The group were among more than 100 people arrested when police raided the Iona School in Sneinton, Nottingham, on the morning of Easter Monday, April 13, last year.
The protesters planned to trespass at the coal-fired Ratcliffe-on-Soar power station in Nottinghamshire and shut it down for a week, a trial at Nottingham Crown Court heard.
In December a jury found all 20 protesters, 18 of whom were sentenced on Wednesday and who are from across the country, guilty of conspiracy to commit aggravated trespass.
Judge Jonathan Teare sentenced the group to a mixture of community orders and conditional discharges and told them the jury rejected their motive behind the aim of shutting down the power station.
He said: “There was substantial material before them that closure of the power station would not only stop the emission of carbon into the atmosphere, but also provide a huge publicity stage for your ambitions, and I join with the jury’s verdict, having heard the evidence, that this was at least an equal aim of your intention to stop emissions.
“It is sad to reflect that this very expensive criminal trial has probably more than fulfilled your ambitions for publicity.”