Japan's government Friday called on the nation's homes and businesses to cut their energy use by as much as 15 percent amid fears that nuclear plant shutdowns will spark power shortages and blackouts.

With the coming of summer, when the draw on the grid increases as air conditioning gets cranked up, Japan's utilities are warning of a shortfall.

Kansai Electric, which serves the commercial hubs of Osaka, Kyoto and Kobe, has warned it will be 15 percent short, with utilities in other areas expected to redirect any saved power to energy-starved regions, the government said.

Power providers in the northern island of Hokkaido and southern Kyushu have also warned they may not be able to meet demand, with customers in those regions being asked to cut energy use between 7.0 and 10.0 percent, though the government has stopped short of ordering cuts.

A power-saving order was issued last year in the wake of Japan's nuclear crisis sparked by the March 11 earthquake and tsunami.

But the Friday call was not mandatory because it may put "too much of a burden on business activity", Economy Minister Motohisa Furukawa told a press briefing.

The cuts last year, aimed at large energy users, sparked complaints from the business sector and fears it may prompt manufacturers to move production overseas.

Japan's power-saving campaign will run from July through September, as Kansai and some other utilities prepare for scheduled blackouts.

The resource-poor country used to draw about one-third of its electricity from atomic power, but last year's tsunami-sparked meltdown at the Fukushima Daiichi plant has generated anti-nuclear sentiment among a wary public.

All of Japan's 50 commercial reactors have now been switched off, and when -- or if -- they will be restarted remains uncertain.

On Friday, the government said it will send out emergency requests for power saving through media and mobile phone text messages if demand places too much strain on local utilities.

Japan's Prime Minister Yoshihiko Noda said Thursday that he will soon make a final decision on restarting reactors at Oi in Fukui Prefecture, which would be the first such resumption since the disaster.

Fukushima operator Tokyo Electric Power, which supplies Tokyo and surrounding areas, has said it expects to be able to meet demand this summer thanks to continuing power saving by businesses and households.

The cost of utilities has soared as companies turned to expensive alternatives including fossil fuel-fired power stations to fill the gap left by the shutting down of reactors.