Real world electrical consumption?

[SY Kelpie]

I've been trying to estimate our likely electrical power requirements so that I can get on with designing/speccing the system. How long is a piece of string!

We're a 39ft mono with two adults, one child, and we plan to keep it very simple- no watermaker or air-con, but no generator either. Quite happy to use manual water pumps although the boat is currently fitted with a pressurised system. Surely not a big user in the scheme of things.

Current fridge is a Waeco standalone unit, not the best efficiency, I would like to replace this with a built in fridge with water cooling and much better insulation.

Anticipated cruising ground is an Atlantic circuit.

It would be interesting to hear from people in a vaguely similar situation to see what the actual power requirements tend to be.

[Cheechako]

My experience is roughly 100 amps a day. Two 45 minute motor charging's a day barely kept things going at anchor. every 3rd day I might run 1-1.5 hours. (100 amp alternator with external regulator) In the am I could be down to 11.5 volts or less even. Your battery capacity need is really limited by your charging capacity.

Of course the fridge is the culprit mainly. Ours was very small, like 2-3 cu ft or so.
Two adults.
Computer charging for Pactor and some video/TV watching. Maybe 1-2 hours of tv 3 times a week at the most if in port a long time.

Will you have anything but alternator?

[Stu Jackson]

30-36 ft. monohull with a fridge will run ~100-120 ah / day. Larger boats don't use exponentially more power, the curve drops off. Some here report 200ah/day on a 38 footer, but they have separate fridge/freezer units. Most compressors use 60 ah/day for budget purposes; better insulation drops that a lot.


How you use the boat makes the numbers grow: TV, devices needing to be charged, computers or MFDs for navigation.


Your boat, your choice.



You need to make an Energy Budget, just Google it, lots of forms and numbers already out there.

[Dsanduril]

Also depends on conditions. We use less at anchor than we do underway. Underway we can easily reach 200Ah/day in boisterous weather, at anchor 150 or less.

At anchor we have cabin lights and maybe the entertainment system on for a few extra hours, but underway we have the AP on 24/7 and running lights 10-12 hours/day. Those add up to more than we use at anchor. Everything else is about the same both ways (fridge, watermaker, instruments, etc.).

If I were ranking things in terms of power hunger:

Fridge/freezer
Autopilot (underway only)
Watermaker (we use a DC unit)
Computers/tablets/phones/monitor
Lighting
Minor stuff

If you don't have some of those items you can knock them off the list.

[Tellie]

This is where most people fail when determining what size battery bank, what size charging device/s on board or will be installed that they need. The simplest way to explain this is to first understand that it doesn't matter what other people do or what power they use everyday. Ten identical boats are going to have ten different energy requirements. Like you stated you don't want a watermaker (shame on you) but another cruiser will insist on one so his electrical needs will be different than yours. The simple answer to your question is to determine "your" daily amp budget. Each one of your electrical devices has an amp/hr rating. You must take every electrical device on your boat, then looking at the label on the device you will see it's amp/hr rating, most manuals on your devices will also have the amp/hr rate. If for example you are using a device that uses 6 amp/hrs and you operate that device for one hour you will use 6amp/hrs from your battery bank. If you only use that device for 30 minutes then it is only 3mp/hrs. Two hours then 12 amp/hrs. Add all this up determined by how long you use each device each day and you'll have your daily amp budget. You'll be quite surprised on how much power you use each day all depending on the equipment on your boat. Once you've figured this out the rest will be easy to determine what it will take to properly power and recharge "Your" boat.

[DefinitelyMe]

We also have a Waeko fridge, manual water pumps. Most of our lights are CFL, not LED. We also have a separate built-in top-loading freezer, air cooled.

Weaco fridge uses about 40Ah per day in the tropics. Freezer about the same (it's much colder but also much better insulated). Lights, phone and laptop charging and any incidentals 20Ah. At sea the radio and AIS transponder add about another 20Ah per day. We don't have a chart plotter or any wind instruments.

[wingssail]

1. Based on our own experience, on a 43' monohull, your big user will be refrigeration. We use about 80-125 AH per day on our smallish Dometic refer with a littler freezer section inside the evaporator plate. The difference depends on the ambient temperature and the sea water temperature. As the water warms up in the summer our electrical usage also goes up. In cold climates or cold water our usage drops way down, to 1/2 half.

The amount and quality of insulation is important. We re-insulated last year and saved about 15% of the electrical demand.

So, keep it small and have very good insulation.

-85 AH

BTW, water cooling isn't worth the complexity in my opinion, others may have different views.

2. Recharging of devices is the next biggest use. We have 2 laptops, two phones, two tablets, and a rechargeable portable VHF.

These take roughly -25 AH

3. All other uses, (LED lights, pumps, electric tools, appliances:

-10 AH.

4. Watermaker. Our watermaker is small, it supplies drinking and cooking water, not washing water, it runs 1-1.5 hours per day at 3 amps, so 3-5 AH

-3.5 AH

So total=-134 AH.

We have 270wtts of solar and get about 75AH depending on the weather and direction of the sun (hopefully the panels are not shaded).

+75 AH

We run the engine daily, 1-1.5 hours at avg 60AH. (We do not have a genset or wind generator or a large solar array.)

+60 AH

Total inputs = +135 AH

Underway we have increased power usage due to the instruments (older B&G which are not low power) and more computer usage. We may run the engine twice per day on passages. We have LED running lights.

We use a windvane so there is no autopilot use unless we are motoring. We have a very high performance sailboat and we rarely motor.

We have a 450AH lead acid battery bank, which is minimum for our usage.

The batteries are only completely recharged if we get to a dock with shore power or motor for long periods, such as 24 hours.

We carry spares for most of our recharging equipment and much of our essential power usage items.

We have a very simple boat and purposefully have avoided adding items which add to the cost, weight, and maintenance. But it all works, we are self sufficient and comfortable. We have frozen meat for a month and ice cubes each day. We use the computers as much as we want, lights are not an issue.

[a64pilot]

Another thing people don’t understand is it doesn’t really matter how much you do or don’t use, your not charging a battery with a 30min or even 1 hour engine run, it takes up to 6 hours to recharge a battery, maybe a little less if it’s in great shape and not deeply discharged, but your not getting it fully charged in an hour.
It takes 6 hours to recharge a 200 AH bank at 50% or an 800 AH bank at 50%, assuming a large enough charger of course
This is where Solar comes in, run the motor or suitcase generator for an hour in the morning to get a lot of bulk charging done and by lunch you may actually be fully charged on Solar. A Honda can add 100 AH to a bank if the bank can take it, in an hour.
Of course if you have enough Solar and don’t deeply discharge, then Solar may get you to full by itself.
But run the numbers, if you use 100 AH overnight, night is often about half the 24 hr day, so if you do anything during the day that uses electricity then you are maybe 125 AH for the day, or 225 for a 24 hr cycle.
Now Solar on average will make 1/3 of it’s rated capacity in watts, in AH, so to provide 225 AH you need three times 225, or 675 watts, but as a battery is not 100% efficient you need to put more back in than you took out so 225 turns into 250, and that’s 750W of panels.
If you can’t have 750W of panels you need to run your motor, accept a walk down in charge every day, or get a suitcase generator, and we aren’t even talking about cloudy / rainy days.

It’s been my observation most are walking down in charge every day and if they stay in an anchorage for more than a week, they end up waking up to a dead bank.
Most motor more than they like to admit and as long as they do motor a decent amount of time when they move locations, the bank gets fully charged.
So to prevent all of that, consider a Honda suitcase generator, for an hours worth of noise a day and less than a quart of fuel, your batteries assuming you have Solar can get to an honest full charge every day, even those rainy / cloudy days.

In my case I use 150 AH overnight, so at least 300 for a 24 hr day, my 1000W of panels can’t usually quite make it, but a 30 to 60 min Honda run gets me to fully charged by noon, cause that Honda does two things, first it extends the charging time by running to before Solar is making any real power, and secondly it’s running my charger at 100 amps as long as the bank will accept that much.
I cruise Winers and of course in the Northern Hemisphere there is less sunlight in Dec and Jan than in the middle of Summer.

[Suijin]

Based on my experience, it's almost impossible to make up an energy budget in the abstract, without real world experience on your boat with the systems that are drawing power.

As noted, refrigeration is going to make up the majority of your energy usage. It's daily draw is going to depend on a myriad of factors: whether you have reinsulated it, how many times a day you open it, how full you keep it, whether your hull is dark or white, ambient temperature, sea temperature, etc.

Your second largest draw is going to be computing, or TV if you have it, and that is going to depend entirely on how much you use either. Some barely use them, others spend hours using them.

Are you going to leave your VHF on 24/7, and do you need to leave your NMEA2000 backbone and instruments on 24/7? I do. That eats up 15 amp hours per day.

LED lights, pressure pumps, and all the rest are virtually negligible, individually, but add up depending on what your habits are. I generally leave one light on in the boat, sometimes two, but I'm alone on the boat. As a result I consider everything other than fridge, laptop, and VHF to be essentially incidental power usage.

You will be much better served by leaving your battery refit until you've spent time on the boat in the conditions that you expect to encounter and actually monitored your usage. In addition, if you have solar and will be refitting it, you'll see the pattern of output based on seasons, cloudy days, etc which will all contribute to your designing charging systems.

Also, battery capacity declines over time. If you have AGMs, they will lose capacity between equalization and obviously over their lifespan. I would design your system with a buffer, basically starting with usable amp hours at least 20% greater than what your "heavy day" needs are, if you can fit the capacity and charging to support it.

[wingssail]

Originally Posted by a64pilot

Another thing people don’t understand is it doesn’t really matter how much you do or don’t use, your not charging a battery with a 30min or even 1 hour engine run, it takes up to 6 hours to recharge a battery, maybe a little less if it’s in great shape and not deeply discharged, but your not getting it fully charged in an hour.
It takes 6 hours to recharge a 200 AH bank at 50% or an 800 AH bank at 50%, assuming a large enough charger of course
This is where Solar comes in, run the motor or suitcase generator for an hour in the morning to get a lot of bulk charging done and by lunch you may actually be fully charged on Solar. A Honda can add 100 AH to a bank if the bank can take it, in an hour.
Of course if you have enough Solar and don’t deeply discharge, then Solar may get you to full by itself.
But run the numbers, if you use 100 AH overnight, night is often about half the 24 hr day, so if you do anything during the day that uses electricity then you are maybe 125 AH for the day, or 225 for a 24 hr cycle.
Now Solar on average will make 1/3 of it’s rated capacity in watts, in AH, so to provide 225 AH you need three times 225, or 675 watts, but as a battery is not 100% efficient you need to put more back in than you took out so 225 turns into 250, and that’s 750W of panels.
If you can’t have 750W of panels you need to run your motor, accept a walk down in charge every day, or get a suitcase generator, and we aren’t even talking about cloudy / rainy days.

It’s been my observation most are walking down in charge every day and if they stay in an anchorage for more than a week, they end up waking up to a dead bank.
Most motor more than they like to admit and as long as they do motor a decent amount of time when they move locations, the bank gets fully charged.
So to prevent all of that, consider a Honda suitcase generator, for an hours worth of noise a day and less than a quart of fuel, your batteries assuming you have Solar can get to an honest full charge every day, even those rainy / cloudy days.

In my case I use 150 AH overnight, so at least 300 for a 24 hr day, my 1000W of panels can’t usually quite make it, but a 30 to 60 min Honda run gets me to fully charged by noon, cause that Honda does two things, first it extends the charging time by running to before Solar is making any real power, and secondly it’s running my charger at 100 amps as long as the bank will accept that much.
I cruise Winers and of course in the Northern Hemisphere there is less sunlight in Dec and Jan than in the middle of Summer.

A64, I am just a little confused about how a "suitcase" genset can perform such miracles. Producing 2000watts continuously for a quart of fuel? OK that's 140amps at 14 volts so, OK, it could drive your AC battery charger at 100 amps, considering losses of two or more power conversions. But why is that better than me running my engine for the same time, using .3 gal of diesel per hour and producing roughly 80-100 amps of DC with my Balmar?

That honda must have a marvelous alternator and an excellent regulator and superb cooling for the whole package.

On the other hand, motoring a lot simplifies the problem. Maybe that's it.

[Pizzazz]

In my case, we have four critical items, fridge, autopilot, radar and watermaker and each consumes around 50AHrs. The radar I usually switch off when not needed.

Then you have the iPads for the kids (play and learn), the TV and the PlayStation. The PlayStation alone draws 18A + 6A for the TV which can kill any energy budget on any small boat if used excessively. Lighting, water pumps, power tools is relatively small.

Then you have convenience items such as the microwave, the pressure cooker, the diesel heater. The good thing about these items is that if you are low on power you can live without them.

Bottom line is that if you want to have conveniences and happy kids, your energy budget can easily go above 300 AHrs. This is impossible to recover without a generator or significant motoring (I chose the latter for now).

[SY Kelpie]

Thanks for all the replies. I had done a back-of-envelope calculation and come to a bare bones figure of about 85Ah, maybe up to 120Ah if I got a bit more generous. So it's good to see that I'm not too far off the mark with this, but of course everybody will come up with a different figure.

As I understand it there are two different but interlinked problems to solve- charging capacity and battery capacity. I plan to install as much PV as can be realistically fitted on the boat- it is so incredibly cheap now that I am tempted to make my bimini out of rigid panels, which would give me nearly 1000w. I already have a 30w panel on the coachroof and a 100w panel on the guardwires; a second guardwire panel would give me a total of about 1230w. All for under £200, if you shop right. Crazy.

Battery capacity seems to be a function of how much power you use in a day, and whether you want the bank to last you through periods of cloud/rain. But batteries are pretty expensive, compared to solar, so I think we will likely find a balance here which involves relying on engine charging through dull days rather than having an oversized battery bank.

[Cheechako]

As with many things in life, getting the last 5% is the hard part. I just never worried about getting a flat out full charge. Why worry about it? It wont be fully charged very shortly anyway, so why spend a bunch of time and money to get it?
Of course I used mostly a engine alternator or 12V generator setup. I like the simplicity of it and I motorsail anything below 3.5 knots of speed.
At anchor though, solar would have been nice, as long as it was minimal area of panels.

[SY Kelpie]

Originally Posted by Cheechako

As with many things in life, getting the last 5% is the hard part. I just never worried about getting a flat out full charge. Why worry about it? It wont be fully charged very shortly anyway, so why spend a bunch of time and money to get it?
Of course I used mostly a engine alternator or 12V generator setup. I like the simplicity of it and I motorsail anything below 3.5 knots of speed.
At anchor though, solar would have been nice, as long as it was minimal area of panels.

Getting back to 100% is pretty important if you want your lead-acid batteries to last a long time.
I am actually very tempted by LiFePO4 as they don't mind what level of charge you get them back to, so long as you don't leave them completely flat. They also accept charge much more readily, so the last 10% of charge is no harder than the first 10%. It's a shame they're so expensive!

[Cheechako]

Originally Posted by SY Kelpie

Getting back to 100% is pretty important if you want your lead-acid batteries to last a long time.
I am actually very tempted by LiFePO4 as they don't mind what level of charge you get them back to, so long as you don't leave them completely flat. They also accept charge much more readily, so the last 10% of charge is no harder than the first 10%. It's a shame they're so expensive!

Yeah, I hear that a lot. Empirical experience doesn't seem to support it, but theory does I guess. My last 3 car/truck batteries have lasted almost exactly 5 years. Those are always charged up of course. My boat batteries always lasted at least 5 years. The gel's I had never did die under my ownership and came to me in the boat used.
The other interesting thing is the last two boat batteries I bought were at 12.45 and 12.46 volts when fully charged. (measured after sitting overnight disconnected) So I just don't worry about it much.