Since most energy sources are not available continuously some energy storage is required to keep plants alive and production processes running.
Food contains energy for human beings, and hence the production of food consumes a great amount of energy. In a static (not growing) colony this seems to be the biggest consumer of energy. Due to the dim sunlight the greenhouses must be lit with additional energy, especially during dust storms.
Solar panels provide electricity out of sunlight. This source of energy is currently utilized by the majority of space probes, including the Mars Exploration Rovers. Maintenance effort is low, but hi-tech processes are required for their production. The availability strongly depends on the weather conditions and the daytime. Dust storms can last for months during which solar panels are useless. Unfortunately, this energy is not available during the time the greenhouses need additional energy for lighting. The solar energy reaching the surface of Mars is less than half of what reaches Earth. Solar Concentrators may be used to increase the efficiency of solar energy devices.
Wind turbines provide electricity out of movements of the atmosphere and can produce lots of energy. Dust storms may have a massive abrasive effect on the rotor blades, increasing the maintenance effort. The availability strongly depends on the weather conditions. Fortunately, this energy is available just during the time the greenhouses need additional energy for lighting.
The atmosphere of Mars is 100 times less dense than the atmosphere of Earth. Thus wind turbines on Mars would generate much less energy than comparable wind turbines on Earth operating in comparable wind velocities. Two things must be modified: The rotor blades must be enlarged by a factor of 100 to make up for the thin atmosphere, maybe with a different shape. Calculations will show if this is feasible regarding stability and material consumption. Secondly, the moving parts must be redesigned to avoid friction in dusty bearings, maybe pure magnetic bearings without direct touch.
Nuclear power has been considered as the preferred energy source for most plans for medium- to long-term human expeditions to Mars. It does not depend on weather conditions. The availability of radioactive resources on Mars is unclear. Due to the vast effort of the nuclear enrichment process the nuclear fuel must be brought from Earth.
Thermocouples provide electricity out of temperature differences. Thermocouples are used is RTGs, but can also generate electricity out of the terperature difference between the Martian surface and deeper layers. The amount of energy produced with this technology is sufficient for powering sensors or small gadgets for information technology.
Newer research is done by scientists of the Fraunhofer Institute. They have found a way to use thin electroconductive polymer foils to generate electricity. Those foils could be laid out on large areas and covered with a thin layer of regolith, which is heated up by the sun during the day.