Posted by: @ringi

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Does the quality of sine wave from the generator then become an none issue?

Interesting question.

The usual assumption is that generation sources based on mechanical rotation will naturally provide genuine sine-wave output.

Do you have experience suggesting that this is not the case?

Posted by: @ringi

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Posted by: @abernyte

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then the plumbing was left with manual change over valves to isolate the heat pump and run the old LPG boiler to provide heating,

Does that setup heat the water inside of the heatpump?

What abernyte proposes here is just using a generator to run a backup fossil fuel boiler.

A heatpump will always need more electricity..

Your backup solution could use a generator to power a heatpump that then trebles the generator output and keeps your home warm. But many heatpumps use a lot more power on startup than after they get going. Adding an inverter that is powerful enough with even a small battery may make the difference. 

@transparent Thanks. Good point about startup surge current. I guess an appropriate battery could absorb that, so that the generator would only need to be sized for normal running. Would need to keep in mind that it could be running in cold weather. I was also thinking of turning off DHW, to limit demand.

Any thoughts about how to determine startup current?

Posted by: @pipcart

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@transparent Thanks. Good point about startup surge current. I guess an appropriate battery could absorb that, so that the generator would only need to be sized for normal running. Would need to keep in mind that it could be running in cold weather. I was also thinking of turning off DHW, to limit demand.

Any thoughts about how to determine startup current?

If the heatpump manufacturers app does not show it, you could use a ct clamp or a smart switch. Most provide that information via their app.

Thanks. Well need a professional in I think. I struggle with Ohms law!

Us Moderators have had a (private) discussion in 2025 about peak current demand from heat-pumps.

It certainly used to be the case that there was high demand for the first minute or so after start-up whilst the motors for fans and compressors reached operational levels.

However, that is no longer the case.

Heat pumps now use soft-start motors and there is no noticeable surge as the HP starts up.
That comment applies both to electrical surge currents, and also the possibility of lower efficiency as the HP water-circuit reaches the required flow-level.

Those issues are now substantially resolved.

If you're reading topics here on the forum which refer to large current draw, particularly when a HP is cycling on/off,
then check the date when that discussion occurred...
... and whether the post refers to a HP system which was of an older design.

Posted by: @transparent

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However, that is no longer the case.

Heat pumps now use soft-start motors and there is no noticeable surge as the HP starts up.

That´s useful to know. 

Given that the peak power has to be determined in order to size the generator or inverter (and the number of queries I read here, where the users cannot find it like from the manufacturers app), a CT clamp or a smart circuit relay still seems to be a cheap and effective way to assess it.

Fortuitously, I've just posted here on another topic about buying a (cheap) clamp multimeter.