How it should work .....
For those new to solar and trying to work out how their system should work this might help. Happy to amend if anything needs adding/amending.
Long post warning...... 🤣
When your solar panels generate it goes to the inverter as DC. The inverter then converts it to AC and sends it to the house consumer unit to be used by the house to meet any demand. If the house doesn't use it then it's excess and that goes out to the grid.
If you have a battery (either on its own inverter or the same one as the solar) then its inverter monitors the houses main grid connection (using a CT clamp) looking for any excess being exported and if there is excess it charges the batteries. When there's no solar and the house has demand, that same CT clamp determines that there's electricity being taken from the grid and it tells the batteries to discharge to meet that demand.
In terms of loads that exceed the capacity of your solar and/or battery, the demand is first satisfied by available solar, then by battery, then by grid. Each adding as much as it can to meet the demand. I say as much as it can as parts of the system have limits which restrict the flow. Think of the inverter and batteries as each having a tap that lets current flow. Each battery and inverter has its own charge and discharge rate and depending on which is the least it becomes a bottleneck.
To try and explain here's a couple of examples:
Scenario 1 - Solar only - a 5kwp solar array on a 4kw inverter with the sun shining brightly giving the full 5kw of generation. When a 10kw load (say an electric shower) comes on the inverter becomes the limiting factor. So even though 5kw is being generated by the solar array, only 4kw is given to the house with 1kw lost through clipping. In this case 6kw would need to come from the grid. On the basis the shower only takes 15 minutes it would consume 2.5kwh, the solar would supply around 1kwh and the grid would supply 1.5kwh.
Scenario 2 - Battery only - a 12kwh battery with a 2kw max discharge rate, behind a 5 kw inverter. Using the same 10kw shower example, the battery max discharge rate becomes the limiting factor. So only 2kw will be supplied by the battery inverter combo and 8kw would need to come from the grid. On the basis the shower only takes 15 minutes, the batteries would drain by around 0.5kwh and the grid consumption would be 2kwh.
Scenario 3 - Solar and AC coupled Battery - as above but with a separate 5kwp solar array on its own 4kw inverter. Again the sun was shining brightly giving the full 5kw of generation. So in the 10kw shower example, 4kw will come from the solar (the solar inverter limiting output from solar to 4kw with 1kw lost through to clipping), and it will be topped up with 2kw from the batteries (limited by the battery discharge rate), leaving 4kw to be supplied from the grid. The 15 minute shower will consume 1kwh from the solar, 0.5kwh from the battery and 1kwh from the grid.
Scenario 4 - as per scenario 3 but it’s really cloudy and the solar is only generating 1kw. Now that 10kw shower will get 1kw from the solar, and 2kw from the batteries (the batteries are limiting the draw due to discharge rate), leaving 7kw needing to be drawn from the grid. The 15 minute shower will now be 0.25kwh from solar, 0.5kwh from the battery and 1.75kwh from the grid.
Scenario 5 - Solar and battery on Hybrid inverter - now imagine the battery is not AC coupled but plugged into the same inverter as the solar - a 4kw hybrid inverter with that 5kwp solar array and the 12kwh battery connected it. Once again the sun is shining brightly giving the full 5kw of generation from solar. The inverter is still the limiting factor and only 4kw of the solar is given to the house, however instead of loosing the 1kw to clipping, it will go to charge the batteries (if they're full it will be lost through clipping). So in the 10kw shower example, 4kw comes from the solar and 6kw from the grid. The 15 minute shower will be 1kwh from the solar and 1.5kwh from the grid.
Scenario 6 - same as scenario 5, but there's no household load. So when the sun is shining brightly giving the full 5kw of generation from solar, the inverter doesn't have to give much to the house, so almost the full 5kw is channelled to the battery - even though it's only a 4kw inverter. Now if the batteries have a max charge rate of say 2kw, then only 2kw of the solar generation can be channeled to the batteries and 3kw will be sent to the grid as export (see note below about export restricted systems)
Scenario 7 - Overnight charging - so it's now off-peak time and your batteries want to charge from the grid. With a 4kw inverter when the charge kicks off it could draw 4kw from the grid, but it will only draw at the rate that the batteries can be charged at. So if the batteries can only take a charge rate of 2kw then that's all that will be sent. In this situation over the 4 hour charge window the batteries will only be charged with a maximum of 8kwh
A note on all the above scenarios when a system has been given an export limit by the DNO as part of a G99 application. When an export limit is set the inverter monitors grid export and will restrict export to the grid. In most cases the full capacity will be usable by the household subject to the above bottlenecks. For example a system with a 5kw AC coupled battery and a 3.6kw solar inverter (8.6kw total capacity) which has approved by the DNO as long as it is export restricted to 3.6kw. In this case the battery inverter would need to be set to zero export, but the full capacity could be used by the household
Next Step: My results after a couple of years of running
One final note
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