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Old 02-09-2009, 12:41 AM   #1
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As far as I can find pretty much everything is rated for its power consumtion based on 110 and/or 220 systems, which is understandable...

for example, my laptop is rated at 1.5amps at 110 - 240v. but how many amps does that pull from my 12v batteries with a 110 inverter in the middle??

all help appreciated.

J
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Old 02-09-2009, 12:57 AM   #2
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(P) Power = (I) Amps x Volts (V) , that the P=IV equation...

So if you go from using 120V to using 12 V, then Amps goes up by a factor of 10 to maintain the same power. In your example 1.5 amps becomes 15 amps. When you think this, also...120 isn't quite the number, and neither is 12 in reality but the factor of 10 is useful.

Ah, but inverters aren't perfectly efficient. The efficiency depends on the details they don't tell you ... and how hard you're using it. The closer to the rated power of the inverter, it is usually more efficient. Efficiency might be between 70% to 90% on the inverter.

Hope this helps!
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Old 02-09-2009, 07:45 AM   #3
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j,

#1 What voltage is your laptop's battery ?

#2 When on AC Input - what does the AC - DC adaptor reduce voltage to ? at what amps ? What watts ?

-----

An example :-

The HP laptop,s adapter allows a wide range of AC input :- 110 - 240 volts - 1.7 A -- 50-60Hz.

It outputs 18.5 Volts --- 3.5 amps - 65 watts .
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Old 02-09-2009, 12:22 PM   #4
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That's almost exactly what mine says... it's an Acer

AC input :- 110 - 240 volts - 1.5 A -- 50-60Hz.

It outputs 19 Volts --- 3.5 amps - 65 watts .

So that's 15'ish amps on 12v?? that's more than a freezer...

aye yai yai...

... as background info to the question... I'm about to buy a boat and am trying to figure out power requirements... she's an old girl so no auto pilot (I'm planning on installing a vane), radar, or other power heavy electrics, just vhf and depth. the only electrics I'll really run regulary are the freezer, water maker, a couple of LEDs, and my laptop (I use Tsunamis with a gps mouse so its all i need for both navigation and telecomuting as long as I have a cel signal... ... my hope was to be able to get totally off grid with just a wind genny and a couple of 100'ish wat solar panels, but sounds like I may have to recalculate if my laptop sucks that much power.... may need two wind gennies! $$$ , ouch.... ... I guess that's the price of freedom...

please someone tell me i'm wrong!....
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Old 02-09-2009, 03:15 PM   #5
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what the battery of your laptop sees after the inline transformer is pretty much no important. what it draws off the line coming, that is important. As earlier stated most inverters operate best at between 65 - 85% of their max capacity (this is their "sweet spot" where there is the most effective exchange).

P = watts = power

V = Voltage

I = Amps = current

R = Ohms = resistance

With AC you also look PF which is the phase between Voltage and amperage. If you go to this web site http://www.sengpielaudio.com/calculator-ohm.htm it gives a very good basis to start from (also useful for teaching children and other adults with no electrical experience).

This is why there still wages the argument of series versus parallel set up on batteries. inverters where supposed to solve part of the problem by having an in line capcitor that allowed for the starting up amps that a cold plate or the radio needs when first coming on line. Not everyone agrees that they do the job well (I think they are just fine but hey I'm not an EE either just had to get through a year of this stuff on route to my degree and use a good bit of it in daily life).

P = IV

look at the wheel and it will explain most of it.

Best of luck,

Michael
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Old 02-09-2009, 05:31 PM   #6
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Quote:
Originally Posted by atavist View Post
... as background info to the question... I'm about to buy a boat and am trying to figure out power requirements... she's an old girl so no auto pilot (I'm planning on installing a vane), radar, or other power heavy electrics, just vhf and depth. the only electrics I'll really run regulary are the freezer, water maker, a couple of LEDs, and my laptop ..
Hi, there, the 15 amps is the current (I), not the power (P), btw. What its telling you is that the lower voltage system (say 12V DC) will have higher current flowing through it to achieve the same power needed by the appliance. What this means to all of us is that 12V wires must be much heavier guage than 24V, 32V/36V, 48V DC, or 120 VAC wires need to be. For this reason, many times folks will choose a 24 V or 32V bilge pump or motor rather than a 12V one. When you get into larger boats that need larger motors for autopilots, windlass, etc, you'll find entire boats wired with 24V for systems. Look in a good reference book--say Caulder's manual for Mech/Elect systems or the West Marine Catalog and you'll find recommendations for size of wire vs current (amps). Then, look at the prices in the catalog or for a real understanding of the difference between say 20A 12V DC and 20A 120V AC go down to your local marine store and check out the difference in size and price for the wire requirements you've determined.

Are you planning on having a manual windlass? These days it seems a good power windlass is easier/cheaper to find new than a good manual one, btw. Depending on size of boat (how big is it? what rig? and what displacement?) this will push you along a path to a certain size motor for an electric windlass and that may actually drive your battery and charging needs--and may push you to a 24V or 32/36V system rather than 12V for some boat equipment. For example, we have 12V lighting, but 32V/36V bilge pumps, autopilot motor, windlass motor, deck washdown pump, pressure water pump...etc. We also have several 15 amp 120VAC/60hz circuits (galley, charthouse, and laundry). Our boat came with a good sized (8KW) genset, so the AC circuits can always be run directly from the genset rather than through an inverter.

Regarding your laptop and other existing 120V AC equipment that you wish to use onboard--you can purchase a little gadget... brand name is "kill-a-watt"...that you plug into the outlet between your applicance and the power source. It will keep track of the requirements of the appliance. With it, you can discover what you're using in the way of current, it tells you the AC cycles the applicance is seeing (eg 60 hz), the power factor, and (cumulative) kilowatt hours used by the applicance. We've used one of these little gadgets to figure out the requirements for our washer/dryer, computer, notebook computer, my iron, a bunch of different tools we have, anything we think will be on the boat and used from time to time. I can tell ya--I won't be using the iron unless in emergency!

Best of luck to you.
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Old 02-09-2009, 06:07 PM   #7
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Quote:
that's almost exactly what mine says... it's an Acer

AC input :- 110 - 240 volts - 1.5 A -- 50-60Hz.

It outputs 19 Volts --- 3.5 amps - 65 watts .

So that's 15'ish amps on 12v?? that's more than a freezer...
5 amps by my reckoning, say 6 allowing for losses.

Think about LED lights to reduce power consumption - they can make a massive reduction possible as they are only about 1.5 watts each
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Old 02-10-2009, 12:39 AM   #8
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One of these set to 19volts with the correct polarity - will enable using one of the boats 12 volt batteries direct.

CLICK HERE
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Old 02-10-2009, 12:48 PM   #9
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While redbopeep's response and calculations are correct, I think that the power consumption of of 1.5 a at 110 - 220 V does not make sense.

First of all, the power consumption is usually specified by watts (w) and not by current in amperes (a).

Second, the any devise will have the same consumption regardless of input voltage, so 1.5 at either 110V or 220V cannot be correct.

At 110 V, 1.5 (165 w) a is too high for a laptop. My MacBook, for instance, is rated at 60 w. This translates at 12 V and assuming a 70% conversion efficiency to about 7 a, at 110 0.54 a.
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Old 02-10-2009, 04:30 PM   #10
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Quote:
Originally Posted by Istioploos View Post
the power consumption is usually specified by watts (w) and not by current in amperes (a)...

At 110 V, 1.5 (165 w) a is too high for a laptop.
Thanks for the reality check on power usage for a laptop. The primary power user will be the display's backlighting so depending upon the size of display...could be alot but my desktop lcd is rated at 60 watts and its a 21" monitor. Fans on laptops shouldn't have huge draw....but maybe...

Further, yes, we boaters like amps (current) don't we easy if you're working 12V only but not so good when comparing your AC loads to DC loads 120V, 12V and of course then if you have other things at 24 or 32/36 it gets a little more dicy...I suppose we like it so we can convert everything into Amp hours--for convenience of the battery sizing I guess--rather than sticking with kilowatt hours. I know the power consumption of many things I own in terms of kilowatts but always have to do the convert over to amp hours to figure what it means to my battery capacity.

Best to all,

Redbopeep
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Old 02-11-2009, 11:25 PM   #11
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Quote:
Originally Posted by Istioploos View Post
While redbopeep's response and calculations are correct, I think that the power consumption of of 1.5 a at 110 - 220 V does not make sense.

First of all, the power consumption is usually specified by watts (w) and not by current in amperes (a).

Second, the any devise will have the same consumption regardless of input voltage, so 1.5 at either 110V or 220V cannot be correct.

At 110 V, 1.5 (165 w) a is too high for a laptop. My MacBook, for instance, is rated at 60 w. This translates at 12 V and assuming a 70% conversion efficiency to about 7 a, at 110 0.54 a.
Yeah that didn't really make sense to me either, how it just had one amp reading for both 110 and 220... i just typed what is says on the adapter... maybe the 1.5a means something other than amps??
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