Hi.
I have looked at the other threads but they are all project specific so I decided to get some information on the minimum battery sizing requirements as listed by Victron for AC coupled systems. The 1.0 factor rule is well explained but the battery storage is not well explained.
In section 3 here: https://www.victronenergy.com/live/ac_coupling:start they say that you need a PV:Storage ratio of about 1kW PV: 5 kWh or 1.5kW PV:5kWh
For Lead Acid:
1 kWp installed PV power requires approximately 5kWh of battery:
-
100 Ah accubank 48 Vdc
and for Lithium Ion:
1,5 kWp installed PV power requires:
-
100 Ah accubank 48 Vdc
Could I please get a detailed response to this maybe with an example (also for the future readers of this thread) so that the reasoning can be understood? In some places it says 'for system stability' in others it says 'for battery lifetime'. Neither is a very good answer for an otherwise detailed knowledge platform.
My thinking on this:
Assuming the batteries could get charged by full power in an uncontrolled manner through the AC coupled PV inverter, for a 1 kW PV to a ~5kWh Battery@100Ah/48V, you would be charging at about 0.2C which is low enough. And for Lithium Ion at about 0.3C which is also quite low.
What would happen if we only had for e.g. 5kWh Lead Acid for 5kW PV output? The charge rate would be 1C, which is high for lead acid but not dangerous.
So what am I missing? Why such a high minimum battery capacity? If the PV power is high enough and the battery is only used to form the grid (but not discharging much since the load is met by PV), then it would be good to save money on storage and instead invest it on PV.
Thanks for spending your time on the answer and for sharing your knowledge.
Stay safe!
M