Plan to spend about $75 billion a year if you want to keep the lights on and your iPhone charged.

Figure on about $9.4 billion a year if you’d like the toilet to keep flushing.

Or, for about $262 billion a year, you can get the whole package, salvaging the electrical grid, repairing water and sewer systems, overhauling decrepit highways and bridges, updating rail systems and expanding overcrowded runways.

A staggering investment is needed in infrastructure that is reaching the end of its life span if the United States is to remain competitive and serve its growing population.

The experts have been saying that for years, and the latest report on an aspect of their concern was released Thursday, when the American Society of Civil Engineers (ASCE) described the nation’s electrical grid as a patchwork system that ultimately will break down unless $673 billion is invested in it by 2020.

Electric lines extend over the hills of Owen County, Ky., in July 2011, when a lengthy, blistering heat wave in the eastern U.S. was straining the nation's power grid. (Ed Reinke/AP)

If investment isn’t increased by at least $11 billion a year, the report said, the electrical service interruptions between now and 2020 will cost $197 billion.

“Electricity was primarily a luxury when the majority of our grid was built 50, 60 years ago,” said Otto Lynch, vice president of Power Line Systems. “Today it’s an absolute necessity. Business comes to a screeching halt when the electricity goes out. We don’t have our computers, the Internet isn’t there, credit cards don’t work, and what would we do if we couldn’t charge our iPhones?”

The report describes an electrical system that dates to the 1880s, just after Thomas Edison invented the first commercially practical light bulb. It grew much as the country grew, in haphazard fits and starts with pieces that now fit together in an imperfect match of power plants, power lines and transformers of widely varying age, condition and capacity.

“If we keep investing as we are today, we’re headed for some serious financial and economic difficulties,” said Jim Hoecker, former chairman of the Federal Energy Regulatory Commission (FERC). “The investment gap that we’re facing is a little scary. In fact, it’s a little more scary than the report indicates. We’ve got a congested system that keeps electricity costs artificially high, and that translates into higher rates for consumers.”

Aging equipment and bottlenecks in the grid that routes power where it’s most needed have resulted in some brownouts and occasional blackouts. Although investment by power companies has mitigated those failures, the ASCE report said, they will get worse unless billions of dollars are poured into the system.

The system that delivers light at the flick of a switch consists of 5,800 major power plants, 450,000 miles of high-voltage transmission lines that connect them to areas of demand, and overhead and underground lines that deliver them to buildings and to that light switch.

ASCE President Andy Herr­mann described the nation’s power generation as “close to adequate” but said the system that delivers power is badly deteriorated. Lynch, whose company provides consulting and software for power line projects, compared the power grid to an overloaded water system where the pressure on the system increases each time a burst pipe shuts down a portion of the network.

“We’re at a state where we can’t take a line out to fix it,” Lynch said. “We’ve got poles rusting, we’ve got towers falling down, that we can’t take out of service.”

As new plants and sources of electricity come on line, and old ones are retired, the patchwork system becomes more problematic, he said.

“Basically, it’s like moving a fuse panel to the other end of your house,” he said, “and you’re going to have to add new wires, and your old wires are going to have to be upgraded to handle the additional current.”

The good news, said Curt Hebert Jr., another former FERC chairman, is that private capital is available for investment in electrical infrastructure.

“The money is sitting on the sidelines,” Hebert said. “Why is the money sitting on the sidelines? For the most part because we haven’t inspired anybody to get out there and build this infrastructure in a way that they might believe there’d be a return” on their investment.