Computer systems generate 20% of carbon emissions
The government hopes to be the first in the world to make its computer system "carbon neutral".
The Cabinet Office hopes measures like switching off computers at night and making sure servers do not stand idle will save 117,500 tonnes of carbon.
Computer systems generate up to 20% of all carbon produced by government.
The environmental audit committee has warned that the government is "lagging behind" its own emissions targets and IT systems are among factors to blame.
The British government says it wants to be the first in the world to make its computer system's energy output "carbon neutral" - through a combination of reducing energy use and offsetting schemes - such as buying carbon credits.
It also aims to make the whole system - taking into account the manufacture and disposal of the machines - carbon neutral by 2020.
Cabinet Office minister Tom Watson will announce later 18 key steps which departments will be asked to take to vastly reduce their "carbon footprint" - the measure of their impact on the environment through greenhouse gases.
Switching off computers outside working hours could save up to 117,500 tonnes of carbon emissions a year - equivalent to taking 40,000 cars off the road, the government says.
Other measures include re-using as much computer equipment as possible - as making the equipment in the first place uses up a lot of energy - and auditing servers to make they are not standing idle.
Mr Watson said that worldwide, computers are responsible for as much carbon emissions as the airline industry.
"That's why I'm so proud that we are the first government anywhere in the world to formally set out exactly what we're going to do to make our ICT systems carbon neutral within four years.
"We won't achieve this just by offsetting but by making serious changes to the way we do business."
The guidelines will be adopted by the Cabinet Office immediately and other departments will be asked to use the rules and report on progress.
In a report earlier this month, the Commons environmental audit committee accused the government of "lagging behind" its own targets to cut carbon emissions and said "rapid progress" needed to be made on cutting electricity use.
It also warned the government not to rely too heavily on carbon offsetting, but to look at reducing its own emissions.
Shadow Minister for the Cabinet Office, Francis Maude said the Conservatives welcomed the measures, but added: "The government's own environmental watchdog has savaged ministers for not doing enough to go green.
"Merely switching off a computer at night may get a good headline for the Whitehall spin machine but isn't enough."