If the lighthouse is on the land then a mains power cable is installed, either by burying it in the soil of the land leading to the lighthouse, or by taking a wire from an overhead pylon. If the lighthouse is out to sea, it may be possible to use an undersea cable, as long as the lighthouse is not too far from the land. However, all remote rock lighthouses need to generate their own power and many land stations do too because if there was ever a power cut, the light must not be allowed to go out. So an electric generator is required.
Most generators are diesel engines such as you would find in cars or trucks. Diesel engines burn diesel fuel and cause pistons to go up and down. As they do so, they cause a shaft to turn. The idea now, as we said elsewhere, is to have a coil of copper wire and a circular magnet. When one is rotated inside the other, electricity is generated in the copper wires of the coil. So lighthouses either get their electricity directly from the mains or they have supplies of diesel delivered. In fact, many probably do both.
Older lighthouses used to burn gas. If there was a supply of town gas (e.g. from coal) they might have a direct supply through gas pipes - just like you may have in your home.
Suppose you are in a lighthouse that has an electric light source but no external supply of electricity. You thus have to generate all your own electricity by means of a diesel-powered electric generator. What happens if the engine breaks down? Many lighthouses overcame this problem by having another generator, but what if your worst nightmare came true and the second one broke down too? Well, now you must resort to batteries. In cars there is a type of battery called a lead/acid accumulator. All the time the engine is running, the battery is being charged so that the next time you need to start the engine, there is power to turn over the starter motor. So in the lighthouse you have a bank of these batteries connected together and all the time the generators are running, the batteries are kept fully charged. If ever the need arises, the batteries can then be used to power the light. This is very much a last resort. Many lighthouses don't have any more than one back-up system and sometimes the back-up may be gas or oil fuel.
Today we know all about solar power. we can use flat, thin panels of silicon with wires attached to generate electricity when the sun shines on the panel. Of course, we could use this at once, but we don't need a light when the sun is shining. So we use the sunlight to generate electricity and charge banks of lead acid accumulators.
Imagine a situation where you have a large optic, perfectly designed to capture all the light created at the focus and throw it out as a powerful beam in all directions. But, during the day, the sun shines brightly. The light is not working and the optic is not rotating. The keepers are resting, or cooking, or painting... the bright sunlight falls onto the powerful optic and, because the light can take the same path in both directions, is concentrated into an intense point of light at the focus where you have the light source. Serious damage could occur. We all know how fires are supposedly started when the sun is focussed through broken bottles acting as lenses, for example.
So another essential task for the lighthouse keepers was to use curtains during the day in order to keep the sun off the optic. This was done for centuries, until the lighthouses were automated. Without the keepers there to look after the curtains, what was to be done? In many cases, the answer was to have the optic rotate all the time - day and night.