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Old 05-04-2015, 15:06   #1
Zai
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The nightmare begins!! Where to start power consumption calcs?

I started an excel spreadsheet to figure out the correct size battery bank and charging system. What became apparant at once is, the difficulty in estimating the power consumptipon of all the various power hungry devices we use. For example a Webasto 5500 air top heater says in the specs. that it will use 15/95 watts. ??? That is quite a range and obviously I must use a figure somewhere in between.

36 foot Bavaria/30 L/H watermaker/Original fridg./auto pilot/Radar-chartplotter/AIS trsp

My original thoughts are a battery bank of aprox. 500 amps (gel or AGM), 450 watts solar (9 hours/day), 400 watts wind (calculate at 200 watts for 4 hours a day).

I am making a power consuption comparison for sailing days vs. at anchor.

Anyone with advice/links to threads (I've searched a bit)/own experience?
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Old 05-04-2015, 15:19   #2
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Re: The nightmare begins!! where to start power consumption calcs?

here you go, knock yourself out!
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Old 05-04-2015, 15:51   #3
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Re: The nightmare begins!! where to start power consumption calcs?

In theory if you are working on an average daily budget, so you estimate average daily usage.

For example...we have an 800W microwave. But if it gets more than 5 minutes of use in a day that is a lot and unusual. At 12V that uses up about 5.5 Ah total ((800/12) * (5/160)) - Peukert is studiously ignored!)

A computer might draw 5-6 Amps (call it 80W), but be on for 8-12 hours per day or even 24x7 if you are passage making. So that is going to take 40-72Ah off of your batteries even though the draw is a lot lower its the estimated time of consumption.

So you need to make some educated guesses about your average time on for all your devices to plug into the spreadsheet above.

I know it is complicated, but you could (or some say should) work your budget around a few scenarios. For us, our energy profile at anchor when the kids are using laptops, the microwave sees use, more cabin lights are on at night etc. Is VERY different than on a passage. We actually use a lot more power on passage as we have instruments, chartplotters, autopilots, etc. running on top of the refrigeration and other baseline systems.

So to get an accurate assessment you probably have two scenarios...three if you want to pick an "average" model.

1) At rest
2) Under way
3) "Average" model if you want only one

Plug in the numbers, make your estimates and make sure your batteries and chargers can handle both scenarios.
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Old 05-04-2015, 15:57   #4
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Re: The nightmare begins!! where to start power consumption calcs?

Zai,

As you will find out there are a number of ways to sort this out - none of which will give you an exact answer, but all will give you something close enough.

You will find a good number of your loads will have a high & low setting. Your webasto obviously has high/med?/low settings as do the fans on our boat. They way I've dealt with it is the following:

Fan #1 0.5A @ low for 4 Hrs/day, 1.0A @ high for 4 Hrs/day => 6AHr/Day

For many of the items on board the best way to get the numbers is to measure it.

With regards to anchor vs under sail - pick the larger numbers, a larger battery bank is better in my opinion as it will last longer as your discharges will be lower.
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Old 05-04-2015, 15:58   #5
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Re: The nightmare begins!! where to start power consumption calcs?

And underway for how long. You will be fine with 500Ah and 450W for at least a few days sailing in most cases and have more than enough at anchor. Also fine for longer crossings but you would need to be more energy conscious that usual and consider maybe having to run the engines for 2 hrs a day in neutral, using 2l of fuel, to keep the batteries topped up.
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Old 05-04-2015, 17:16   #6
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Re: The nightmare begins!! where to start power consumption calcs?

If you have a way to measure current consumption, that can help you fine-tune your numbers. My battery monitor/charger/inverter has an Ammeter I use to measure the consumption of my various devices. Make sure you turn off all charging sources when measuring current consumption.

Many devices will have an intermittent consumption (refrigerators, heaters, autopilots, electric windlass, etc). You can watch the devices as they cycle to get an estimate of the average load. Remember that your refrigerator will cycle on more often in the tropics, and less often in cooler climates.

I've found that there are a few devices that use most of the power. At sea these will be chartplotter, computer, refrigerator, autopilot, nav lights, etc. My masthead light used to be the biggest drain (or close to it), until I switched to a LED tricolor. Likewise my computer. As technology has progressed I've been able to find good low-power computers, which has helped. I've been able to arrange my nav gear so I don't have to leave the chartplotter running 24/7.

My power budget has shown me where the low-hanging fruit is in my quest to run with minimal use of the engine for battery charging. Careful current measurements have made my power budget more accurate. Not perfect, but much better than a wild guess!
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Old 06-04-2015, 02:47   #7
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Re: The nightmare begins!! where to start power consumption calcs?

Thanks for all of the info!

Helps me re-think things a bit, for example: that the chart plotter uses much more of the daily budget than I thought, what about durring long passages to switch it off for the most and only using it to verify once in a while (durring daylight hours)? Running the radar on standby for the most and transmitting on a regular schedule (every half hour for example)? On longer passages not running the fridge (dried foods and/or canned). On cloudy days doing more manual steering, etc.

Now I am getting inspired, thanks again!
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Old 06-04-2015, 05:46   #8
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Re: The nightmare begins!! where to start power consumption calcs?

You will find that the underway consumption is the controlling amount to plan for. Attached are the sheets I made up and use.
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File Type: xls Power comsumption @ mooring weekdays.xls (57.5 KB, 141 views)
File Type: xls Power comsumption underway spreadsheet.xls (55.5 KB, 130 views)
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Old 06-04-2015, 06:20   #9
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Re: The nightmare begins!! where to start power consumption calcs?

Whatever you decide ... right or wrong ... high or low ... you'll adjust your power usage to what you've got, and you can always add more later, or decide to live within your power capability.
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Old 06-04-2015, 06:40   #10
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Re: The nightmare begins!! where to start power consumption calcs?

Quote:
Originally Posted by sailorboy1 View Post
You will find that the underway consumption is the controlling amount to plan for. Attached are the sheets I made up and use.
Here's the link to the power consumption worksheet I created back in 2009:

Amp Hour Load Calculator - Did I Miss Anything?

There's also a good discussion from from knowledgeable people on that thread in regards to power consumption. As you can see the spreadsheet I created is the same one that sailorboy1 is now claiming to have created.

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Old 06-04-2015, 06:41   #11
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Re: The nightmare begins!! where to start power consumption calcs?

Just forget the spreadsheets and get a battery monitor, or better still a SmartGauge and an Ah meter that measures charging and discharging - then average the Ah discharge only that you get over 24 hours, both at anchor and under sail. Make sure you don't run the engine or have any charging from wind or solar during these 24 hr tests.
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Old 06-04-2015, 06:43   #12
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Re: The nightmare begins!! where to start power consumption calcs?

Quote:
Originally Posted by GeoPowers View Post
Here's the link to the power consumption worksheet I created back in 2009:

Amp Hour Load Calculator - Did I Miss Anything?

There's also a good discussion from from knowledgeable people on that thread in regards to power consumption. As you can see the spreadsheet I created is the same one that sailorboy1 is now claiming to have created.

I didn't claim to have created them. But to be clear I didn't "create" them just modified them for my use (must be important to someone)
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Old 06-04-2015, 06:49   #13
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Re: The nightmare begins!! where to start power consumption calcs?

Grab a copy of Nigel Calder's Cruising Handbook (this one has a whole section in chapter 4 on energy audits) and his Mechanical and Electrical Manual....I don't know what I'd do without them....


www.learntoliveaboard.com
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Old 06-04-2015, 07:33   #14
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Re: The nightmare begins!! where to start power consumption calcs?

Quote:
Originally Posted by Zai View Post
Thanks for all of the info!

Helps me re-think things a bit, for example: that the chart plotter uses much more of the daily budget than I thought, what about durring long passages to switch it off for the most and only using it to verify once in a while (durring daylight hours)? Running the radar on standby for the most and transmitting on a regular schedule (every half hour for example)? On longer passages not running the fridge (dried foods and/or canned). On cloudy days doing more manual steering, etc.

Now I am getting inspired, thanks again!

Exactlyz Zai. One problem with our Raymarine plotter. It has a standby function, but won't go into standby when radar is running, even on timed transmit, which is unfortunate. That was my plan also crossing the Atlantic but we ended up running both a lot to track squalls and chewing a fair bit of power. Same with the fridge. We have two so the plan was use them both the first week then transfer the content,s if the second to the first and turn it off, then turn the second off after a week if need bel we ended up leaving both on all the way but it's good to plan different scenarios. Yes to manual steering and also making sure the ap doesn't work harder than it needs to by best sail trim. We have over 1 kW of solar and 800Ah batteries and at anchor we never go below 90% charge. Last boat had 600W and 540 Ah and also fine at anchor. I think around the same W solar as Ah batteries is a good rule of thumb. We had a wind gen before but not now.
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Old 06-04-2015, 08:21   #15
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Re: The nightmare begins!! where to start power consumption calcs?

It's an estimate. Trying to be too accurate is a waste of time because you will never have a day that exactly matches. Make some reaonable assumptions, add 20% (to cover new or missed items) and move on.
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