Is it permissible to use a stratagem to pay for the electricity debt of the pump connected to the building's electricity, by collecting money from the residents without their knowledge, or by paying more or less than the value of the debt?

If your electricity consumption was unauthorized, then the cost of that consumption is a debt upon you to those residents who paid for it. You must hasten to pay them back. You can pay their electricity bill on their behalf to the extent of what you consumed, if informing them of the truth would cause harm. If you pay more than the amount you owe, that is permissible. The fundamental principle is that rights should be returned immediately to their owners and not delayed except in cases of inability or permission from the rights-holders. Your mere apprehension that they might think ill of you does not permit you to delay returning this right to its rightful owners.