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Turn as much as possible off except his shed, take a meter reading, wait an hour or two, take another meter reading. Scale that up for daily/monthly use. If you want to go full nerd, do it a few days on different temperatures, eg now, mid frosty season, etc.
If it’s his business use, I’d imagine there’s some way for him to offset even partially as a business expense.
Also, I’m concerned for your wellbeing if your husband is willing to use anger to stop you investigating this, or to force you to shoulder the costs for his office running. There’s a book by Lundy Bancroft on angry men. It might shed some light on things.