Is working from home a more sustainable choice?

Here's a piece of the answer for the U.S. Hopefully others can provide additional details covering other countries or regions.

MAJOR EDIT: Comments pointed out that something seemed fishy. I double-checked my numbers, and at one point I multiplied by 1,000 when I should have divided. Turns out that makes a HUGE difference.

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Office building emissions

According to the U.S. Energy Information Agency (from Commercial Buildings Energy Consumption Survey 2012), there are 33.8 million office workers using 1.5 billion m² of non-vacant office space in the U.S. (looking at office work only since it is the most likely candidate for the switch to telework).

CBECS also provides data on energy usage for office buildings:

• 773 TWh of electricity (including transmission and distribution losses)
• 83 TWh of natural gas (for heating and cooking)
• 5 TWh of fuel oil (diesel, also for heating)

On average in the U.S., emissions factors for each of these fuels are as follows:

• 0.45 kg CO₂ per kWh -- total 347 million tons of CO₂ (source)
• 53.07 kg CO₂ per million Btu of natural gas -- total 15 million tons of CO₂ (source)
• 78.79 kg CO₂ per million Btu of fuel oil -- total 1.4 million tons of CO₂ (ibid.)

Thus total annual emissions from office buildings is 364 million tons, or 10.77 tons per office worker per year.

Office commute emissions

According to a 2005 poll, the average American commutes 32 miles (51 km) a day. According to the same poll, 4% use public transit, and 8% carpool. If we assume the average carpool is two people, this means that 92% of office workers are driving 51 km a day -- about 31 million workers driving a total of 1.6 billion km per day. There are about 250 working days per year, so that equates to 400 billion km per year.

In the U.S., the average efficiency of vehicles on the road was 22.3 miles per gallon in 2017. This equates to 10.6 L per 100 km. Combined with km driven, this means Americans are using 42 billion L of gasoline per year to commute.

Combustion of gasoline results in emissions of 8.89 kg CO₂ per gallon, or 2.35 kg CO₂ per L.

Thus total annual emissions from workers commuting to offices is 99 billion tons, or 2.93 tons per office worker per year.

Comparison

There are series of assumptions and approximations here (please check my math, again!), but for the average office worker who commutes in a private vehicle in the U.S., emissions from the office are nearly triple the emissions from the drive.

If we say (for arguments' sake) that emissions from a home office per capita are the same as an office building per capita, then eliminating the commute results in a 20% reduction in emissions.

To fully understand the question, we'd need data on increased residential energy usage from teleworking. There are some estimates (here and here), but nothing thorough that I could find.