It's simple: even when idling with no or very little load, an inverter must produce AC sine wave voltage. This means the power devices are switching constantly, and the output filter has ripple current. So there will be switching and conduction losses. There's no way around it. It's AC so it cannot go to sleep in the middle of the sine wave. Basically it means for the same design scaled up, idle power loss will be proportional to max output power. To get lower idle power, you have only three options:
- Cheat, as Victron does, and output a potato shaped sine wave at very low power to reduce losses, it seems to work
- A better design (silicon carbide, etc)
- Two inverters in parallel, a low power one with low idle losses, and a high power one that only starts up when needed (more expensive, and it needs integrated so the big inverter starts instantly)
On the other hand, DC-DC converters can scale down as much as needed: if there's enough capacitance at the output, when running at low power, the converter can switch between bursting some power into the output caps and sleeping. This increases output voltage ripple, but that's not usually a problem. This means, if the only loads you have that need to be constantly powered can run on DC, for example lights, network, etc, then powering them from the batteries via DC-DC converters and turning the main inverter off at night can be an option. But it's a hassle.
Further complicating the matter, efficiency isn't just a number, it depends on output power. Victron publishes it:
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They are excellent for idle power, also efficiency at low power is good, but at high power it's not that good.
For hybrids the data is hard to find, and they will often give the PV->AC efficiency which only considers MPPT and inverter, but not battery DC-DC conversion, so it is useless for you. That said a HF inverter like Deye should have higher idle losses than Victron (50-70W) and a flatter efficiency vs power curve (better at high power).
