To my mind, there's only one energy source that's reliable, renewable, safe and efficient enough for my liking: geothermal.
No political candidate has yet promised the public more volcanoes, but when they do, I will vote for them.
Admittedly, geothermal is great, but there isn't enough of it. (It's also, technically, a form of nuclear power.)
Nuclear power is an extremely safe and efficient power source. Chernobyl, the big disaster everyone thinks of that pretty much ruined nuclear power for the public, was a
predictable engineering failure caused by the Soviets
not even trying to be safe. Nuclear plant failures under reasonable safety measures have so far killed...
maybe one person that I know of, at Fukushima Daiichi, whose cancer was declared to have been caused by the radiation by the Japanese government. Now, there may be others, for sure, but there are also a surprising number of people who have been killed in geothermal plant accidents. Meanwhile, nuclear waste, the other big scary thing people tend to mention, is a solved problem with breeder reactors and processes like REMIX and PUREX, to which the primary opposition has been "OMG plutonium!"
It's still not a magic problem-solving button and won't absolve us of the need to reduce the human population by more than half. But it'll be an effective way to power the civilisation that exists afterward.