Date: Sat, 10 Jul 2010 09:59:35 -0700
Reply-To: "Mike \"Rocket J Squirrel\"" <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Mike \"Rocket J Squirrel\"" <camping.elliott@GMAIL.COM>
Subject: Re: Battery Demand Calculator
In-Reply-To: <AANLkTil4JtRz0bOnnYA0QNOxwN59rIVFBw9EuRlp3bYP@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
As the calculator page says:
When "120 Volts" is selected, calculator assumes battery will power an
inverter with 80% efficiency. In other words, 20% of the power is lost
in the inverter as heat. This may be pessimistic, and your inverter may
have different characteristics. See, for example "Inverter (and other)
Efficiency." [link] In general, inverters have much higher efficiency
near their maximum power output and often very inefficient at low
values. A 1000 watt inverter that is 85% efficient at 900 watts may only
be 60% efficient at 50 watts, so it's generally better to get an
inverter that's not horribly oversized for the intended load. A higher
rated inverter may also have higher quiescent (idle) current, but it may
not be a significant factor; for example, my inexpensive "Chicago
Electric Tools" 1000W inverter from Harbor Freight only draws 0.1 amp
when idling.
--
Rocky J Squirrel
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR
neil n wrote:
> On Sat, Jul 10, 2010 at 8:15 AM, Mike "Rocket J Squirrel"
> <camping.elliott@gmail.com> wrote:
>> Neil was asking an Ohm's Law questions about calculating appliance current
>> draw.
>>
>> Herewith, formerly on the old wiki, then lost, and now placed back on the
>> wiki for all to use, is the handy Battery Demand Calculator:
>>
>> <http://www.vanagonwiki.net/wiki/Living_Off_The_Grid_Articles>
>>
>> Written for the fine folks in the VW Vanagon Westfalia community,
>> but useful wherever fine living from lead-acid batteries is found.
>>
>> --
>> Rocky J Squirrel
>>
>>
>>
>>
>> neil n wrote:
>>> Hi all.
>>>
>>> Viewing this: http://tinyurl.com/23utmeo in the store, I wondered how
>>> many amps it would use. On the back it says: 12 Watts, 12 Volts. Only
>>> vaguely recalled Ohm's Law at that moment, didn't obviously know the
>>> resistance of the unit, so couldn't calculate amps used. Bought it
>>> anyway.
>>>
>>> At home checked current draw. Low - 0.45 amps. High - 0.6x amps
>>>
>>> But entering 12 volts and 12 watts here,
>>> http://www.the12volt.com/ohm/page2.asp shows 1 Amp. Here:
>>> http://www.csgnetwork.com/ohmslaw2.html 0.58 amps.
>
>
> hey neat. Thanks for posting that Mr. Mike.
>
> I found it interesting to compare the same wattage appliance on 12
> volt and inverter operation. Though as the page say's, "measure,
> measure, measure", the amps drawn are more than double on inverter
> operation. This is something I was already aware of, and it has been
> pointed out in the past (inverter inefficiency), but great to get a
> ball park comparison.
>
> Neil.
>
>
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