07272008, 05:57 PM  #1 (permalink) 
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Inverter Calculations
Watts = Volts x Amps, 1000 watts is a kilowatt,
To use the full potential of a 1000 watt inverter (12 VDC to 120VAC), do I need 12VDC and about 1000 watts going in or do I need 10,000 watts going in, at a given amperage. It doesnt seem that 10k watts would be required as that is over 13hp, but the equation above suggests that is the case... 12 volts is 10% of 120 volts and 120 is 10 times 12...so to make the math work, it seems the wattage would be times 10 also. Does the 12VDC just get converted to 120VAC and the wattage stays about the same?? The inverter is supposedly 90% efficient. thanks Last edited by 85blue4runner; 07272008 at 08:12 PM. 
07272008, 07:11 PM  #2 (permalink) 
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Ohms Law is your friend, a Watt is a Watt no matter if it is AC or DC at any voltage; 1000 Watts @ 120V AC is going to give you 8.333 Amps @ 120V AC
Assuming that your inverter is ~90% efficient 1000 Watts out is going to be 1100 Watts in so 1100/12 (Watts/Volts)=91.666 Amps @ 12V DC.
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07272008, 08:11 PM  #3 (permalink) 
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for some reason, i was not changing the amperage.. that makes sense now thanks.. i knew 10k watts didnt sound right, but i guess it could be mathematically correct if amperage stayed the same...
thanks again.. 
07272008, 11:46 PM  #4 (permalink) 
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Actually a watt is not always a watt when considering AC power. 120VAC is a reference to the RMS voltage. Generally, there's a power correction factor that comes into effect depending on whether the load is resistive or inductive. It's also dependent on how many phases are in use.
When considering an inverter it's generally easier just to look at the volt/amp ratings on the output for peak and continuous load and how that translates to DC input. Most massmarket inverters won't really function continuously at the advertised watt output no matter how one calculates it. 
07282008, 06:58 AM  #5 (permalink)  
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Quote:
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07282008, 12:31 PM  #6 (permalink) 
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Honestly, it's easiest and fairly accurate just to use a factor of ten. This will factor in the fact that your batteries are actually operating at 12.212.6 volts (assuming you aren't over discharging them), your inverter efficiency, the fact that inverters sometimes output 110 or 115 instead of 120, etc. I haven't had this method fail me yet, especially considering you're a fool not to factor in SOME margin of safety.
So...you need 7 amps AC and you use the factor of 10...that's 7*10 or 70 amps@12vdc. Watts are watts as far as this calculation is really concerned so you can arrive at that number either way. 7*120=840 watts or 70*12=840 watts.
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