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Old 06-07-2008, 05:55   #1
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Power requirements for pc..

Hi everyone!

Could someone please tell me, I am buying a small 25' boat. I will need to run a pc for nav. purposes, and some other small electronic gear, maybe a small tv/radio etc..

Would a solar panel, wind generator and 2 batteries suffice? How do I find out know how much battery power I would need?

Any advice most gratefully received,

Mark.
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Old 06-07-2008, 06:42   #2
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If you are not trying to run refrigeration, you can probably get by with 2 good 100 amp hour batteries and 2 solar panels of at least 60 watts. If its not enough, put on another solar panel. You will ocasionally need to charge the batteries some other way, like with your engine alternator or a shore battery charger.

My Fujitsu laptop with a core duo processor is currently drawing about 2.5 amps from the batteries through a 100 watt inverter. I have another laptop with a P4 processor which draws 7 amps, so be careful to get a low power laptop. You can also cut power consumption by using flourescent and LED lights, rather than incandescent bulbs.
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Old 06-07-2008, 07:39   #3
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As above.I tested my micro pc ( cross between a laptop and a domestic pc) with a 12v direct power supply. 2.7 amps.with the hard drive running. To that add a screen of some sort. It is about $100 cheaper to run one 160 watt panel than two 80s, BUT I wouldn't recommend it. If you have one panel and it stuffs up.... And two panels gives you more sun angle choices in semi fixed positions. As above with lighting absolutely. especially with nav lights. Incandescent bulb Nav lights running all night use a huge amount of power...
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Old 08-07-2008, 18:14   #4
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Damn, my AC only GE refrigerator with seperate freezer, double door, and defrost only draws 155 watts. It runs 20 min an hour. That is one hour every 4.

But computer time isn't all day and night. I just found the draw to be a lot.
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Old 08-07-2008, 18:16   #5
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I was really surprised how much a laptop draws... about 4 to 5 amps! That's the same as my fridge/freezer.
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Old 08-07-2008, 19:45   #6
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Laptops as well as desktops vary quite a bit in how many watts they draw. Its not very accurate at all to label them all as drawing a certain amount.
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Old 09-07-2008, 03:39   #7
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Originally Posted by majdrew View Post
Hi everyone!

Could someone please tell me, I am buying a small 25' boat. I will need to run a pc for nav. purposes, and some other small electronic gear, maybe a small tv/radio etc..

Would a solar panel, wind generator and 2 batteries suffice? How do I find out know how much battery power I would need?

Any advice most gratefully received,

Mark.
Calculating power needs is a bit of math. You can create an excel spreadsheet for this. The basic formula is that Watts = volts X amps. This is consumption per hour. 40w light takes 3.3 amps. A 12V DVD player with LCD screen might take 3.0 amps.

List all the 12 V things you have on the boat and then put in a "hours per day" factor - i.e.

nav lights = 3 lights X 4 hours X (40W/12) = 40 amp hours
VHF radio = 2 amps X 8 hours = 16 amps

For AC stuff it gets a little trickier - watts is a measure of "work" if you have more volts you need less amps. So my Dell laptop has a 65 watt power supply.

65 / 110V = 0.6 amps
65 / 12V = 5.4 amps

What this means is that running my 110V laptop power supply through a 12V inverter will draw 5.4 amps. A 400W inverter running at max load is drawing more than 33 amps off your batteries not counting the inneficiencies of converting the power from DC to AC - yikes...

So Inverter for laptop = 5.4 amp X 6 hours = 21.6 amps

When you add it all up you will know your daily usage. Then you have to have enough storage so that you don't run out of amp hours before you can recharge. Then you have to figure out how to put the amp hours back in. Most commonly we know about alternators. however Solar and Wind are alternatives.

I am assuming you don't have inboard engine power?

We have a small boat and have added lot's of gizmo's - fridge, twin DVD players, 400W inverter, fans etc. I added a third battery and have them split between starting and house.

We don't run the fridge much as I have been trying to figure out how to recharge the batteries (after a weekend) while we are away from the boat.

With our size boat (yours and mine), lot's of solar starts to become problematic. I have figured out that I want two ~50-65 watt panels that will mount on the bimini frame - which I may need to reinforce. The reason for the smaller panels is that I can't fit one 85 watt or bigger panel anywhere that makes sense. Two smaller panels will fit on the bimini frame on each side of the backstay which runs through the bimini.

For reference (probably optimum numbers)

50 watt solar = ~120ah week
80 watt solar = ~200ah week
110 watt solar = ~280ah per week

If we run our 4 amp 12V fridge for 48 hours we need 192 AH to fill up the batteries that coming week. I'd love to get about 110 watts of solar on our boat.

I reckon with that we can stick the fridge on Friday afternoon, turn it off Sunday and be all charged up next Friday. Especially if we count on some motoring time with the alternator running.

Now I just need to come up with the ~$1,000+ boat bucks - LOL.
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Old 09-07-2008, 08:21   #8
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Dan,

Thanks for clearing that up for me. I have a house battery bank of 6 - 6 volt Trojans 240 amps ea. I calculate that to be 720 amps combining batteries to make three 12 Volt batteries. I have an 1800 Watt Inverter that is about 95% effecient so the draw is somewhere near 54.3 Amps a day and answers why I am almost depleting my batteries in two days of fridge running.

I have found that AC powered battery chargers are not the answer and engine run alternators do the fastest recharging. You have the ability to add solar which is awesome. They make a small single cyl diesel engine tied to an alternator which is another choice.

Mark,

Sorry for jumping in on your thread.
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Old 09-07-2008, 08:47   #9
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Majdrew: If you are running an outboard you might consider a portable gas generator to keep the charge up on your house batteries. Solar panels take up a lot of real estate and a mare's nest of cables. After you squeeze two big deep cycles into your boat you might have to start evicting guests. If you're lucky, the generator might store in the same hole as you gas tank (vented overboard of course.)
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Old 09-07-2008, 09:56   #10
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I have an 1800 Watt Inverter that is about 95% effecient
Be careful with the published efficiency numbers -- they are usually spec'd at full load and will not be as efficient at lighter loads. The numbers may be close enough, but perhaps not.

I would either put in a fudge-factor or measure the actual input/output at the load in question (CaptHead, I realize that you might have already done this).
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Old 09-07-2008, 10:08   #11
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Thanks Paul, Coming from a sales background I look at a lot of claims as "sales pitch" and as you suggested I used 90% as my real world numbers to adjust for the "sales pitch".

Also, I use a little under the 240 Amp rating on my batteries so I don't use too much before the charging.
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Old 09-07-2008, 10:53   #12
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Computers & displays run at 12v or less. By converting battery to 110v back down to the voltage the device uses, you lose a LOT of energy. If you are using a laptop, use a car adapter directly from 12v. For your display, MOST are 12volt, with a built in converter - find one you can plug directly into.
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Old 09-07-2008, 11:37   #13
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Our Eee PC uses LESS than 1 Amp when plugged into a small 75 Watt cigarette plug type inverter. We can use a Radio Shack converter to drop the 12 volts ships power DOWN to 9.5 volts!


Only a 7 inch screen, but LOW LOW power draw! Only US$400.00 each. Solid state drive, so it's drop tolerant!


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Old 09-07-2008, 19:43   #14
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Excellent advice regarding the 12V (or less) PCs. Converting DC to AC on the boat and then running the AC into the computers 12V converter is tragically inefficient. Getting a 12V supply for the PC input makes excellent sense.

Capthead - The other rub on storage is that you really should only consider half the amp hour rating of a battery as being available. Purpose designed deep cycle batteries a little more. So my 120 ah batteries really only have 60 or so available. If we've been mooored a couple of days on the 2 battery house with the fans, DVDs and lights on and the battery indicator starts to go into the red it is really tempting to use the start battery... Bad idea Dan!

The portable genset idea is valid but on small boats there is not much real estate to store it and you also have 2 fuels to deal with if you have a diesel donk.

I suppose my next mod might be an inverter for shore power, which we don't have yet. That's pretty cheap and straight forward to add.

But for us here at the equator solar is probably the right answer.
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Old 09-07-2008, 21:15   #15
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It should be noted that the common claim that the voltage converters that get from 12v DC to your notebook's voltage (aka car adapters, step up converters, etc) are more efficient than using an inverter because it misses out the inverter step (ie the claim is they do it all at DC) is entirely wrong.

The step up voltage adapters actually work much the same an inverter as the first stage in them takes the 12v DC then chops that up into AC, then they convert that back to the desired greater than 12v DC voltage. So just like using an inverter with the notebooks own AC power supply attached.

It is likely that the step up voltage converters are more efficient than using a large inverter and then the notebook's normal AC power supply to get back to DC again (particularly if the large inverter is just driving the notebook power supply), but that I expect mostly just a scale thing. A small inverter and the notebook's AC power supply may or may not be more efficient that the car type adapter but I suspect that the difference is hardly worth worrying about.

Whatever, the claim that the car type adapters are DC only devices is entirely incorrect - they go 12v DC in -> AC -> DC out.
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