(NewsNation Now) — As extremely cold winter weather and snowstorms slam the U.S. Monday, some cities are experiencing widespread rolling blackouts.

But what is a rolling blackout and why is it necessary?

According to Direct Energy, an energy provider in the U.S. and Canada, rolling blackouts are “systemic, temporary power outages that help bring balance to the supply-demand of electricity in the market.”

Generally, rolling blackouts occur as the “last step in a series of emergency procedures” when a power supply shortage is detected in a market. They help to prevent widespread blackouts across a region.

Rolling blackouts generally occur in one area at a time and last for a few hours, but the length of the blackout depends on the severity of the event.

Utility providers decide which parts of the city will be subject to rolling blackouts and try to avoid areas with hospitals, medical centers, or downtown regions, for which electricity is vital.

With higher-than-normal generation outages, it’s important to conserve and reduce electricity during high usage times.

In Texas, more than 3.5 million customers were without power as of 6:15 p.m. EST, according to poweroutage.us, a utility tracking site.

Reliant Energy, a Texas utility provider, provided these tips to NewsNation affiliate KIAH to help Houston-area residents prevent rolling blackouts, regardless of their electricity provider:

• Set your thermostat four degrees lower than usual.
• Avoid the use of large electric appliances, including washer/dryers and dishwashers, during the early morning and late evening hours, when demand for electricity is highest.
• Close heat escape routes in your home.
• Set your ceiling fan to rotate clockwise, which helps force warm air down into the room.
• Close blinds and shades to reduce the amount of heat seeping out of windows.

NewsNation affiliate KXAN reports some Texans without power Monday afternoon could be forced to remain without it through Tuesday, according to an emergency update from Oncor.

It’s important to use caution while heating your home if there is a power outage. In Texas, six people were hospitalized due to carbon monoxide poisoning after using a charcoal grill to heat their apartment.

Nexstar Media Wire and NewsNation affiliates KIAH and KXAN contributed to this report.