Solar-power arrays have sprouted up on rooftops and parking garages across Tucson and Arizona.

The next big wave of solar development will be across expanses of desert, as Tucson Electric Power Co. and other Arizona utilities bring online hundreds of megawatts worth of large, utility-scale solar-power plants.

Last year, state regulators approved some 150 megawatts of renewable-energy projects to be owned by TEP or developed under power-purchase contracts with the utility, including more than 130 megawatts of new solar plants by 2013. Six of those projects are expected to go online by the end of this year.

One megawatt, or 1 million watts, worth of solar capacity is roughly enough to power about 160 typical TEP homes, the utility estimates.

On top of those projects, the Arizona Corporation Commission has approved TEP's plan to spend up to $28 million annually to add about 7 MW of new company-owned or contracted solar-power capacity each year through 2014.

And the state's biggest power company, Phoenix-based Arizona Public Service Co., plans to increase its solar output more than sevenfold by 2013, with major utility-scale projects including an 18-MW photovoltaic plant and a massive, 280-MW concentrating solar thermal power plant, both in the sun-soaked desert near Gila Bend.

The Big Solar buildout is happening nationwide.

Driven by taxpayer subsidies, state renewable-energy mandates and plunging equipment costs, the U.S. market for utility-scale solar energy farms is expected to double each year between 2010 and 2015, to $8 billion, according to a study released in November by GreenTech Media, a Cambridge, Mass.-based research group.

In 2010, 274 MW of utility-scale solar plants were expected to be connected to the grid - a 370 percent increase from 2009. GreenTech projects are planned to double in 2011 and reach 5,600 MW by 2015.

Utility-scale installations will help TEP and other utilities meet a state standard that regulated utilities generate an increasing amount of power from renewables, including solar and wind.

"We are planning a mix of resources to meet the mandate," said TEP spokesman Joe Salkowski.

Utilities must generate energy equal to 15 percent of their retail power sales by 2025, in increasing increments that will hit 3 percent this year. The goal is supported by a system of incentives - cash rebates - funded by ratepayers through monthly surcharges.

The renewable energy standard, adopted by the Corporation Commission in 2007, requires that utilities get at least 20 percent of their renewable energy from so-called "distributed generation" - installations on customer sites that feed into the main power grid but offset each customer's individual usage.

Utility-scale installations - typically ranging in size from a few megawatts to hundreds of megawatts - supply power directly to utilities that own the plants or the rights to the power they generate.

Since the latest state renewable-energy standards went into effect in 2007, much of the development has taken place on customer rooftops, driven by incentives paid to customers, the expansion of solar installation companies and a relatively streamlined approval process.

Because of their size and cost, utility-scale solar projects have taken longer to develop - some current projects have been years in planning - but their contribution to the power grid will dwarf that of rooftop systems.

Through the third quarter of 2010, distributed systems installed on TEP's system contributed about 9.7 MW of generating capacity, with nearly the same amount in the pipeline for the next year or so.

Multi-megawatt, utility-scale solar has some distinct advantages over rooftop systems in cost and available technology.

While TEP estimates the cost of rooftop photovoltaic systems at about $5.50 per watt, not including incentive payments, utility-scale solar runs about $3.50 per watt, Salkowksi said.

Because utility-scale solar power plants are larger and their power is fed directly to the utility, it's easier to manage to help fill peak power needs than thousands of smaller systems channeled through customers' meters, Salkowski said.

"We have to have that energy handy, the moment it's needed," he said.

While rooftop solar systems are almost invariably fixed photovoltaics, utility scale allows developers to economically add such features as sun-tracking panels, which can add 25 percent to a panel's output.

Large scale also allows other technologies - like solar thermal systems, which harvest the heat from sunlight to produce power from steam generators - that are too costly or impractical for rooftop installation.

TEP is adding several utility-scale plants using tracking systems and advanced technologies, including tracking systems and photovoltaic concentrating technology, which focuses light on solar cells to minimize the use of photovoltaic materials.

TEP also is planning one solar concentrating thermal plant, a 5 MW system with heat storage, at the Solar Zone at the UA tech park.

The system, being built by Bell Independent Power Corp. with expected completion in 2013, will store heat to keep generators running hours after sundown.

That would help the utility handle peak-power demands in the late afternoon and evening, when photovoltaic panels peter out, Salkowski said.

"Storage is a long-term strategy, as renewable energy plays a larger role in our generation resources," he said. "In the meantime, we're waiting to see the technology develop further."

But APS has made a significant bet on stored thermal solar energy with its Solana plant, which recently broke ground about 6 miles north of Gila Bend after finalizing $1.45 billion in federal loan guarantees in mid-December.

The plant, being built by Spanish-owned Abengoa Solar and expected online in 2013, will store heat in molten salt tanks for up to 12 hours of generation, said Eran Mahrer, APS director of renewable energy.

That allows the plant to provide "dispatchable" power while the summer heat lingers into the night, when photovoltaic power is unavailable, Mahrer said.

APS has about 55 MW of solar online now, both distributed and utility-scale, and another 470 MW in development.

But not every grand solar project sees the light of day.

In October 2009, Lockheed Martin and a solar development partner canceled plans to build a in a 290 MW solar-thermal plant in western Maricopa County to serve APS, calling the project too costly.

Contact Assistant Business Editor David Wichner at dwichner@azstarnet.com or 573-4181.