Solar energy is most efficient in terms of environmental impact. Coal and natural gas are more efficient by reliable applications, though.
Solar panels typically have between 15% and 20% efficiency, while coal has an efficiency of up to 40% and natural gas reaches up to 60% — the remaining energy in fossil fuels and coal is lost in the form of heat — and it’s lost forever. Optimize your solar panel efficiency by installing a solar concentrator or backup batteries and by not running too many devices at once.
Solar energy requires a hefty upfront payment, but it's more efficient in the long run. You also offset electricity costs for the life span of the panels, which can be as long as 35 years.