On Fri, 10 Jul 2015 11:01:54 +1000, John G wrote:

It happens that Ned Simmons formulated :
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.
are you sure?

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.

How many people who ask Electric Questions in a Metalwaork Group have a
Variac sitting around for such experiments?
I've gat 4 or 5 of them- from itty bitty to HUGE.