>What's the running cost of a huge ups?

A UPS will require a battery replacement every 10 years or so, regardless of whether you ever have to go on battery power or not. The UPS should automatically sense the status of the battery and warn you when a replacement is needed.

Pick a model from one of the major manufacturers and look up the replacement battery costs to get an idea of what it will cost.

I don't understand. Why would it cost anything to run it? It's just a big battery. Maybe a small charge to keep it topped off?

It was a genuine question, I have ever used or needed one, therefore I have little to no idea how they operate.

It's just a big battery that you can plug things into full time. When power goes out it instantly switches to battery power. It recharges when the power comes back.