I have little experience in this business back in the early days.
The Americans (hippie types) saw an opportunity and built some of the first off grid equipment.
This article by Bob Gudgel about his career I found interesting:
I still use an Outback MPPT and have a working Morningside PWM on a shelve 
I can also add this, when it started because of fish in a tank…
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“Xantrex had done a fine job building small disposable high frequency inverters, but had virtually no presence in the off-grid market.”
Clearly inverters had been around prior to this off grid market but technically what is the difference between high frequency and off-grid inverters?
That’s a bit like asking what the difference is between a Diesel car and a 4x4 (I could have a Diesel 4x4 or a petrol not-a-4x4).
LF topology is generally favoured for off-grid, but it doesn’t have to be.
The difference is essentially how the boost stage works.
LF topology takes your 12VDC, and turns it into something slightly less than 12VAC at 50Hz (because losses through the components), and then sends it through a big iron transformer to turn it into 230VAC. That is, the boost stage runs at 50Hz, like grid transformers.
HF turns your 12VDC into AC at a much higher frequency, typically above 25Khz (so that human ears cannot hear it). Then it uses a much smaller transformer to boost that to a higher voltage, and then it rectifies it back to DC, so you end up with about 400VDC. Then it changes this 400VDC to 230VAC (because the 230V is an RMS value, the peak is higher), this time at 50Hz.
The HF topology is typically a lot more efficient, and also lighter.
There is no reason you can’t use HF off-grid. All those cheaper Mecer/RCT inverters people were so fond of at one point were HF topology.
There is also no reason you cannot use LF grid-coupled. Victron Multiplus, as well as MLT Drives’s inverters, are all LF, and work fine on the grid.
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Correct me if I’m wrong but the inverters used in our applications are all LF??
Depends what you mean by “our”. Most people (as in slightly over 50%) on this forum probably have a Victron Multiplus II, and those are indeed LF. I have a Multi-RS in my house, which is HF. The rest likely have Sunsynk/Dye inverters, which are HF too.
So there is a good chance it’s about 50/50…
Interesting!
Given that electronic DC switching has replaced 50Hz transformer based power supplies I presume that this must have an influence on the choice between HF and LF inverters…
An HF inverter is effectively an SMPS (switch mode power supply). It uses a boost converter to get the voltage high enough to make 50Hz AC. And an isolated(1) boost converter is effectively still a transformer… it just runs at higher frequencies. All that stuff about “transformerless” inverters is, technically, BS 
I don’t think the downstream loads factor much into the choice of inverter topology. The simple reality is that HF tech is now as reliable as LF tech, and it is a lot more efficient.
1) You can of course make non-isolated boost converters as well, but if you used that with an inverter, it creates all sorts of new safety issues with residual current monitoring, not being able to earth the battery, and so forth…
And is there no difference in price between HF and LF inverters??
Not so much anymore… in fact HF can be cheaper sometimes. Less material. That big old transformer for the LF topology is quite expensive.
Yes, and mostly a toroidial one as well.
The overload capability on my Sunny Island I have been impressed with. As yet I’ve never managed to trip it with an overload fault. (The batteries die first so I get a battery fault)
How do the HF versions do in this department?
I asked my bud in the US his opinion:
LF uses large LF transformers that are a lot more robust and can start induction motors etc much better than HF. HF is more efficient and lighter making them better for RV’s etc. Untill they made the HF ones larger than they need to be they only used the HF ones on small inverters that did not start big loads. The trend is now towards HF but I still use LF which is much more reliable. You can see a test with a LF Schneider VS a HF Solark on youtube. Guess which one smokes first.
It depends… as always. The engineer can design for that. With an LF design, you get some of it for “free”, as there is generally some margin in all that material for overdoing things for small amounts of time, but a properly designed HF inverter works just as well.
If you look at the datasheet of the Multi-RS for example, which can be overloaded by 15% for 4 minutes and by 50% for 3 seconds, although that is not quite as good as the LF design (which can do 200% for 1 second, and 15% to 20% over for as much as 30 minutes, depending on ambient temperature), it is nevertheless a healthy margin that you won’t find on cheaper HF designs.
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