The number of solar power systems across the island state has doubled every year since 2007, with nearly 20,000 units installed. But with homeowners and businesses now producing nearly 140 megawatts of their own power — the equivalent of a medium-size power plant — and solar tax credits biting seriously into the state budget, Hawaii legislators and electrical utilities are tapping the brakes.

Solar tax credits cost the state $173.8 million this year in foregone revenue, up from $34.7 million in 2010, prompting state tax authorities to announce this month that they will temporarily cut the tax credit in half, effective Jan. 1.

Hawaiian Electric Co. on Oahu, which oversees subsidiary utilities on Maui and the Big Island, has warned that the explosion of do-it-yourself solar could threaten parts of the power grid with the possibility of power fluctuations or sporadic blackouts as the power generated by homeowners — unpredictable and subject to sudden swings — exceeded output from power plants in some areas.

So rapid is the growth that Hawaiian Electric at one point proposed a moratorium on solar installations, a plan that met with immediate outrage and was quickly withdrawn. But utilities are requiring expensive “interconnection” studies, such as the one the Lees had to do, in solar-saturated areas to analyze what impact a new unit is going to have on the utility system before it can connect to the grid.

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