My mum has a hybrid system (air source PV on roof, Inv and batteries) with the specs below:
* Solar - 2.4kw
* Inverter - 2.5kw
* Batteries - 5kw
Currently it's sunny-ish (overcast though on/off), and the specs on the app are showing:
- PV -> Inv/charger = 0.900kW
- Load -> 0.071kW
- GRID (IN to house/load) - 0.02kW
- Battery 0.794kW
- Battery SOC = 49%
What I'm trying to understand, is why there's 0.9kW coming in, charging the batteries, and yet for the 0.071W of load, 0.02kW is coming from the grid? Surely the system is producing enough power from the small PV array (0.9kW) to power the load?
Or does it favour charging the batteries to ensure they're maintained, when there's only a bit of PV power coming in and there's a load?
Thanks.
Why drawing from grid whilst charging batteries on sunny day?
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Dave Foster
- Posts: 1328
- Joined: Thu Oct 13, 2022 7:21 pm
If the inverter is in Self Use mode the PV will firstly be used to cover the house load, then any spare will be used to charge the batteries - once the batteries are full, the inverter will export anything left.
To answer the question about why it's drawing from the grid, it's a grid tied inverter that means it is constantly connected to the grid so that it can either draw power from it or export to it instantly - this requires it to draw a small amount of bias current - it's only a very small amount, usually just a handful of watts.
If you had a larger array the batteries would be charged much quicker and the moment it goes to export it no longer needs any bias current as it will be exporting (biased in the opposite direction).
You can change the work mode to feed-in first, after covering the house load that sends all available PV generation to export which you will get paid for, but obviously the batteries won't have any power to cover the night time running. In feed-in first the inverter doesn't draw as much from the grid as it is expecting to be exporting.
If your mum has a time of use tariff such as economy 7 (or an EV tariff), then you could charge the batteries overnight during the low tariff and set the work mode to feed-in so that all solar is exported - that way your batteries would be full to start the day, any solar will cover the house load and spare exported and the batteries would have the charge needed to run the house during the dark hours.
To answer the question about why it's drawing from the grid, it's a grid tied inverter that means it is constantly connected to the grid so that it can either draw power from it or export to it instantly - this requires it to draw a small amount of bias current - it's only a very small amount, usually just a handful of watts.
If you had a larger array the batteries would be charged much quicker and the moment it goes to export it no longer needs any bias current as it will be exporting (biased in the opposite direction).
You can change the work mode to feed-in first, after covering the house load that sends all available PV generation to export which you will get paid for, but obviously the batteries won't have any power to cover the night time running. In feed-in first the inverter doesn't draw as much from the grid as it is expecting to be exporting.
If your mum has a time of use tariff such as economy 7 (or an EV tariff), then you could charge the batteries overnight during the low tariff and set the work mode to feed-in so that all solar is exported - that way your batteries would be full to start the day, any solar will cover the house load and spare exported and the batteries would have the charge needed to run the house during the dark hours.