SAN FELIX, Venezuela (Reuters) – Despite having some of the world's largest energy reserves, Venezuela is increasingly struggling to maintain basic electrical service, a growing challenge for leftist President Hugo Chavez.

The OPEC nation has suffered three nationwide blackouts this year, and chronic power shortages have sparked protests from the western Andean highlands to San Felix, a city of mostly poor industrial workers in the sweltering south.

Shoddy electrical service is now one of Venezuelans' top concerns, according to a recent poll, and may be a factor in elections next month for governors and mayors in which Chavez allies are expected to lose key posts, in part on complaints of poor services.

The problem suggests that Chavez, with his ambitious international alliances and promises to end capitalism, risks alienating supporters by failing to focus on basic issues like electricity, trash collection and law enforcement.

"With so much energy in Venezuela, how can we be without power?" asked Fernando Aponte, 49, whose slum neighborhood of Las Delicias in San Felix spent 15 days without electricity -- leading him to block a nearby avenue with burning tires in protest.

Just next door, Carmen Fernandez, 82, who is blind and has a pacemaker, says she has trouble sleeping through sultry nights without even a fan to cool her.

Experts say Venezuela for years has skimped billions of dollars in electrical investments, leaving generation 20 percent below the level necessary for a stable power grid and increasing the risk of national outages. Officially Venezuela has a capacity of 22,500 megawatts for a population of 28 million people, but a sizeable proportion is not working, analysts say.

And while Chavez has won praise for investing in health and education, his government has done little to repair local distribution systems that deliver electricity to end users, from barrio residents to business and industries.