The power theft caused annual losses of about $35m (£27m), Liberia Electricity Corporation officials told state radio.
This was robbing the utility of cash for extending power supply, they said.
Liberia is trying to rebuild its power sector, destroyed during a civil war which lasted from 1989 to 2003.
The US is giving financial and technical aid to the West African state to increase connectivity, as part of the Power Africa initiative launched by former US President Barack Obama to bring electricity to 50 million people in sub-Saharan Africa by 2020.
The government has set itself the target of rolling out electricity to 70% of Monrovia’s population of more than one million by 2030.
Why people steal electricity
By Jonathan Paye-Layleh, BBC Africa, Monrovia
Power theft is a problem among both the rich and the poor. It is not because people do not want to pay, but because the power utility has not been able to meet the huge demand for electricity.
As a result, if a person sees an electricity cable running over his home or shop, he will connect his own wires to the cable to give himself electricity.
o tackle the problem, the power utility has established a taskforce, which patrols neighbourhoods and cuts off illegal supply. It has also set up a hotline, urging people to report power theft.
The Mount Coffee hydro-electricity plant, destroyed during the war and subsequently rebuilt with donor aid, generates a lot of electricity, but the problem is that the state utility has limited capacity to distribute it. And until that changes, power theft is likely to continue.
Some people see privatisation as the solution, pointing out that the telecom sector has boomed since it was liberalised almost 20 years ago. But there is no indication that the government intends to do this in the power sector anytime soon.[[Jonathan Paye-Layleh, BBC Africa, Monrovia]