Hi Chris, We thought about having electrical underfloor heating mats put in our extension but after we spoke to a consultant it made a lot of sense to use wet underfloor heating. There is a lot more to it than just how many watts the mat draws. You also need to take into consideration insulation and heat loss calculations. IE: The type and number of windows you have in the house the thickness of the walls etc etc... All of this will have a huge impact on your electricity bill.
Also the chances of an electrical heat mat going wrong over a 10 year period is alot greater than a wet underfloor heating system having issues and wet underfloor heating systems tend to be a lot easier to fix. (Not very easy to pull up an electrical mat and change it if it short circuits or malfunctions.)
Most water based underfloor heating systems have a 25 to 50 year guarantee on pipe so definitely something to consider.
Good luck with the project! 🙂