Electricity generation in photovoltaic stations depends on the light intensity. In foggy conditions almost no electricity is generated. At night there is absolutely no electricity generation due to the absence of sunlight.
Solar water heaters also depend on light intensity level. Water hardly gets warm in foggy weather. Depending on the cloudiness level, water will heat up to different temperatures. In winter under direct sunlight the temperature of water will reach the level usable at home or higher even if it’s extremely cold outside. In summer water temperature reaches the boiling point.