question

I wasn't aware there is a 1:1 rule PV power : Battery capacity.

And if there is, it needs a lot of clarification.

E.g. which 1 stands for what, the rule does not specify "exactly 1:1" but probably more like "at least 1:1". And of course we need to be benevolent with the units. kWp ~ kWh

Let'a assume the rule is called X:Y, where X stands for "PV power" and Y for "Battery capacity" and the rule suggests that the quotient of X divided by Y should be at most 1.

It makes sense for a Lithium battery if you want to limit potential charging current to 1C.

10kWp PV power -> "at least" 10kWh Battery capacity, because then on a sunny day and with the great 99.9% efficiency chargers from Marketing Inc. you keep just unter 1C. Plus your battery is charged within one such sunny hour.

Certainly that doesn't apply to Pb batteries, because if you treat these with 1C, they will pretty soon start to show their discomfort by farting hydrogen in your face. So if the rule exists, it exists for quality Lithium batteries.

If a battery has some BMS and your charger can talk to the BMS (and does listen to it), then it actually doesn't matter how big or small your battery is. It should be always protected. Similar applies if you are able to limit your charger to a max. current.

In that respect, a "1:1 rule" PV-Array:Battery capacity is more of a "rule of amputated thumb".

To give concrete answers to some of your questions:

Does it matter if PV charging is done via an AC grid inverter or DC ( MPPT ) or combination setup.

Electrons are electrons. ;-) As battery is DC, charging it with a DC system (SmartSolar) is more efficient. If your off-grid household is AC, then of course it is more efficient for immediate self-consumption to have AC as efficiently as possible. That's why we use an "PV AC arm" in our installations (self-consumption) and a "PV DC arm" (efficient battery charging). Needs to be well dimensioned - of course.

Why does one need higher battery capacity in an offgrid Multiplus configuration?

For the same reason a plane or a ship that is about to cross an ocean needs bigger fuel tanks than a city-hopping car. You may need to last longer without a gas station on your way. If you are off-grid and your PV-Array(s) are your sole means of energy income, you need more storage reserve.

If you have a Gas/Diesel generator, your battery can be smaller. If you are in an area with reliable and strong solar yield (Arizona), your off-grid battery can be smaller than - say - an off-grid cottage in Canada.

Its not unique to SolarMD, BYD and probably others the same but not given in Victron documentaion. There is a clue here in the Freedomwon documentation it is not permissible to install an inverter larger than stated in the Lite specification sheet regardless of whether the “Limit DC Discharge Power” ESS feature in the Venus device is used. For instances where the grid is not available this power limiting feature will not operate and the battery could thus be overloaded, causing it to trip.

I'm not an expert on the topic, but this is my understanding of what off-grid is why it requires a bigger battery.

When they refer to "off-grid" they refer to an installation that has no grid connection at all. In cases where the grid is usually present they refer to that as "Self-consumption" when tied to Solar and or generators and "backup" is when the inverter is used to supply power on a grid failure only.

I can't speak for all BMS's but in general the BMS will protect the battery and control the charge voltage passed by the MPPT (from panels) or inverter (from mains). The configuration and settings on your system will control when and what the source of the charging is.

Why do you need more batteries on an "off-grid" installation:

• Most of the manufactures recommend the same practice e.g BYD - https://www.victronenergy.com/live/battery_compatibility:byd_b-box
• Spikes and sustained in consumption (e.g starting a motor). In a grid connected system the inverter can supplement the load with mains power. The battery combined with the mains and solar (if applicable) can buffer the increased consumption. In an off-grid scenario the battery and solar must be able buffer the consumption without support of the mains. The sustained consumption of a battery is usually higher in an "off-grid" scenario.
• Long periods of low Solar generation. In this case the battery needs to be able to keep enough charge to span this period. This is more important when you have no mains to back you up.
• Battery life optimization. Having more battery capacity means you reduce the cycling of the battery, this in-turn will generally lead to a longer life ( This is one of the reasons the manufacturer often links this to their warranty ) . In an "off-grid" scenario you are going to be cycling the battery more.

When sizing the batteries for the system I think of it like this. If the grid is reliable or only has short/rare outages then you can align to the "self-consumption" guidelines. If the grid is unstable with long outages I would align to the "off-grid" recommendations for batteries.

Hope that helps