Inverter size calculator
A 1,200 W load on a 12 V battery pulls 111 amps of DC once you allow for inverter efficiency — and on surge, over 330. That is the reason 12 V inverters above about a kilowatt need battery cable as thick as your thumb and a fuse to match. Moving the same system to 48 V drops the running current to 28 A and turns an awkward installation into an easy one.
Estimates for planning and study. Fixed wiring must be designed and installed to the wiring regulations that apply where you are, by someone competent to do it.
Size an inverter for continuous load plus headroom, and separately for surge — motors need three to six times their running power for a second or two. A 1,200 W load wants a 1,440 W continuous rating and a 3,600 W surge, pulling 111 A from a 12 V bank.
How to size an inverter
Inverters are rated twice for a reason. The continuous figure is what it can sustain thermally; the surge figure is what it can supply for a second or two before it folds back. A fridge compressor starting is a genuine short-lived multiple of its running power, and an inverter that meets the running load but not the surge will simply refuse to start it — repeatedly, and confusingly, because the load looks well within its rating. The other half of the job is the DC side, where the current figures get large enough that cable and fusing become a real design task rather than an afterthought.
Questions
Continuous rating above your simultaneous running load plus headroom, and surge rating above the biggest motor start.