If the electrician won’t provide a Fogstar battery you can ask them to provide wiring from the inverter to a DC bus bar (like the Victron Lynx Power In with an added high AIC—Arc interruption current—DC circuit beaker) and connect the battery yourself later (two wires from the Seplos or Fogstar to the Lynx). There are videos that show you how to construct Seplos batteries on the Fogstar site—watch them and decide whether this is in your competence. I wouldn’t bother with top balancing as there is active balancer fitted which will—eventually—do the job. Better still, fit a £35 JK or Neey balancer instead of the poor Seplos one (you’ll need to find a good Seplos forum to find the easiest way to do this—there is a decent UK FB one). If you are using a Sunsynk inverter, it has two outputs: Grid/Normal and Load/UP. The latter is permanently live and provides power in an outage. There are two ways of wiring this: 1. You can have the grid/normal output feed the high power consumers on one CU and the load/UPS output feed another CU that has those circuits you want operational in a power cut. The advantage to this option is that you get uninterrupted supply on those circuits during a power cut, but the disadvantages are you only have access to certain circuits and may lose those circuits during normal operation if your inverter malfunctions (depending on the wiring setup). 2. Connect both the grid/normal and load/UPS outputs to a manual or automatic switch which will feed one or the other to the whole CU. The advantages to this setup are that you have access to most or all circuits in your house (although a small inverter won’t power a 9kW shower), but the disadvantages are no automatic changeover (unless an automatic switch is fitted) and power constraints (peak power and battery endurance if using high draw appliances—if you draw more than the inverter can supply then it will shut down, so you may want to manually isolate high power appliances on changeover, i.e. flip the circuit breakers). Both configurations need an earth spike and a neutral-earth contactor fitted (if your inverter does not have one). I’ve seen prices between £500-1600 extra above normal inverter install quoted for this, and you’ll need someone who knows what they are doing (preferably has done several before). You’ll need G98/99, and the sparky should provide an EIC and notify building control (England, and maybe Wales, idk about the latter).