I'm not entirely sure if this is the best place to put this, but I'm having an issue with a mega-battery I'm working on.

The battery is two AGM 12v batteries in parallel, with an inverter stepping up the voltage to 120VAC. However, I only measure 90V on the output.

When I connect the battery to a fan, the fan buzzes loudly, and does not spin quite as fast as usual. I measured the voltage as 90V, while the mains in my house is 240V, with split-phase power so 120V at each outlet.

Sound difference: From outlet: https://drive.google.com/file/d/1Nxzmv7j3tZUn3u-P-dO2HALBsROkLexd/view?usp=sharing The sound is the normal slight hum on startup and low wooshing noise from the fan.

From battery: https://drive.google.com/file/d/14qAOo2tkDCe9d_dRLISRUL3OLf2PiKGT/view?usp=sharing The sound is a loud buzzing sound with the fan turning on much slower.

The fan is just a normal fan, no heating done.

The batteries are good. 13.2V no load, drops to 12.9V under load. They are just a few months old.

The inverter is one I found in the garage, Vector VEC024. The wiring is 12 ga, capable of handling up to 20A through it. Inverter is rated for 400W. The fan is a Lasko U15617.

What's going on here? Why is the inverter putting out less power than it should? How can I fix it?

  • Your fan, presumably designed to run at 120v doesn't like running at 90v, so it hums. That's not surprising. Do you have an actual question?
    – FreeMan
    Oct 28, 2021 at 15:14
  • Added questions
    – Proxy303
    Oct 28, 2021 at 15:40
  • Is this just a fan or a fan-heater? What are current ratings of the fan & inverter?
    – SiHa
    Oct 28, 2021 at 15:54
  • 2
    You may want to include the specs for the inverter, or at least a make/model number. Right now, it's kinda like me saying "my car doesn't accelerate as fast as I think it should". Well, if I've got Ferrari expectations when driving a Yugo...
    – FreeMan
    Oct 28, 2021 at 16:14
  • 1
    @FreeMan Didn't even realize I put that there. Meant to say 12 AWG.
    – Proxy303
    Nov 3, 2021 at 13:17

1 Answer 1


There can be a variety of issues at work here:

  • Vector is a low-price brand; we should expect it provides a modified sine output.
  • if your volt meter does not use "true RMS" methods for measuring AC volts it'll give an incorrect reading when measuring a non-sine-wave signal. A post on practical machinist explains it in slightly more detail.
  • I didn't see a power rating listed in the fan owner manual, but it might be on the high end of the inverter's capability. This could be why it is slow to start.
  • Motors generally are expected to run slower and with more noise when powered from a modified sine source.

In summary: it's possible there's something wrong with the inverter, but also possible that this is just the way things are with modified sine waves. You could do the same test with an incandescent or halogen light bulb (ie not an LED nor compact fluorescent) to see whether its brightness is about the same on wall power and inverter power. It should be fairly comparable.

  • The fan draws 100W on high speed mode constantly. Not sure how much on startup.
    – Proxy303
    Nov 3, 2021 at 13:20

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