Thread Tools Search this Thread Display Modes
Old 03-03-2016, 03:48 PM   #1
Veteran Member
City: Naperville
Country: us
Join Date: Dec 2015
Posts: 45
Another electricity question

Sorry guys, need your help again. I've read some books, now trying to practice my understanding of batteries. Could you please verify my calculation below? Nothing is real, purely trying to work out the formula:

If I want to calculate how much battery do I need for overnight staying on a boat, suppose I don't run generator, one "night" I mean 12 hrs(some of the numbers are rounded):
One Refrigerator: 150w x 12hr = 1.8kwh per night(the 150w number I found on line, average)
One Freezer: 100w x 12hr = 1.2kwh per night
Ten Bilge pumps(assume they runs twice an hour, for 5 minutes per time): 100w x 10 x 24 x 5mins / 60mins = 2kwh per night
One Air conditioner(assume it runs continuously): 900w x 12hr = 11kwh

So in total I need: 1.8+1.2+2+11=16kwh

If I buy battery rated 300ah, I need: 16000wh / 12v / 300ah = 4.5
So I need to install at least 5 batteries(again, not counting discharge rate, 50% reserve etc).

chicagoq is offline   Reply With Quote
Old 03-03-2016, 04:06 PM   #2
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 5,709
Yes, with the numbers you have presented, I think your calculations are correct. In practice, you would need double the batteries to keep discharge level to 50%. And you probably already know this, but a bunch of your numbers are much higher that they should be in any real situation.

twistedtree is offline   Reply With Quote
Old 03-03-2016, 04:19 PM   #3
JDCAVE's Avatar
City: Lions Bay, BC
Country: Canada
Vessel Name: Phoenix Hunter
Vessel Model: Kadey Krogen 42 (1985)
Join Date: Apr 2011
Posts: 2,379
The bilge pump calculation surprises me. Mine almost never come on. Even for a wooden vessel, that's a lot of time on the pumps. Can you provide the details on these pumps? Also, the fridge and freezer won't be running continuously, if they are AC units. Also, I think it would be very taxing on your batteries if you were running an AC unit continuously.
JDCAVE is offline   Reply With Quote
Old 03-03-2016, 04:30 PM   #4
Veteran Member
City: Naperville
Country: us
Join Date: Dec 2015
Posts: 45
Thank you guys! Yeah, the refrigerator/freezer numbers I found on line, for Bilge pump and air conditioner, I really have no idea how long/often they would need to run, I guess it would have to depend on the condition of the boat(which I don't even have one) and weather etc, hence just throwing some random numbers just to get the calculation going.
chicagoq is offline   Reply With Quote
Old 03-03-2016, 04:34 PM   #5
Britannia's Avatar
City: SF Bay Area
Country: USA
Vessel Name: Stillwater
Vessel Model: Kadey-Krogen 54
Join Date: Dec 2014
Posts: 782
I also agree that the calcs look correct but the numbers are high. If you have ten bilge pumps running a total of 10 mins/hour then I think your boat may be sinking!

I don't think most people consider running an Air Conditioner from batteries - I think that they fire up a genny for that. Mine run on 220V so I have run a genny.

Britannia is offline   Reply With Quote
Old 03-03-2016, 04:48 PM   #6
ranger42c's Avatar
City: Maryland
Country: USA
Vessel Model: 42' Sportfish
Join Date: Aug 2013
Posts: 5,232
The math looks right enough, but...

An AC would be a big draw. Believe that's where most would fire up the genset, charge batteries while at it. Ideally an AC wouldn't run continuously, though. But it also needs a water pump; can't tell if that's included in your theoretical 11kwh for AC.

(Fans instead would be a different but postulate-able draw.)

Our fridges don't run very often. although that's influenced by how many beers I retrieve (how many times we open the doors).

Our bilge pumps (almost) never run. If/when they do, I'm all over that to figure out why.

If you were to leave any electronics components turned on -- AIS transmitter for example, or a GPS/plotter so you can use the anchor alarm, maybe a VHF on monitor -- that'd be a continuous draw.

An incandescent anchor light can be a big draw (LEDs much better for that role).

Any other cabin or deck lighting would be a draw, assuming you stay awake -- and use lighting -- after sunset.

Water usage, fresh water pump would be a draw.

TV? Stereo?

Much of what I'm adding assumes you're actually alive and doing stuff on there... so the boat's just not out there enjoying the evening all by itself.

If it helps, I can tell you I didn't beat myself up compiling an energy budget (although I did eventually get a round tuit, posted it here somewhere). I just improved the quality of batteries in our existing banks (600 Ah in 2 banks) and got on with life. Gradually worked out that we can get by quite easily by running the genset for an hour in the am while cooking breakfast (electric cooktop, toaster, microwave, coffeemaker) and then again for a couple hours in the early evening while we prepare dinner. That keeps us more or less above 50% SOC on each bank all the time (and that's with a fridge on each bank).

AC is a separate deal, for us; we only run that when the genset is running... but we also tend to not need it toooo much when we anchor out... except for sometimes in July and August, and we can often just choose to not anchor when conditions aren't wonderful for that.

South River, Chesapeake Bay
ranger42c is offline   Reply With Quote
Old 03-03-2016, 05:04 PM   #7
Senior Member
bilge53's Avatar
City: Oriental, NC
Country: USA
Vessel Name: M/V Major Award
Vessel Model: Senator 35 w/single Lehman
Join Date: Oct 2007
Posts: 423
Just a suggestion, but have you considered chartering a vessel and experiencing actual usage of the boat's systems? It might help you get everything in perspective. Even though boats are all different they do all have some similarities.
bilge53 is offline   Reply With Quote
Old 03-03-2016, 05:17 PM   #8
Veteran Member
City: Naperville
Country: us
Join Date: Dec 2015
Posts: 45
I did realize the AC would be a huge draw comparing to other appliances. Even if it doesn't run continuously, likely need a gen.

Thanks for the advice! I plan to just read books, post insane questions on line for a couple of years(so pls bear with me for a while , then definitely join a club, then eventually buy my own boat and move off land. Not in a hurry!
chicagoq is offline   Reply With Quote
Old 03-03-2016, 05:19 PM   #9
DavidM's Avatar
City: Litchfield, Ct/Punta Gorda, Fl
Country: USA
Vessel Model: Atlas Pompano 23
Join Date: Aug 2012
Posts: 5,418

Your calculations are ok as far as they go, but your assumptions are off the wall:

1. Nameplate running amps on refrigerators assume full time operation. Most home, ie 120V refrigerators operate about 20% of the time. Marine, DC Danfoss compressor types operate 30-50% of the time. So your calculation of refrigerator watthours are several times too high.

2. Same with freezers.

3. If your bilge pumps are running that often and for that long, you need a new boat. A bilge pump should run for maybe 5-10 seconds every 3-4 hours if you have a conventional dripping stuffing box. Otherwise, bilge pumps should never run.

4. 900 watts is a pretty small A/C. The typical marine A/C is 16,000 BTU and takes about 15 amps at 120 V to run or 1,800 watts. A/Cs typically run only half the time or less, particularly at night.

5. I don't know of any 12V, 300 amp hour batteries that are commonly used in marine service. Even a big 8D (which I sure don't want to haul around) is only 220 AHs.

6. You absolutely can't ignore the 50% discharge rule of thumb and you should include inverter efficiency in your calcs as well.

So come up with some realistic duty cycle and wattage values for your expected usage and calculate again. You will probably never be able to run a decent size marine A/C from batteries/inverter. Maybe a small portable one like the carry on type, but it will still take some big batteries to do it all night.

DavidM is offline   Reply With Quote
Old 03-03-2016, 05:33 PM   #10
MYTraveler's Avatar
City: West Coast
Country: USA
Join Date: Apr 2014
Posts: 1,227
FWIW, I size my house bank to permit a discharge to 35%, which is admittedly more damaging than 50%, but only a little more so. Xantrex, which I consider to be an authoritative source, has published that recommendation.

More importantly, if you have 10 bilge pumps each pumping for 10 minutes per hour, you need to deal with that leak (or maybe multiple leaks?), even if your batteries can keep up. I have cycle counters on my 6 bilge pumps. I go months at a time without any cycles (and usually when it does cycle it is because a cleaner has open a laz hatch and sprayed water down there).

From what I have seen, not many people run AC off of battery power.
MYTraveler is offline   Reply With Quote
Old 03-03-2016, 05:40 PM   #11
BruceK's Avatar
Join Date: Oct 2011
Posts: 11,238
Originally Posted by chicagoq View Post
Sorry guys, need your help again....
Ten Bilge pumps(assume they runs twice an hour, for 5 minutes per time): 100w x 10 x 24 x 5mins / 60mins = 2kwh per night....
10 bilge pumps??!! I`m genuinely concerned for you going boating. Unless of course, we are being played with here.
2005 Integrity 386 "Sojourn"
Sydney Australia
BruceK is offline   Reply With Quote
Old 03-03-2016, 06:10 PM   #12
High Wire's Avatar
City: Cape May, NJ and Englewood, FL
Country: USA
Vessel Name: Irish Lady
Join Date: Dec 2007
Posts: 3,336
Who on this forum has 10 bilge pumps? Figure one pump a few minutes a night worst case unless your packings are leaking like crazy. Even an hour a night is not much load percent wise.
Your math works but your total load is way high.
I suggest you drop the AC on batteries idea. That's one sure way to prematurely kill your bank. If you want AC away from shore, run the genny.
A typical fridge/freezer found on a 35-40 ft boat draws 5 or 6 amps at about 50% on, 50% off duty cycle.
Irish Lady
1984 Monk 36 Hull #46
Currently in New Jersey.
High Wire is offline   Reply With Quote
Old 03-03-2016, 06:16 PM   #13
River Cruiser's Avatar
City: UMR MM283
Country: US
Vessel Name: Northern Lights II
Vessel Model: Bayliner 3870
Join Date: Jan 2013
Posts: 1,357
If you don't have a gen I would consider a Honda eu2000, a lot lighter than all those batteries and will run a AC all night. I have one we used on our 32' gas powered boat for running the AC at night, no noise in the boat and used less than a gallon of gas. The boat had a 6.5 Westerbeke we used when cooking and hot water but then we shut it off and fired up the Honda to watch TV, charge batteries and for AC. I've read on forums of people not liking the noise in a anchorage but we always anchor in areas with no other boats.

Sent from my iPad using Trawler Forum
Ron on Northern Lights II
I don't like making plans for the day because the word "premeditated" gets thrown around in the courtroom.
River Cruiser is offline   Reply With Quote
Old 03-03-2016, 06:54 PM   #14
TF Site Team
FlyWright's Avatar
City: California Delta
Country: California Delta
Vessel Name: FlyWright
Vessel Model: Marshall Californian 34 LRC
Join Date: Apr 2008
Posts: 12,876
All boats are different, but here's a shot at my numbers as a point of comparison. The items below are items that are typically operated at anchor:

12V: LED interior and exterior lights, stereo, TV, freshwater pump, multiple fans, multiple laptop and device chargers, electric head, 12V Norcold fridge/freezer, 2nd fridge running on 1000W inverter. Two bilge pumps are 12V but they never come on so they're not a factor in my electrical budget. I have lots of minor loads like tank monitors that really don't affect the numbers.

Propane: stove, oven and heat.

110V (Shore power or Honda Generator): 55A shore charger, 110V feed to Norcold Fridge, microwave, electric ceramic heaters.

My house bank is a 660AH bank of 6 golf cart batteries which gives me 330AH to play with. I use 150-180AH per day, so I can get 2 days without recharging if I'm careful. Typically, I run the generator 2-4 hrs per day to keep up, usually in the morning for coffee and evening for shower and dinner prep. If I had solar, I could probably do without the Honda gen.
My boat is my ark. It's my mobile treehouse and my floating fishing cabin. It's my retreat and my respite. Everyday I thank God I have a boat! -Al

@DeltaBridges - 25 Delta Bridges in 25 Days
FlyWright is offline   Reply With Quote
Old 03-03-2016, 11:56 PM   #15
TF Site Team
City: Ex-Brisbane, now Bribie Island, Qld
Country: Australia
Vessel Name: Now boatless - sold 6/2018
Vessel Model: Had a Clipper (CHB) 34
Join Date: Jun 2008
Posts: 7,966
Send a message via Skype™ to Peter B
Originally Posted by BruceK View Post
10 bilge pumps??!! I`m genuinely concerned for you going boating. Unless of course, we are being played with here.
I agree. Now I realise you haven't even got a boat yet, I have to say it is hard for anyone to make solid recommendations, but for example re this bilge pump issue. I have an old fashioned stuffing box type stern prop-shaft gland, and it drips slowly, but I have only two electric bilge pumps, (one manual) and only one of those ever runs unless I activate the other manually, and the one that does only runs for say 1 minute about every 3-4 hours at most - less when at anchor or in the berth. My boat is described in the avatar, so that will give you some sort of idea re bilge pumps anyway. Others have already commented on battery and generator issues, but again, even there you might be getting several carts before their horses...just sayin'...

Peter B is online now   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

All times are GMT -5. The time now is 03:54 AM.

Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012