Global investment in coal-based power plants fell to its lowest level in 10 years during 2017, led primarily by fewer such plants being commissioned in China and India, a new report shows.

"Retirements of existing coal-fired power plants offset nearly half of new coal plant additions," according toWorld Energy Investment 2018, produced by the International Energy Agency (IEA), which has 30 member countries including the U.S.

2017 saw 24 gigawatts of coal capacity retired globally, and over half of that was more than 40 years old. Nearly 70 percent of the affected plants were in the United States and Europe, and were "retired on environmental and economic grounds," IEA said.

"By contrast, investment in gas-fired generation capacity rose by 40 percent, led by the United States and the Middle East/North Africa," the report said.

Investments in both solar PV and offshore wind hit record levels last year, but despite that, the report found overall investment in renewables-based power generation capacity fell 7 percent. Hydropower investment alone slumped 30 percent to its lowest level in more than a decade.

And where solar PV is concerned, the report noted that China accounts for more than 40 percent of the global investment. That raises the specter that recent policy changes there could lead to a slowdown in investment this year. IEA said this highlights findings in its past reports that policies driving investments in renewables are of critical importance.

Nuclear power saw a 45 percent decline in global investment last year, with IEA noting that "a growing share" of that money "is going to upgrades of existing reactors, which now represents around half of total nuclear investment." Only four new reactors were commissioned in 2017, three of them in China. The report also pointed out that Europe's decline in nuclear generation since 2010 has offset more than 40 percent of the continent's growth in solar PV and wind output.

Investment in grid-scale battery energy storage was $600 million last year, compared to $1 billion in 2016—but that's actually good news, as the report noted that "stemmed largely from the reduction in battery costs."

And after being overtaken by South Korea the year before, the U.S. regained the lead in battery storage deployment in 2017, with IEA noting "state policies are driving activity." It cited California in particular, where utilities are under a mandate to install 1.3 gigawatts of storage by 2020. The Golden State saw 100 megawatts come online last year, raising the total to 139 MW, and "hundreds more megawatts are in the interconnection queue," the report said.

About 1.1 million electric cars were sold worldwide last year at a total purchase cost of $43 billion. They represent a record 1.3 percent of all passenger car and truck sales.

The report noted that "most of these purchases benefited from some kind of government incentive (national or local)." And it said continued expansion of the electric car market will depend on declines in both car prices and operating costs that meet customers' expectations, while avoiding "unsustainable increases in government budgets for purchase incentives."

Overall, IEA said 2017 global energy investment totaled $1.8 trillion, down 2 percent from 2016. The largest share—$750 billion—went to the electricity sector.