I grew up in Guernsey 1946-1958 and remember frequently seeing many of the Dorey coal ships in St. Sampson's Harbour, they were at that time all named after bays in Guernsey, thus :- Fermain, Perelle, Rocquaine, Belgrave, Havelet, Belvedere, Lancresse, Portelet, and possibly others. Sarnia was the Roman name for the island.
During the last century there was a thriving horticultural industry growing first grapes then tomatoes and flowers which were mainly shipped to England in British Railways ships. The tomatoes were grown in glass houses, which were heated by coal (and filled with CO2), which was imported in Doreys colliers; there was also a gasworks which used a large quantity of the coal.
In the latter part of last century when the UK joined the European Common Market, the horticultural industry was adversely affected and less coal was required, many glass houses became derelict. Electricity replaced gas in many uses, and I believe that Guernsey now receives electricity via an underwater cable from France, where it is generated in a nuclear power station not far from Cherbourg.