Motors, pumps and inverters are always a touchy situation. Aquarium pumps will probably be less touchy. A motor could require 1 to 5x capacity to start. That's why generators and UPS's are rated in VA.
300 Watts for 17 hours is a lot of time. The inverters will probably kick out at 8V or so, so you have to look at the discharge curves carefully.
DIY is kinda tough.
I'd go with the human approach for now. Notification systems when the power goes out could be useful too. A simple technique is to grab an auto-answer modem and place on the phone. Set up a drop out relay. If it answers you have power, if it doesn't then you don't and have to go set it. This can work if your caretaker is far away.
A more permanent solution may be on the order of a solar system or backup generator with an automatic transfer switch where the generator is powered from natural gas. Fuel cell systems do exist too.
For reference, UPS combined with generators have to be special to handle the erratic frequency. Been there. Done that.