Both of those issues have contributed to a declining interest in the energy source, but Toshiba’s announcement this week that it would scale down its business building nuclear power plants underscores the real challenge to the energy source: it’s really expensive.

The Japanese multinational company said Tuesday that it would take a $6.2 billion write-down of its nuclear power plant construction business after several construction projects ran far behind deadline and deep into the red. Poor accounting contributed to Toshiba’s trouble, but the company also faces challenges given the sheer cost of building nuclear power plants and growing interest in cheap alternatives.

Today, the biggest downside to building new nuclear power plants in many developed countries is sheer cost. Data from the Energy Information Administration shows that building a new plant costs more than $5,000 per kilowatt of capacity compared to around $2,100 for the primary type of solar power plants and less than $1,000 for the most common type of natural gas plant. (These figures vary by region within the U.S.). A nuclear power plant also requires six years of lead time while a solar plant can operate in as little as two years.

Nuclear power plants do provide some advantages over other sources, namely that it provides consistent baseload energy that provides a consistent power source at any time of day or night. But because of the costs the debate over nuclear has shifted from whether to keep old nuclear power plants operating to whether to build new ones.