How much of my electricity bill can I safely deduct off my taxes? 20 - 25%?

How much of your DEDICATED FOR NO OTHER USE  space in your home by square foot is the determining factor.  If your "home office" also is used for family game night or watching the Late Show with Stephen Colbert then you cannot deduct anything.  The IRS has specific rules and I doubt anyone here will advise you on a blanket deduction without further particulars. In fact, specific tax advice is beyond the scope of this forum.

https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction

Please read in detail what the IRS has to say, and pay particular attention to the rules disqualifying home expenses if you maintain a separate main office.

There is also an allowed Simplified Method where you can deduct a flat $5 times UP TO 300 square feet of dedicated business-only space. Under the simplified approach you cannot deduct any actual expenses (like electricity) but you can depreciate a portion of teh premises.

However if you are going to deduct any electricity it means you are going to determine Actual Expenses and thus would include an allocatable portion of other utilities, property taxes, etc.