Does a solar geyser work in winter?

Yes, but less in foggy or cloudy northern winters. Plan for a gas or electric backup.

Does a solar geyser need electricity?

Basic thermosiphon systems do not; backup elements and pumped systems do.

Solar Geyser in Pakistan: How It Works and Trade-offs

Solar geysers in Pakistan: how they work and what to expect

A solar geyser heats water with sunlight using collector tubes or panels on the roof and stores it in an insulated tank. Running cost is very low and it is unaffected by gas load shedding, but it has the highest upfront cost, needs roof space and sun, and needs a backup for cloudy days and winter evenings.

Types

Evacuated tube: glass tubes that heat well even in cool weather; the most common type sold in Pakistan.
Flat plate: sturdier panels, often pressurised; performance drops more in cold weather.
Solar with electric or gas backup: a solar tank with an element, or solar water feeding a gas geyser.

How well it works across Pakistan

Karachi and southern Sindh get strong sun most of the year, so solar covers much of the demand. In Lahore, Islamabad and further north, short foggy or cloudy winter days reduce output exactly when hot water is needed most, so backup heating matters more.

Practical points

Needs unshaded roof space facing south.
Hard or salty water causes scaling in tubes; regular flushing helps.
Tubes can break in hailstorms and need replacing.
Water is hottest late afternoon; mornings depend on tank insulation.

Solar and gas together

A common setup feeds solar-heated water into the existing gas geyser, which only tops up the temperature. If that gas geyser has auto ignition, it also restarts by itself after load shedding, giving a reliable backup without someone relighting it.