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Assistance Required: Axpert mains power usage when using PV and battery


Schnavel
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After 4 months of following up with City Power, they finally came on Friday to downgrade me from 3 phase post-paid, to single phase prepaid.

I have an Axpert 7.2kW MAX unit installed:

For some reason, when running in SBU mode (completely on solar and battery), the system is still using roughly 1kWh per day. According to ICC, I haven't used any grid power.

If I switch the mains breaker off, forcing the inverter to be completely disconnected from the mains, no mains power is used.

Does anyone know if the inverter draws a continuous small amount of power in order to keep it synchronised with the mains?

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On 2021/12/06 at 5:41 PM, Schnavel said:

the system is still using roughly 1kWh per day.

 

1000 Wh per 24h ≅ 42 W continuously.

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Does anyone know if the inverter draws a continuous small amount of power in order to keep it synchronised with the mains?

It would use a microscopic amount of power for that, nothing like 42 W. 42 W may be the switching losses of the DC-AC converter, but I suspect that a part of that will be a "bias" towards power drawn from the AC-in, so that no significant energy (Wh) is seen to be exported from the AC-in port. If they are seen to export significant energy, then these inverters would require a heap of expensive regulatory approvals.

So that seems to be the price of being able to blend utility and solar power. A reasonable price, I'd suggest.

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2 hours ago, Schnavel said:

if the Axpert draws a small amount of mains when in operation?

Some models do; it's hard to keep track of which ones have a AC-in power supply. The idle power for a 5 kVA model is about 50 W; it would possibly not be much more for larger models. But 72 W continuously (scaling up linearly with rated VA) comes to 72 x 24 = 1,728 Wh per day, or 1.7 kWh. So I suspect that this idle power still comes from the battery, or solar when available.

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Thanks for your response!

Having watched it the past few days, it appears to only happen when running on battery (at night). During the day, when there is PV available, the system doesn't seem to use any mains power.

Considering the cheap price of the inverter, it's capabilities and the huge monthly saving, having gone from post-paid to pre-paid, it's a small price to pay.

Thinking about it, I could stop this from happening by installing a contactor, and using the dry contacts on the inverter to switch the mains on and off, based on the voltage of the system. For the majority of the time, the contactor would remain open, but when the batteries are running low, either due to use, or lack of PV on rainy days, the contactor would close before the inverter switches to mains (would have to play around with the voltages to establish the current cut off points).

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