Kavesh

Hi all I am new here and really looking for some honest advice. I have searched some of the other posts regarding inverters which was quite useful. I am looking for a backup power system for my home to keep the lights on and a few plugs. (Nothing heavy like stove, ovens, geyser) I was hoping to have my fridge connected as well but various people have said that this is not a good idea as the power draw would be too much. I was really keen on a generator backup but with the high costs of fuel that idea went out the window quickly. Are there better brands to look for over others? How best do I size the inverter. I am not looking for solar panels at this stage as my budget may not allow for it. My budget is in the region of R15k-R20k. Hopefully that is enough. I have a modest house, but is a 2 storey building with 2 DB's. (Have a 40amp municipal breaker) The one issue with this is if I put the inverter downstairs it won't power anything upstairs and I would think vise versa. This would be an issue which I need to resolve. I know I will need to engage an electrician but I like to know what is required prior to getting them on site so I pay for only what work is needed. Would really appreciate any advise you can give me. Many thanks Kavesh