I have a 2000 w inverter that I use as an emergency generator. It has a dedicated connection to the battery. In the instructions it says to connected to the car battery and to keep the car running. I want to know, how many amps is the inverter actually receiving? Do the amps produced by the alternator to charge the battery jump to the inverter since both are connected thru the battery poles? I want to Install a male/female plug between the battery and inverter so I can easily connect and disconnect the inverter(it is not permanently installed) I need to know the amps running between the battery and the inverter so I can get the right plug. ie. 20 amps plug

  • does the inverter really not have a label that shows the amp requirement?
    – agentp
    Mar 30, 2018 at 22:22

2 Answers 2


This depends on the load you connect to it.

The Formula is P=U*I

Meaning that if you draw P=2000 Watts and on you primary side you have U=12 Volts you will need I= ~167 Amps + any inefficiencies your Inverter might add (calculate about 10 % to be safe).

Be sure to have a fuse of the size appropriate for the power cable on the battery side!

  • Thanks for the fuse suggestion! Maybe I wasn't clear, my worry is that the cable connecting the battery with the inverter is going to carry the same amount of amps that the alternator will feed the battery. I don't want the plug between the battery and the inverter to catch on fire since it is only made to handle about 20 amps. Mar 28, 2018 at 19:33
  • 20 amps will get you 240 watts at most - you should not connect more than ~200 Watts consumer. And for a 20 Amp cable use 20 Amp fuse, of course.
    – Daniel
    Mar 29, 2018 at 7:25
  • The alternator is a fixed voltage source. The current (amps) are dependent on the applied load. Similarly, the inverter appears to the alternator as a variable load. The idle current may be a fraction of an amp; beyond that entirely depends on the attached appliance.
    – DGM
    Mar 30, 2018 at 9:51

Remember that the inverter will only draw as much power as your drawing from the inverter.(plus some 10-20% extra) If it's rated at 2000W but you never use more than 50 there's no problem. I presume the inverter delivers either 115 or 230VAC, so the amps are about 10 to 20 times as low as for the 12V circuit, keep that in mind. Rather calculate with Watts and use amp ratings to check for crossing design limits.

If the inverter will make use of the cigarette lighter mount, that is usually rated at 100-200W and thus fused with 10-20A,(@12V). You can fit a bigger fuse, but it's not designed to deliver much more than 200W. At 12V the amps to sustain that power are high, and the produced heat with that is also relatively high.

If you use more than 200W of power you'll need to make a dedicated connection to the battery(or alternator but battery is nice central point) with (properly fused)wires that are able to carry the expected current. Your alternator is likely not able to deliver much more than 1000W. And it still has to run the car's needs from that. You can ask more power for a short period of time, but the alternator will overheat or stop, and the battery will drain if you do that for too long.

And 2000W is just way too much to ask from an alternator. So don't run hairdryers or heaters from your car while camping, you'll likely destroy the alternator and drain the battery.

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