As standard they are gas only but there is a supplementary, thermostatically controlled electric heater called the 'Utraheat' that fits inside the heater casing and can be used independently or together with the gas.

Quite a few of us on here have fitted them and find them very useful. They are best used with fan circulation but can be used without.

Do a search for Ultraheat and you'll find a number of threads on the topic.

Here's one example: Ultraheat