What should I do about the amount of electricity consumed from a meter that is not mine, which the guard took possession of to use for finishing works?

What the guard did is an aggression against public funds and an unlawful consumption of wealth. The guard must be stopped, and the حرمة (prohibition/sacredness) of his act must be explained to him. If it is possible to return what was consumed to the managing company, ensuring it reaches its proper destination, then this is obligatory. If it is impossible to return it, or if returning it would cause harm, then estimate what was consumed and place it in public welfare projects and charitable causes. As for anyone who has unlawfully taken from state funds, they must repent to Allah by feeling remorse, resolving never to return to such an act, and abandoning it by giving the taken money in charity if they are able. If they are unable, it remains a debt upon them that they must repay when they are able.