IMHO, the 55G would be a better choice. You get 1-1/2 times the power output, and a twin cylinder engine that operates at a much lower rpm (read less noise). With only 3kw, the 45G may not be able to keep up with A/C, microwave and other appliances without having to shut down one or the other. Any time you operate at or near maximum load, the wear on the engine and other components increases greatly, and time between repairs could be shortened considerably. An additional load you may not have considered is the Converter/Inverter battery charging draw, which could be as much as 1kW by itself (500W for a 40A converter) Bite the bullet now rather than later wishing you had gone larger in the first place. Removal and replacement is double the work, and you would still have the old 3kW unit to (hopefully) sell, but at a large loss.