BANGOR – Powerful lightning storms that have jolted Mainers around the state this week shocked some area computer owners Tuesday.

They had discovered their machines didn’t work.

Repair experts on Tuesday advised there’s only one sure way to prevent damage to a computer from lightning: Unplug it.

At All PC Computer Repair & Sales in Orono, manager Jamie Rosebush reported 18 power supplies, eight motherboards and 16 modems destroyed by lightning since June 1.

While surge protectors and uninterrupted power supplies help, neither option can completely halt thousands of volts from a lightning strike.

“Surge protectors and [uninterrupted power supplies] aren’t going to fix the problem,” Rosebush said Tuesday afternoon.

He expected about 10 complaints of lightning damage within the next few days.

Bunker Herbest, co-owner of Computer Renaissance in Bangor, said three customers already had dropped off computers in need of repair by Tuesday afternoon.

All three were minor cases of fried modems. When a lightning-induced power surge reaches the motherboard or even scrambles hard-drive data, however, greater problems can arise, he said.

Computer owners must constantly stay alert when thunder rumbles in the distance. When lightning strikes, the surge can travel for miles, according to Herbest.

“Unplug everything,” Herbest said. “A spark can travel through anything plugged into the wall. Nothing is going to stop a direct lightning strike.”

Many people don’t unplug their computers when storms approach “because it’s a pain,” he said.

Uninterrupted power supplies offer a battery backup that takes over during electricity outages.

During a lightning strike, they also clamp down on any spikes in a computer’s power supply. Even though the UPS device is more effective than a surge protector, neither is guaranteed, Herbest said.

Uninterrupted power supplies cost about $100 while surge protectors can run from $3 to $50, according to Herbest. High-quality protectors include a phone jack to prevent surges from entering through a modem.

“Most people go to the store and buy the cheapest surge arresters that don’t offer much protection at all,” he said, reiterating that none is 100 percent guaranteed.

Herbest said his company used to handle more lightning destruction when dial-up Internet connections were common. With the switch to high-speed Internet, however, many surges stop at modems designed for digital subscriber lines. Cable modems are also susceptible, he said.

All cases Herbest handled Tuesday involved destroyed modems. With replacement parts and labor charges, repair costs for those machines will average $60, according to Herbest. When a surge spreads to other internal computer devices and external accessories, damage can reach the thousands.

“Lightning can do some weird stuff,” he said.

Excel Computer in Bangor had received no calls for lightning-related repairs by Tuesday afternoon, according to a staffer. Owners often don’t discover problems with their machines until they unsuccessfully attempt to turn them on, she said.