These may help you with a rough estimate.
https://www.reddit.com/r/Iota/comments/9b6vom/how_can_iota_claim_to_be_free_when_there_is_pow/
https://www.reddit.com/r/Iota/comments/8y831e/iota_costs_000166_in_electrical_fees_to_submit_a/
https://www.reddit.com/r/Iota/comments/8rkwr8/i_always_hear_iota_transactions_are_free_but_are/
https://www.reddit.com/r/Iota/comments/8p29kd/iota_as_the_greenest_crypto/
https://www.reddit.com/r/Iota/comments/84te7o/are_there_any_statistics_on_how_much_energy_the/
https://www.reddit.com/r/Iota/comments/7ih0rn/bitcoins_carbon_footprint_249kwh_per_transaction/
https://www.reddit.com/r/Iota/comments/78mvb9/cost_per_transaction_no_comment/
https://www.reddit.com/r/Iota/comments/6yvmlq/how_much_energy_does_one_iota_transaction_cost_in/
https://www.reddit.com/r/Iota/comments/74t9sc/how_much_energy_does_one_iota_transaction_consume/