The free version doesn’t put any thumbscrews on you. You can store all your meters and any number of readings in the app. There is no restriction. Provide the readings with the date and exact time.
For an overview of costs, you should enter the current prices from your utility contracts. Otherwise, the app calculates with average values, which show whether savings measures help, but only the concrete prices give you a precise view of your situation.
The item “Activities” is interesting: Here you can create activities such as washing, bathing or ironing and find out how much it costs to fill a bathtub or a load of colored laundry. To do this, enter the current meter reading and add the action to the meters involved.
Good idea: the developer lets users vote on the next features. As it looks, there will probably soon be the possibility to deposit a photo with the meter readings as security in case there are disputes with the supplier.