Dig out the EPC and find the annual kWh for heating and hot water.
Then double that number.
Then times it by £0.33 plus a daily charge of £0.54. Add those up and add 5% for VAT.
Then consider this:
https://www.theguardian.com/money/2023/oct/24/higher-energy-bills-forecast-for-uk-households-next-year
Someone posted on here yesterday trying to sell her son's fully electric flat. She claimed it was costing £100 a month in winter and £50 in summer with him being frugal. It was 45 sq m and had people either side and above.
The number and position of neighbours matters. If it is top floor, you lose heat. If it's a top floor penthouse with no neighbours either side, you lose even more heat.
An EPC C is pretty decent, but EPCs are often vastly, vastly wrong, and TBH, I struggle to believe a fully electric flat could even achieve a C because of the running costs.
Usually a new build gets downgraded after its first EPC, and as it's 20 years old, unless they have made energy improvements, it should really have fallen into the D category, imo. This is because when flats are first built, developers literally hand over blueprints and all the fit out proof to assessors. When they next come to assess, this proof is not available, the assessor has to make assumptions, and it falls down a category.
Would it put me off? If gas doesn't even enter the building, yes. Because energy is so desperately volatile at the moment.