# Amp to KW?

New UserPosts: 10Registered Users
I am not sure where this question would be best posted, so I will try here and see what happens. I am hoping that someone can either tell me or point me in the right direction. What I want to be able to figure out is how many amp hours it would require at 12 volts to produce one KW at 120 volts. I am hoping to be able to figure in loss in the batteries and going through the inverter. I do not wish to account for any lose in the system before the charge controller. If you think that you can get me started in the right direction that would be helpful I can try and put the peices together from what you give me. If you know the answer that would be even better. Thanks

• Just some guy Posts: 23,740Super Moderators admin
Re: Amp to KW?

Starting with the math:
• Power = Volts * Current = Volts^2 / Resistance = I^2 * Resistance
• Energy = Power * Time = Power * Hours (for house/solar usage)
• 1,000 Watts = 1 kWatt (kilo=1,000x)
• Power = Volts * Amp
• Amp*Hours = Amps * Time = Energy (Watt*Hours) / Volts = Amps*Hours
So, for example:
• 1 kW = 1,000 Watts
• P=V*I
• I=P/V= 1,000 watts / 120 volts = 8.33 amps @ 120 VAC
• I=P/V= 1,000 watts / 120 volts =83.33 amps @ 12 VDC
Now the details... A 12 volt inverter may be around 85% efficient and operate from 10.5 volts to 15.5 volts... You should design the wiring to manage the flow at minimum operational voltage (or you will blow fuse/melt wire when the battery is low):
• 12 volt circuit current = 1,000 Watts * 1/10.5 volts * 1/0.85 efficiency = 112 amps at 12 VDC
Now, the circuit and fuse/breaker should be rated (per NEC) to 1.25 the maximum continuous current you plan on operating at:
• 112 amps * 1.25 safety factor = 140 amp minimum circuit rating
Round up to next standard fuse/wire size or ~150 amp fuse and fuse holder.

Does this get you started?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• New User Posts: 10Registered Users
Re: Amp to KW?

This is very good. When I did the math the other day I came up with 83.33 amps for one KW as well. So that would be 83.33 Amp hours= 1 KW hour? I believe my inverter is rated at 90% efficient at 2/3 power. I am normally using it well below that 2/3 so if would be less then 90%. 85% is a good starting point. So if I understand correctly then I would take 115% of 83.33 and that would tell me how many amp hours = 1 KW hour? Or at least get me close enough.
• Just some guy Posts: 23,740Super Moderators admin
Re: Amp to KW?
• 1,000 watts * 1/12 volt nominal * 1/0.85 * 1 Hour = 98 AH for 1,000 watts at 12 volt nominal for 1 hour
Of course, you plug in your numbers as needed...

Also, I did 1,000 watts for 1 hour which is 1,000 Watt*Hours or 1 kWH.

I try to pull all the variables out and label them so you can see exactly what is going on--There is no "real difference" in the result.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset