October 4, 2010

Power, Power

Since it's a topic of interest to many cruisers and would be cruisers, I (Evan) thought I'd do a post on electricity generation and consumption aboard.

Electrical Generation and Storage

Our last boat had one computer, no SSB, no watermaker, no refrigeration, radar, etc. etc. This one has 3 computers, a SSB + Pactor modem, refrigeration, radar, a watermaker.

On little Ceilydh (our old 30' monohull) we could get by quite fine with a 50 W solar panel and 220 A.hrs of batteries.  The new Ceilydh has 375 W of solar panels and 450 A.hrs of batteries (4 x 6V golf cart batteries). Both amounts of battery and solar panels seem to be about right, with the big Ceilydh having more margin in real world conditions for cloudy days. Multiple cloudy days in a row means we might have to cut back on computer time. But so far we have only had to run the engine for power a few times last Christmas during rainy, short December days. Peak power we have seen from the solar panels is 23 Amps, but typical mid day output is around 19A for a few hours, dropping off to 5 or 6A at the end and beginning of the day.  On a sunny day in summer, where we have many sunny days in a row we'll be charged by mid morning.

With all the equipment on new Ceilydh we needed serious solar panel capacity; running an engine driven alternator an hour or more a day, having a wind generator in a windy anchorage, a separate generator or some combination of these. We like solar panels because they are quiet, low drag, long warranty periods and minimal maintenance (dusting is about all you do to them). We have a stock 55A alternator on our engine.

Electric refrigeration aboard might take 50 A.hrs a day (more on really hot days of 35C where we are adding many litres of drinking water to cool down). When I built our fridge, I used 6" of insulation everywhere except the door (3" there). I also kept the box size down. It's about 3 cubic feet.

Our stripped down Spectra watermaker runs at 8 Amps, for about 3 or 4 hours, but not every day, usually every 4 or so days. It's not that big a consumer overall.

Our desktop computer almost deserves a whole posting on it's own. Diane is a freelance writer and on the days she is sitting in front of the computer for 8 hours, we do notice the power drain. It's a small case desktop that I built using a lot of research to keep the power down. Here's what makes it tick, and what we save power on:

  • Intel P8400 Core 2 Duo CPU. This is a low voltage laptop chip that is actually quite powerful. I can use it for Autocad and Rhino 3D work and also processing photos using Lightroom. If you are choosing a chip, look up the maximum TDP wattage; ours uses 25W running flat out but many other are 35W.
  • a mini ITX motherboard that actually supports this mobile type of chip(hard to find). We used an industrial type with lots of serial and USB ports. MSI brand, model IM-GM45
  • A 12V power supply that directly gives the clean 12/5/3 V that motherboards require. It's about 90-95% efficient. Ours snaps directly onto the motherboard power supply connector
  • a 2.5" laptop hard drive. Uses lots less power than standard 3.5" hard drives
  • slim DVD writer (like that found in a laptop); again less power than standard DVD drive
  • 15" LCD monitor - that uses 12V directly. These are getting harder to find. Most LCD monitors have built in 120V power supplies these days instead of 120V/12VDC "wall warts" but newer larger monitors are lower in power. [update - in 2013 we now have a 21" LED backlit monitor that uses less power than the old one]
What's the bottom line on the computer power consumption? About 3.5A, including the monitor. Better than many laptops these days!

On a passage we're not using the computer that much (checking weather only via SSB; no electronic charting), but our autopilot is the big user of power. It's a Raymarine ST4000+ tiller pilot. I'm not sure about the consumption because it varies so much with conditions, but I can say the rate gyro in the control head makes a big difference in how much the ram operates (more operation of the ram but maybe the boat doesn't get as off course as a dumber pilot).

We have a masthead LED tricolour that sips electricity when sailing. Steaming and side lights that we use under power use incandescent bulbs - no need to save power when motoring.

Most of our interior lights are LED and these use almost no power. We have fluorescent lights in the galley but these are only on for about 1 or 2 hours at most. Bathroom lights are incandescent because they are only on for 5 or 10 minutes and the higher cost of LED lights do not make sense with lights that are seldom on. Our anchor light is also LED and it is around 0.1A.

Fans on boats in the tropics can run 24 hours a day (that's why it is important to get the ones rated for 5000 hrs!). Here's my take on the fans we have:
Hella Turbo 0.5A on full, somewhat noisy, but move a reasonable amount of air. One in the galley which can blow out the portlight for exhausting hot air or right at the cook. Another one at Maia's bed.
Caframo "Bora" uses a piddly 0.25A on full, reasonably quiet, moves 90% of the air flow compared to the Turbo. Our current top choice for fan that is on for many hours (like over the bed). One on our bed, two in the saloon
Caframo 747. Uses a hefty 0.59A on full, but moves a ton of air. Open blades are easy to clean but we keep whacking ourselves on them. Integral blade guard prevents injuries. This one sits beside the computer and is used on really hot days for people working on the computer for hours or sitting at the settee. A good choice for this application
Caframo $25 cheap fan that I am too lazy to look up the model name. Noisy, moves neglible air. We have this on the guest bed :)
(if you have a limited fan budget then consider getting a few fans and equipping them with 3.5mm stereo plugs and have a number of 12V DC stereo plugs around the boat to plug them into. These are much cheaper, smaller, and more secure than 12V cigarette lighter plugs).

So the essence of a happy electrical system is generation of sufficient power to meet your needs - and make sure your electrical demands are kept in check by careful equipment selection.
Questions? Please leave a comment!
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Jonathan said...

Evan, thanks very much for the summary of the power consumption. If at some point you want to repost, here are my questions:
Do you know what the average daily total amp hours is?
Did you determine this by estimation or with an amp hour meter?
What kind of frig system do you use? (Danfoss, water cooled, air cooled, keel cooled? Brand?) 50 amp hours sounds very good in such a high heat environment.
I have an old water cooled Grunert and both a big fridge and a small freezer and it is pulling more than 100 amp hours and we are still in "cool" water. Currently also have 4 x 6v but am considering adding another pair. Also have 380 watts of solar.
Thanks again. Yours is one of the absolute best blogs out there!
s/v Calypso

Bill K said...

What make and model LED reading lights do you use ?

I have not been able to see any in person to know if it is worth the cost.

Thank You

Bill Kelleher

Solar garden lights said...

Such a very informative post on power consumption...........

Diane, Evan, Maia and Charlie the cat said...

The power consumption of the fridge is an estimate only, based on the running amperage (~4A) x the amount of time it seems to be on (< 12 hrs / day).

It's a Nova Kool system using a Danfoss 35 compressor, air cooled, with a tiny computer fan as aux. cooling in the compartment it is in.

A small well insulated (3 cu. ft, including small freezer) box sure helps.

Make of LED lights - something by Dr. Led that fits twin prong halogen MR-11 bulbs. Very easy to read by. Most of the LED bulbs other cruisers have seem to be equally good. Sensibulb got an excellent review in P.Sailor.

ChrisS said...

What a practical post. Thanks for all the real world data.

On an unrelated note, I saw Rich and Lori from SV Third Day at the boat show. It was great to meet them after reading their blog for so long!


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