On Fri, 10 Jul 2015 09:14:47 +1000, John G wrote:

wrote :
On Thu, 9 Jul 2015 02:47:20 -0700 (PDT), robobass
wrote:

I know this is a metalworking site, but there is quite a bit of expertise on
subjects like this here.

I just bought a 200/300Watt 230v inverter. I attached it to a car battery on
the bench (nothing else attached) and plugged in a few lamps. The output
measured 330v with loads of 50 and 150 watts. The lights did seem brighter,
but not excessively so. The voltmeter is a cheapie, but seems generally
accurate. I know that these devices produce a modified sinewave, and I
should expect a different reading, but I thought the reading should be lower
if anything. Is this inverter a dud, or do I just not know how to measure?

Thanks
You should read about 0.7 x the rated voltage on a modified square
wave inverter.

I am amazed no one has suggested using a good old Simpson 260 or
similar ANALOGUE meter and get the real answer.
All this waffle about transformers and peaks and average is typical of
the impractical answers so common in some of the electronics groups.
KISS Keep It Simple Stupid. :-?

Won't work any better than the DVM, it'll just give yet another
misleading value.

My impractical waffle is to use two identical lamps with clear
envelopes, a variac, and a voltmeter. Connect one lamp to the inverter
and the other to the variac. Arrange them so the filaments are nearly
superimposed as you look thru both envelopes. Adjust the variac so
there's no difference in the color of the of the two filaments.
Measure the output voltage of the variac - that's the RMS output of
the inverter, within, I'll wager, a couple percent.

Switch the lamps and repeat the experiment to check for any detectable
difference between them.

--
Ned Simmons