Can I put an internal invoice on Purchase Ledger to balance the account?

I wouldn't put an Invoice on as this would indicate a sale - you would most likely be better off (if the amount is not material) accruing the amount so it sits on the balance sheet rather than your P&L. You can always release the accrual at a later date should a large overspent occur the utility account. Always make sure you can justify your accruals though.