Landlord trying keep the entire deposit

Hey guys,
Last year I lived in an apartment with 2 other guys for 6 months in Frankfurt. I paid 615€ for the first month and next 5 months I paid 15€ extra cuz winter was coming. Now, that I have left the place and landlord hits me with an email that we used too much electricity (our heaters are also electric) and that he is going to keep all of the deposit (1000€). I looked up in the attached bill that the charges shown for the whole year, it shows the 6 months I wasn’t renting the apartment are comparable with the rest 6 months which looks weird to me. And he trying to charge me for the whole year. Could you please tell me if there is anything I can do about it.
Ps: my contract says all costs are inklusive in the rent