Why is my sim being charged power on an off the grid home?

The easy way to remember is anything with a minus/subtraction sign (-) in red on the left, is a credit/discount to you. Anything with a plus/addition (+) sign in green on that side is a debit, or in other words an amount you owe. The same applies to the rate of change on the left, red is money back to you, green is money you owe.