and before I get comments such as ‘ you are too pessimistic , we do not use that much energy , you better adjust or lower your consumption instead of increasing power generation etc etc etc “…. Yes that is all correct , but you will have to make a start somewhere first ?  A so-called ‘zero measurement’ before we can build an informed installation

And this is how you can approach this and adapt it to your own situation .

Step 1 – calculate how much your boat consumes while sailing and when you are anchored

Step 2 – how much energy storage (batteries) do I need and what type of batteries do I need (spoiler, you need lifepo4)

Step 3 – how much energy generation do I need?

step 4 – calculate spreadsheet


Step 1 – calculate how much your boat consumes while sailing and when you are anchored

Probably the most important thing is to sit down and write down all electrical and electronic consumers.  An ‘average’ long-distance sailboat has a refrigerator, an autopilot with controller, navigation lighting, computer laptop, tablet, telephone etc and the somewhat better equipped boats watermaker, freezer, inverter, espresso machine etc etc etc.  Now write down everything you have on board , and write down everything ! I think you will end up on a list of which you think ‘ That’s more than I thought!”  Then you will try to determine how long the various items are ‘really’ used and/or are switched on for, for example a refrigerator uses somewhere between 3.5-6A but it is only ‘running’ for 30 minutes per hour, ie when it is cold enough it turns off and when it gets too warm it turns on again.  Another example is your autopilot, which is ‘on’ 24/7 while sailing, but you can turn off your indoor lighting or navigation lighting (if you don’t forget, with us, for example, the navigation lighting is also on 24/7 because we forget to turn it off) and charging a phone from 10% to ‘full’ takes an hour and then you don’t have to recharge it for 12 hours.  If you have now written down everything you have and how long it is ‘on’ for, you write down , from your manual of the equipment (or google) in a spreadsheet or paper what your actual total consumption is during 24 hours and that will look something like this:

-refrigerator 5A * 30 minutes ( = 0.5 hours) * 24 hours = 60Ah

-Plotter (older type Raymarine for example a C90W,
Power Consumption 32 Watts (typical)
That’s 32 watts / 13 volts = 2.46A * 24 hours = 59Ah (almost as much as a refrigerator!!)
-Typical Raymarine controller , st6001 for the autopilot 0.2A * 24 hours = 4.8Ah
-Typical Raymarine autopilot (S4 or 300G) and depending on the drive unit! 10A * 24 hours = 240Ah
-Typical AIS transponder : 2.5 watts / 13 volts * 24 hours = 4.6Ah

and as you can see with a fairly ‘basic’ sailboat with a refrigerator, an autopilot and a plotter & AIS you already have a consumption of 60+59+4.8+300+4.6 = 368.4Ah per 24 hours!  And I did not pull the above figures out of thin air , that was a selection of the equipment we had on our previous boat .  On our current boat we have newer stuff and I have to say that they, especially the autopilot, uses much less.   But sit down and calculate it.   And be honest, because if you start ‘calculating optimistically’  you will only get disappointed and you can buy new AGM batteries much more often than you anticipated.

Now do the exact same thing again but now go over everything you use while at anchor. For example you do not use your auto pilot and navigation equipment but you may use your watermaker a lot more ? so sit down and think what the differences are between sailing and anchoring and then come up with an ‘at anchor list’ as well

If you have written everything down and have made a pessimistic calculation of what your consumption is per 24 hours, we will look at how we can generate this use.

Step 2 – how much energy storage do I need and what type of batteries (spoiler, you need lifepo4)

In the example above we use 368Ah per day. If you are considering going with AGM / GEL batteries, this means that you need double the storage because you can only empty AGM / GEL etc by 50% otherwise the batteries will not even run the 300 to 500 cycles (days) and you can replace them again, so 368 * 50% = 800Ah @ 13v of battery capacity! lifepo4 you can drain up to 5-10% with impunity so in this case you need 400 a 500Ah @ 13v to get through the day But still, the above battery capacity is again entirely dependent on your consumption, and if you have calculated that you only use 50Ah per 24 hours, so 100Ah AGM/GEL etc or 60Ah lifepo4 is sufficient. So calculate it!

step 3 – how much energy generation do I need?

I think (hope?) that most long distance sailors / cruisers install one or more solar panels to supplement daily use. I have tracked the solar yield for the last 4 years (between the Middle Sea and Panama) and come to a yield of 350-450Wh per installed 100Wp of solar panels. So 1 solar panel of 100Wp delivers 400-450Wh per day, and if you have a 12v nominal system this is 350Wh / 13v = 30-34Ah per day . And coming back to our example above of a use of 368Ah this means that we have to install 1200Wp of solar panels only to supplement our daily use during sailing!  And even that is quite optimistic when you know that AGM/GEL batteries from 80% SoC (State of charge) accept drastically less power to get ‘full’ again. Lifepo4 on the other hand accepts all the current you give them linearly up to 100%, so very little load loss.

And don’t forget that during an ocean crossing , east to west, you put your solar panels in the shade of your own sails between 11:00 and 15:00 and you will miss the best 4 ‘production hours’

“But Rene!!! , we also have an engine with an alternator, a generator and a windgenerator!”

This is of course absolutely wonderful! If you want to use your main engine to recharge your batteries while sailing there is of course nothing against that and if you have a wind generator that also helps! A typical alternator / alternator delivers a maximum of 80A and an average of 50A, if you consume the above 368Ah per day during a crossing, you can run the boat engine for about 8 to 9 hours a day to refill the batteries and compensate for charging losses with AGM/GEL batteries . Unfortunately, we cannot improve the charging efficiency of AGM / GEL / lead batteries, but you could consider mounting a larger alternator or alternator with an external regulator, the more current you can pump into the batteries, the faster the batteries are charged to 75-80%.  Above 75-80% the power acceptance of agm/gel/lead batteries drops drastically and a larger alternator unfortunately no longer makes much sense because the batteries simply do not accept more power.  A typical wind generator can deliver 250 to 400 watts, a wind generator however, needs at least 15 knots of apparent wind and will only deliver ‘real’ power from 20 knots of apparent wind, and if you cross an ocean with an average of 15-25 knots of wind, then the wind generator unfortunately does very little because the apparent wind is <15 knots.  The only wind generator that I am aware of and does significantly better is the D400 but it still needs between 10 and 15 knots of apparent wind to provide some power

step 4 – calculate spreadsheet

Once you have all the nubers , calculated everything :

Below is a link to the document / spreadsheet you can use
Please note that this is made for lifepo4 batteries! An AGM/GEL/Lead battery has a much less charging efficiency, about 75-80%, and you will have to take this into account in the calculations, so to get an AGM/lead/GEL battery ‘full’ again you will have to Have to put in 30% more energy than you took out.  So if you have an AGM battery and have a consumption of 100Ah you have to put in 130Ah to get the battery as full as you started.

Google Doc Spreadsheet