Yes this might be very well due to loading order.
Where sometimes Powercalc starts earlier than Tuya and sometimes the other way around.
I had another proposed fix in the past, which never ended up in a release yet, but might be a fix for your case.

Could you please try applying this patch to your config/custom_components/powercalc/sensors/power.py file?
https://github.com/bramstroker/homeassistant-powercalc/pull/2926/files

Restart, and see if everything works and keeps working after multiple restarts.

I did the change and first did a restart of HASS, everything kept working.
Secondly, I did a complete reboot of the VM/HASS OS and the Tuya's were again without entity.
I reloaded only 2 lights by hand and then did a reload configuration which still didn't give entities (although the 2 started by hand kept on working), but...
after this, I did a restart of HASS (without restarting the VM) and then everything came back up again. Although I'm not sure if the change helped in that matter, as I've seen this before also.

It would seem that with a hot reboot everything starts up just a little faster so Powercalc sees the devices, while with a VM/HASS OS restart they just aren't ready in time.

But given the above, I'm quite confident this then could be solved by introducing a small delay in starting Powercalc (or do a second reload after a few seconds). Can I put a timeout somewhere in the code maybe to try (and if so how/where)?

Reading your initial bug report again you don't have any entities at all added to HA, which is kinda strange.
My first suggestion for the patch, was only a potential fix for sensors keeping unavailable state after startup. But that does not apply to your problem.

Could you enabled debug logs and share logs after startup?

You could try placing await asyncio.sleep(30) on this line: https://github.com/bramstroker/homeassistant-powercalc/blob/master/custom_components/powercalc/sensor.py#L303.
But that won't be something I will add in the integration code itself. There must be another solution, adding sleeps is never a correct solution.

We just need to figure out what exactly is unavailable during setup which prevents powercalc to create / register the entities correctly. Hopefully the logs will provide insights.
Would be nice to have a log when the entities are created, and one when the entities are not created.
That would allow us to compare.

Lucky strike, the first cold reboot after turning on debugging everything worked, then did a second cold reboot and then it didn't.

Enclosed the 2 debug logs (one working one not) for comparison

Edit log removed

Very strange, as I also see these entries in not working log:

Creating power sensor (entity_id=light.fl06_fittinglamp_gang_bg1 entity_category=None, sensor_name=FL06 - Fittinglamp Gang BG1 power strategy=lut manufacturer=tuya model=ANTELA-9W-RGB+CCT unique_id=c2b31ff5-efd1-4554-b402-2f2d9da2e720)

Which indicate the sensor should be created.
Seems something goes wrong between this and the actual creation of the entity in HA.

Could you try adding following to this line: https://github.com/bramstroker/homeassistant-powercalc/blob/master/custom_components/powercalc/sensor.py#L366?

_LOGGER.debug("Adding entities: %s", [entity.entity_id for entity in entities_to_add])

I changed it like this (hope that's what you ment and I did it right)

         if existing_entry and existing_entry.disabled_by == RegistryEntryDisabler.DEVICE:
            entities_to_add.remove(entity)
            _LOGGER.debug("Adding entities: %s", [entity.entity_id for entity in entities_to_add])

    async_add_entities(entities_to_add)

The log download gave 2 (see enclosed). They are both from the same startup, as I can see in the TS, whereby the Hue entities did but the Tuya didn't come up.

** Edit: remove logs **

@Dream1975 Sorry, you should not put the log inside the if.
But just above async_add_entities line. With same indenting as that.

Changed it and made a new log of a start with no Tuya entities

** Edit: remove logs **

@Dream1975 I see following in the logs:
Focussing on sensor.fl06_fittinglamp_gang_bg1_power atm, because that's the same as I checked in last log.

2025-01-31 19:13:26.572 DEBUG (MainThread) [custom_components.powercalc.sensors.power] Creating power sensor (entity_id=light.fl06_fittinglamp_gang_bg1 entity_category=None, sensor_name=FL06 - Fittinglamp Gang BG1 power strategy=lut manufacturer=tuya model=ANTELA-9W-RGB+CCT unique_id=c2b31ff5-efd1-4554-b402-2f2d9da2e720)
2025-01-31 19:13:26.573 DEBUG (MainThread) [custom_components.powercalc.sensor] Adding entities: ['sensor.fl06_fittinglamp_gang_bg1_power']

Also the new Adding entities is being logged now, which means powercalc completed preparing the entities to be added and registered to HA, and delegates the entity creation to HA.
After this sensor.fl06_fittinglamp_gang_bg1_power should be created, but it's not in your case, right?
Seems something goes wrong in HA core, but I have no clue. Will ask some HA core dev if he has any idea what can be the cause.

Indeed, with a startup that has the problem for your example sensor.fl06_fittinglamp_gang_bg1_power is not there (like FL02 in the screenprint in my first post). WHen I do the 3 dots and reload it is immediately there like FL01 in the screenprint (as is it with some starts).

Thank you and I'll wait patiently (it has been like this since I started using Powercalc so am in no hurry.

I noticed something new today. When all the Powercalc sensors are working, but then I reload the Tuya integration (to update devices) they are gone again. So the startup issue is probably related (that Powercalc doe make the entities but in the startup procedure Tuya is sometimes just before and sometimes just behind it).

Very interesting find.
Maybe Tuya integration has some logic to remove all entities from a device.
Powercalc adds entities to the same device as Tuya. This device is managed by Tuya.

Could you test creating a HA helper, for example Integral? Pretty sure these are also added to same device as the source entity.
When this is the case please try reloading Tuya again and see if those disappear as well.

Something interesting happens there also. When making the helper and connecting it to a Tuya sensor leads to the entities for all devices being gone also.

But I have other helpers also connected to Tuya devices (fe an energy integral vor the power on smart plugs) and these never had any issue. What I see in the Tuya integration if I compare:

The helper made in HASS itself is linked to the device

And if I look at a powercalc device (with, off course, making sure the entity is running) that sensor is not visible under the device

Something interesting happens there also. When making the helper and connecting it to a Tuya sensor leads to the entities for all devices being gone also.

What do you mean with this exactly the entities for all devices being gone?
Do you mean the helper entity is also removed from the system, and not working anymore?