calculating power

Here's the scenario: I've got a little heater that is rated at 900w. It currently has a normal house outlet plug on it. After reviewing OHM's Chart, I figured it that it drew something like 8 amps. My question is whether or not I can run it off 12v with a fuse rated at 80 amps.

Does this make sense or am I way off track?

900/12= 75 Amps?

Thank you

If the power inverter were 100% efficient, your math would be accurate.  Power inverters are one of the most efficient electronic devices out there, but they are not great.  I am thinking about 75%.  So divide your 75 by (.)75 to get a number closer to an actual number.

On a side note, heaters, blow dryers and toasters are very demanding on generators and inverters. 

Or are you intending just to connect it to 12V?

If so, if it is a simple resistive heater, it is not a constant power load - it is resistive.

Hence....

900W @ 8A => 112.5V; hence I assume a 110V supply? Or 120V => 7.5A (from P-VI).

I'll assume 120V.

from V=IR & V=IR, P=VV/R hence R=VV/P

Hence R = 120x120/900 = 16R (Ohms) [ Check = V=IR = ~8A x 16R = 128V =~120V. Cool!)

On 12V, 16R draws 12V/16R = .75A.
But lets assume up to 16V, I = V/R = 16V/16A = 1A.
So a 1A fuse should do. (That's if you are trying to protect the heater.)

Total power output (assume charging at 14.4V) = VV/R = 14.4x14.4/16 =13W.

So a 120VAC 900W resistive heater might/should put out 13W in a running car (14.4V).
At 12V it'll be 12x12/16 = 9W.

But if running off an inverter, yes - a 1500W inverter should be ok, and that will consume around 90A - probably the equivalent of 1/4 of a starter motor.


FYI - for heating a car - forget it! Water heating has something like 2-4 times the efficiency, but it is a byproduct of combustion anyhow - you are not "adding energy" (pollution, greenhouse gases etc) by using engine coolant for cabin heating.
But for (fast) window demisting - yes - but use a 12V hair dryer.

I assumed you were trying to use an inverter.  Without an inverter, theoretically it will produce 1/10th of the heat it does when run on 120 volts.

That Idiot means 1/100th, not 1/10th.

IE 9W, not 900W.

Well my exact use for it would have been helping to keep a porter machines' cab somewhat comfortable in lower temps. I just picked up a small 12v heater, but I don't think that'll cut it. Maybe I should keep looking for factory 12v heaters?

Thanks again.

Like I said, it's only 9W.

But even 900W ain't much heating - use and extension cord and try it.

i am an idiot wrote:

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If the power inverter were 100% efficient, your math would be accurate.  Power inverters are one of the most efficient electronic devices out there, but they are not great.  I am thinking about 75%.  So divide your 75 by (.)75 to get a number closer to an actual number.

On a side note, heaters, blow dryers and toasters are very demanding on generators and inverters. 

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Thanks for sharing.

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Not that I like replying after spamsters, but I was thinking the other day... (WARNING - run for cover!)

A 120VAC heater with wire elements.
Cut into 10 segments for 120/10 = 12V segments.
BUT, since a vehicle is typically significantly higher than 12V - eg, 14.4V (20% higher than 12V BUT 44% more power! ie - heat).

So cut into 8 hence 120/8 = 15V segments.
That might be a bit cooler, but 8 is convenient - ie, cut in half, and then halve those again for 8 pieces.   

9 segments is 120/9 = 13.3V each which might be a bit low, though heating elements tend to be self stabilising which helps prevent over-voltage burnout (ie, higher voltage means more current means more heat means higher resistance so current drops, hence a 20% voltage increase will be far less than 44% heat increase).

And 10 is 12V each which might be okay even with 14.4V.

But 900W is a big load for 12V (ie, 60-75A) so vehicles without HO alternators (at least 120A) might not even output 13.5V....

And you need fast fans else good heatsinking to cool the elements.
And a fireproof enclosure....
You'd probably be better off with lots of 10W or 20W resistors (eg; 22R/10W; 10R/20W, 4R (Ohm) 50W resistors etc).


But I do recall a guy insisting on a 2kW electric heater - his heater tap or core had failed. So, a 200A alternator JUST for the heater and/else an extra BIG battery, a >2.5kW inverter, and more than 3HP from the engine.
But whilst a 1kW or 2kW heater might be okay for my cabin (a very small 2-seater ute); for a passenger vehicle... whilst moving... in colder climates (we rarely reach 0C (32F) here)... I have my doubts.   
When an engine producing 50HP (~40kW) puts out well over 5kW (probably above 10kW) of heat, it's easy to realise why engine heat is used for cabin heating. Even the coolant carries more heat than an equivalent cross-section copper cable (unless high voltage is used).