Amp Hour Load Calculator - Did I Miss Anything?

[GeoPowers]

Hi everyone,

I put together the attached spreadsheet to calculate my daily power consumption, and to a lessor degree as a tool to figure out possible house bank configurations. There are some loads on here I don't have onboard yet, but figured I'd plan for them now rather than later. I think I've covered all the major power draws, but any input as to any missing items or errors in the spreadsheet would be helpful!

I was especially surprised at the heater's power usage, which I got from a 6.5 amp draw from the Webasto 90S itself, and 2.5 amps for the 5 heater fans throughout the boat. I'm pretty sure this is right, but it is very high!

Necessity is the mother of all invention- this all came about after a 4-day weekend on the hook. Even before this, I suspected my 4-year old house bank (464 Ah) was very underpowered with the heater, and the cold weather meant the heater was in use pretty steadily. You can guess the rest of the story- a depleted house bank! On a side note, I also discovered that my starter battery wasn't isolated from the house bank in the manner I thought it was. AND, I also discovered it is a little over a mile to the nearest place to buy a new starter battery from the dinghy dock at Poulsbo, which doesn't have cabs or bus service on Sunday mornings... on the bright side, I got my exercise for 2010 already done!

Thanks, Frank

[hellosailor]

Nice worksheet, Frank!

I think there's one error, the "gas valve" normally should only need power to stay open, when you are using the gas. I assume you mean it is a solenoid valve, and normally those will "failsafe" and shut when power is removed from them. So, that should not be using power 24 hours a day, but perhaps one or two hours a day, depending on how much you cook. I know some boats use a spring-loaded switch (the type sold for bathroom vent fans, or used in Europe for stairway lights) so that you can turn the switch up to 60 minutes--just like a kitchen timer--and if you forget to shut the solenoid power, it gets cut off when the switch runs out anyway. A matter of both safety and power consumption. Also not expensive.

[cdennyb]

I was thinking that the nav instruments excluding the radio and gps and/or radar plus the engine instruments would each take about .5 amps instead of .1 amps (did you include the nightly illumination lighting?). Correct me if I'm wrong.
Nice worksheet, may I use it for my own personal needs?
Thanks so much in advance.

[btrayfors]

Very nice job, Frank!

Two observations.

1. The daily power consumption is WAY high for a 40-footer. Yes, the Webasto will run it up when in use, but it would be prudent to look for ways to cut down on power consumption, since this is often much easier than trying to generate the power needed to replenish the batteries; and

2. Replacing 600AH per day ain't easy. And, if you're really going to go to a huge battery bank, you'll need a much beefier alternator. 80A just won't cut it, nor are the solar panels likely to do much to help...especially in the Pacific NW. This will entail some engineering, good planning, and a lot of $$$.

Bill

[Jentine]

Frank,

The most glaring error I can see is that you expect to use all of the equipment on board each and every day. Do you really make 36 gallons of water per day? Do you have the radar and other navigation equipment and lights on at anchor. Your chart is wonderful, but your assessment is skewed. We currently live aboard in a similarly equipped boat but the main difference is that we are currently in the Florida Keys. My daily consumption is generally replenished by the sun (only minimally, one sunny day in five), the wind (most generously) and when all else fails, by the Honda 2000 generator that sits quite nicely on the swim platform.
We have found that relying on the sun and the wind and the engine will not provide the necessary power when there is no sun and no wind and who, but a fool, would run a main engine for battery charging. We have, only once, after sitting in an anchorage for two weeks with little sun or wind, depleted the batteries to the point where it was necessary to start the engine.
I have 460w. of solar, a KISS wind generator, the standard Hitachi alternator with a Balmar MC-612 regulator (you really need this type of regulator), 660 ah of battery capacity (880 if the starting pair is included), and of course, the Honda generator. We lack no power for any usages aboard, nor do we live like we are camping but power management is a prime concern daily.
I think you have the right equipment but need to work on your conservation techniques. All the power generation in the world is worthless if conservation is not part of the equation.
Should you have any specific questions, PM me.

[goboatingnow]

Theres an old saying that " logic is a way of going wrong with confidence". Your chart is undoutably comprehensive, but very very misleading,

Questions, Chartplotter for 24 hours , why, given the autopilot is being used for 5?, (

Chartplotter looks way too low ( ok for a pure GPS model).

Autopilot is way too low, looks like a steady state figure, in my experience on long distances this is the biggest power draw after reeffers.

For example 8 hours of TV, ( is this at anchor or underway?). If at anchor why autopilot comsumption ( or chartplotter for that matter). DO you need all that VHF or hours heater hours?.


WHat you need to work out is underway comsumption and at anchor comsumption, bring any conservation figures to bear, also look at typical comsumption versus worse case comsumption. There little point in trying to generate X amp/hs as a worse case when all you really need is x/3 amphs usually.

And good look generating 600ah per day.??,

[Hud3]

Pretty spiffy spreadsheet!

I would come up with an energy budget for offshore sailing and one for cruising amongst the islands. Sailing 24/7, you'd have your running lights on all night, and be using the radar from time to time, but much of the other stuff wouldn't be used at all. Cruising and anchoring for the night would have an entirely different energy use profile.

460 AH/day does seem awfully high. In the Caribbean, we used about 150 AH/day on our 38-footer, and the bulk of that was the Frigoboat reefer/freezer. We had 600 AH of AGMs for the house, and could generally charge them in an hour or less with the diesel and the 210 A alternator.

[CharlieJ]

As noted by other posters, your estimate of 460 amp-hr/day is very high.

Key to battery bank design is to make it big enough so that you rarely, if ever, dip below the 50% DOD (depth of discharge), but not so large that your charging sources cannot bring it back to 100% SOC (state of charge) on a regular basis without having to run a lightly loaded diesel excessively.

As a data point, when we cruised the Caribbean in our trawler, we had a new household reefer that accounted for 80% of our amp-hr usage. We consumed 500 amp-hrs per day from our bank of 12 series-parallel T105 batteries for a total house bank of 1,320 amp-hrs. The bank was replenished in about 5 hours with a Balmar unit comprised of a 6.5 hp Kubota diesel directly coupled to a 190 amp large case Balmar alternator.

[hellosailor]

denny-
Instrument loads can be deceptive. The first time I measured a LORAN I found out it drew 440mA. And then, if the LED lighting on the panel was shut, the LORAN itself only drew 40mA! A tenfold difference between what the instrument needed, and what the lighting (night use only and we were daysailing) needed.
With autopilots, even identical autopilots, one user may have a well-balanced boat, well-trimmed, in light winds, while the next guy has an autopilot constantly thrashing about trying to fight the helm. Again, very different loads versus the "rated" power draw. And with radios, of course the question is how much are you transmitting versus receiving? Odds are the rx rating is the one you'd put in the daily spreadsheet, most of us wouldn't be transmitting a whole lot.

Electric heat has GOT to be a killer, but I'll bet it feels good in the morning.

[GeoPowers]

Thanks for everyone’s input! I agree that 460 Ah per day is really high. I put the toggles in the spreadsheet to see what my consumption was during various scenarios. Here’s what I’m thinking I use-

During summer, at anchor only: 180-200 Ah
During summer, sailing and at anchor same day: 225 Ah
During winter, at anchor: 300-315 Ah
During winter, sailing and at anchor same day: 340-350 Ah

These #’s don’t take the radar or water-maker into account, as we don’t have them now. Realistically, we only have use for a radar for the next few years, as we will always be close to good shore water. Right now we coastal cruise. We also don’t have the solar or wind, so for now we are stuck recharging using the 80A alternator. I’m looking at both right now, but that’s another topic…


I revised my sheet so the heater and heater fans are in separate lines to account for the different run times and use of “free” heat when the engine was running. Still, that heater is an energy hog, especially when you consider it is a “diesel heater”! I’m just not sure I’m figuring this right. I still get a little over 100 Ah per day during the winter from it, based on 14hrs on the heater and 6hrs on the blowers. So really this means a 200 Ah increase in the size of my house bank? If so, we obviously need to move south for the winter!


Thanks for the input on the values, which was my main concern. To answer your specific questions:

hellosailor, it is a gas sensor and valve control, but you’re probably right on the rating being a little high. I got the number off of the manual, which I assume is its max consumption.

cdennyb, Nav instruments for me is my tridata and wind display. I got the values from the power consumption off the raymarine website, which are 45 and 65 mA. I thought this was low, too, is this an accurate way to est. the power usage? And of course you can use the sheet, glad you like it.

goboatingnow, I also got the power consumption for the chartplotter/GPS from the specs on the raymarine site. Does this work? The raymarine site shows 2.75-6 amps for the autopilot as you say, so I picked 5 amps. Am I missing any components here? I leave the chartplotter on at anchorage for the anchor alarm, unless it is a place I’m comfortable with. I fixed the TV hours, that helped!

The biggest danger I see in the sheet is verifying run times for the big draw things like the fridge and heater, and the amperage ratings for continuous use things.

Thanks again for your input- Frank

[Hud3]

If you install a battery energy monitor like the Link 10, you can find out how many amps each piece of equipment draws by turning them on, one at a time. The ones that cycle rapidly, like the autopilot, will require a little interpolating by eyeball.

[Bright Eyes]

Great spreadsheet, I got excited when I saw it as I have been trying to decide the best way to keep track of this stuff.

One fun item I purchased was a watt meter, which you just plug things into it and it tells you the voltage, wattage, ampage, time, and keeps a running total.

Amazon.com: P3 International P4460 Kill A Watt EZ Electricity Usage Monitor: Electronics

[hellosailor]

Short of buying a Link10...you can also buy any $20-25 multimeter that has a 10-amp (rather than 2-amp) maximum DC-amp range on it, loosen one wire or pull the fuse behind any instrument, and insert the multimeter in line to directly measure the current. If you exceed the rating of the meter, expect to blow an obscure fuse that is IN the meter, so also try to find spares.

There are also clamp-on ammeters, which used to cost a fortune but even Sears has a model well under $100 bucks now. They'll do alright for higher loads (i.e. 1-100 amps) but for measuring little loads like instruments or the chartplotter--I'd stick to the multimeter. Odds are that most of the stuff will come close to the spec in the manual in "typical" use.

I'm not surprised by the low numbers for the wind and tridata. LCD displays often run on flea power these days, as do the electronics behind them. The lighting is more likely to draw more power.

[Stillraining]

Scratch 100..

[Paul Elliott]

Regarding the use of clamp-on DC ammeters, for measuring smaller values of current I have a ten-turn coil of wire in my kit. You have to disconnect one end of the wire whose current you are measuring (or bridge across an opened breaker or pulled fuse), but the ten turns through the clamp gives you a 10x multiplication factor, so (for example) 0.1A reads as 1A.



It's no more convenient, and probably less accurate than using the 10A range on your DVM, but this trick can still come in handy.

For what it's worth, on VALIS we consume about 200Ah/day when sailing, and significantly less when anchored. Of course we are anchored in the tropics, so don't need to run the heater, and we don't have a TV, but the refrigerator is running at close to a 50% duty-cycle (5A when running).