You can’t use a SMA Sunnyboy by itself for an offgrid supply.
NOT TRUE......Read on....
On some models of the Sunny Boy grid tie inverter there is an option called SPS. Secure Power Supply. This is a factory installed option only, it cannot be retrofit. It is activated by a external spst toggle switch which allows one of the inverters to power an auxiliary outlet. this is limited to a 15 amp 120 volt single circuit. It is available to power a single circuit only when the grid is down. The newer transformerless models have this feature. I have installed 3kw. 120/240 volt USA models which have this feature. On these models there are 2 MPPT front ends, each one drives a 120 volt inverter at 1500 watts. Only when this feature is turned off by the external switch can the unit be “in grid tie mode” . When the external switch is turned on one inverter is shut off and the other inverter’s output is rerouted to the SPS outlet which is located in the external junction box along with the SPS switch.
When in SPS mode the normal output connectors of the inverter are disabled to prevent backfeeding the grid.
It is not a true “off grid system” as it has no battery and can only use the available power from the string of panels feeding that inverter, not power from both strings.
It is not a “Hybrid System” It is meant to power a minor appliance and small loads only such as your referigerator and a few lights, radio etc. Obviously this does not work through the night.
I have the manuals as the Sunny Boy is the only grid tie systems that i service and install.
AFAIK.....Only SMA America Sunny Boy models have this feature.
It is really not meant for your proposed use but as an emergency source of power when the grid is down.
I have installed a (USA model) which does have this feature and it does work as advertised by SMA America. This was installed by myself in Santa Cruz County , California.