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Old 15-09-2012, 09:19   #1
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Energy consumption & generation calculation

Hi guys,
I am trying to spec a new 40ft catamaran and need someone with experience to check my assumptions. I don't have the exact consumption of each appliance as yet so am guessing a bit:

Equipment:
- 1 electric winch
- double drawer fridge
- 1 freezer (cupboard size)
- all lighting LED based (includes navlights & anchor light)
- 2 bilge pumps (also pump out showers)
- 1 electric toilet
- 1 freshwater pump
- windlass
- nav electronics including 2 plotters, VHF, autopilot, 4 displays
- radio/mp3 player
- 5 fans
- laptop (2 hours per day off inverter)
- 2 mobile phone charges daily
- 1 ipad charge daily

I am thinking this adds up to around 2,000 WattHrs, add a 25% margin = 2,500.

Assuming a 40% battery capacity utilisation this means a battery bank of at least 520 AmpHrs (2,500 / 12 / 0.4)?

Then to keep the batteries topped up without running engines this would require a bank of solar panels generating 625 Watts (625W @ 85% efficiency = 500W x 5 hours sun = 2,500WattHrs)?

Does this make sense or am I making a mistake somewhere?
Have I forgotten anything obvious?

Thanks!
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Old 15-09-2012, 09:40   #2
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Re: Energy consumption & generation calculation

You figures look ball park correct, but to get a more detailed answer you need to supply more information on the high consuming equipment such as the freezer, fridge and laptop.

On the production side the location and time of year will make a difference on your solar panel output.

Working in AHrs is generally easier and helps compensate for the battery inefficiencies, but you still need to add another 5-10%.
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Old 15-09-2012, 09:41   #3
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Re: Energy consumption & generation calculation

You are along the right lines. I found it helpful to express everything in amp hours so I didn't get confused between 24V and 220V (my boat's voltages). I have two main sheets in my spreadsheet, one for consumption and one for production.

On the consumption side I have two different sets of assumptions, one for at anchor and one for on passage.

Similarly I have two different sets of production - I have a hydro generator which produces 500W at 7 kts (I have a fast cat), and solar. Obviously the hydro only works when under sail
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Old 15-09-2012, 10:06   #4
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Re: Energy consumption & generation calculation

You are probably on track for passages, but consumption seems high at anchor unless the fridge/freezer is a real hog.
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Old 15-09-2012, 10:13   #5
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Re: Energy consumption & generation calculation

What Don says...

As long as you're spec'ing the boat, see if you can set up your electronics to minimize energy consumption underway. For example, in benign conditions it's nice to be able to use the plotters independently, with one of them off, or navigate with an Ipad (via a Muxbus) with both chartplotters off.
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Old 15-09-2012, 10:30   #6
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Re: Energy consumption & generation calculation

Me I would not count the windlass as most of the time the engine(s) are running making electrons. Nav equipment adds up, but I've found that the laptops are not on when moving. so no net extra there either. Though larger autopilots need to be added

Yes, fridge(s), freezer and laptop are the big energy hogs at anchor. Led Lights and fans are lesser, Fresh water pump is intermittent it hardly matters, unless you have lots O people taking showers.

You'll get a bit more the 5 hours of sun. My panels start putting electrons out about 8am and by noon the batteries are getting to 80%. Power is still being produced at 4 pm though not anywhere near rated capacity.

While my calc's showed I needed 100 ah per day. I find that I get by on 70-75 ah a day just fine.. Got to factor in the diversity too The fridge / freezers don't run all the time after all.
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Old 15-09-2012, 12:06   #7
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Re: Energy consumption & generation calculation

The West Advisor: West Advisor Articles

Scroll down to the electrical section, includes worksheets

Here's my spreadsheet, click on the PDF

http://c34.org/bbs/index.php/topic,3976.0.html
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Old 15-09-2012, 12:15   #8
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Re: Energy consumption & generation calculation

Here are my numbers based on your needs:

Refrigerator 4 amps @ 50% = 48 AH
Freezer 3 amps @ 50% = 36
Lighting 2 amps 12 hrs = 24
Nav Elect 4 amps 24 hrs = 96 (only while undeway)
Laptop charging 4 amps * 2= 8
Phones, iPad = 8
Radio, MP3 1 *12 = 12
Fans 2*.5*12 = 12

Total 244

All other loads are trivial. So you are in the ballpark. And FWIW a good rule of thumb is that an X watt solar panel mounted horizontally will make 1/3X in amphours on a sunny day in moderate lattitudes in the summer. Again you are in the ball park.

David
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Old 15-09-2012, 12:32   #9
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Re: Energy consumption & generation calculation

I think you're on the right track, reasonable first approximation, you'd need more detail on both equipment and usage to get much further.

One caution on the solar output, at utility-scale plants we use a factor of 5 to 5.5 times rated output to give us estimated total daily output. That's in line with the value you used, however, these plants are generally at least optimally tilted and pointed or single-axis tracked. On a boat, given all the less than ideal conditions I think I would use something like 4 or possibly even 3.5 to estimate overall solar generation capacity.
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Old 15-09-2012, 12:56   #10
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Re: Energy consumption & generation calculation

Quote:
Originally Posted by cfarrar View Post
What Don says...

As long as you're spec'ing the boat, see if you can set up your electronics to minimize energy consumption underway. For example, in benign conditions it's nice to be able to use the plotters independently, with one of them off, or navigate with an Ipad (via a Muxbus) with both chartplotters off.
Your LED lights need to be PWM or they will suck power. The fridge system you pick is important. Some are really good and a few are cheap up front and costly long term. You can't install too much battery & also assume you only draw down to 50% - no farther, depending on the battery type. I agree regarding the windlass, it only operates when the engine is on.

Selection of the instruments is critical. They will run a lot. I chose Simrad NSE-8 because, at the time, they were the only guys with an LED back-lit monitor. The power used was 1/2 that of any other plotter. We installed broadband for the radar - very low power use compared to conventional. All instruments are networked BUT all can run independantly. This means we have more than one GPS antenna as there is one for the VHS, Chartplotter, AIS. We can choose to not run parts of the system if conditions allow. You will want a GOOD DC-DC converter to power instruments unless they are picked so that they tollerate a wide range of input. Some instruments require 12 volts. Higher end stuff will accept any boat DC (9 to 30 or so).

Will you ever want to add a watermaker?, Air conditioning? Probably need a generator then.
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Old 15-09-2012, 13:06   #11
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Re: Energy consumption & generation calculation

What, no radar? Heh.

I agree it's hard to estimate stuff like the windlass (do you have a mechanical option?) or the electric winches (are they used all the time or just for mainsail furling?). As it's a cat, you have a vested interest in keeping the batteries few and located centrally, so you might consider keeping your rough numbers until you can tally real-life ones, and installing "dock" and "anchored/at sea" practices.

For example, there's nothing to prevent you plumbing a second fitting into your water tanks and having foot and/or hand pump water in galley and head. Some heads, for that matter, can be run in a dual fashion. Catamarans are good candidates for wind scoops, and because they have quite a bit of speed potential underway, wind gens on stern poles can work better as well. The wide sterns of cats can also support multiple solar panels in the form of a bimini, and the arch supporting them can be cambered (rather than dead flat) to increase useful solar harvesting time.

So I am saying you have options to significantly reduce your reliance on amps with a little planning and not much cost, and to increase your ability to make amps in the first place without adding much weight. I haven't even touched on beefing up the insulation in order to reduce refrigeration cycles.

Calories into muscle is almost always more efficient than amps into lead, however. If this is an option, take it.
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Old 15-09-2012, 13:32   #12
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Re: Energy consumption & generation calculation

Quote:
Originally Posted by Nicholson58 View Post
Will you ever want to add a watermaker?, Air conditioning? Probably need a generator then.
Air conditioner? You will need a generator (or something that makes a lot of power).

A watermaker can be pretty frugal. I have a Spectra watermaker that burns 15A @ 12V and makes 12 GPH (15Ah makes 12 gallons). This isn't too bad.
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Old 15-09-2012, 13:51   #13
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Re: Energy consumption & generation calculation

Quote:
Originally Posted by noelex 77 View Post
You figures look ball park correct, but to get a more detailed answer you need to supply more information on the high consuming equipment such as the freezer, fridge and laptop.

On the production side the location and time of year will make a difference on your solar panel output.

Working in AHrs is generally easier and helps compensate for the battery inefficiencies, but you still need to add another 5-10%.
Not to be argumentative, but I like Watts or WattHours myself. If you don't know if he is running 12V, 24V, 48V or 120V/240V. Watts are pretty much the same since V*A=W

The insulation, and indoor/outdoor temperature will impact the amount of energy the refrigerator/freezer uses too.
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Old 15-09-2012, 14:24   #14
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Re: Energy consumption & generation calculation

I would say that you are being overly optimistic on how much power you'll get from the solar panels. I have 300 rated watts and averaged about 55 ah per day over the winter in the Bahamas and about the same this summer in Maine. The reason we did so well in Maine is because we had less clouds than in the Bahamas last winter. Using your formula we should have had about 1275 watt hours available but in actuality we had about 660. Our very best day was in the Bahamas in May when we got 88 ah or 1056 wh. That was a very sunny day with a very high sun angle and few clouds and the wind blowing the right direction so that we had few shadows from the rigging on the panels.
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Old 15-09-2012, 14:45   #15
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Thank you for all the valuable comments and ideas.
I am going to have to fine tune the consumption figures.
I'm not sure to what extent the choice of batteries and panels affects these numbers. I assume some brands are more efficient than others.
Would it be a good idea to have more battery capacity / AH than I need or is it best to try to balance the consumption, generation and storage?
What happens if my panels generate more power than I can store?
Or if my batteries are underutilized?
Maybe not so important....?
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