In Taschen's Ingmar Bergman Archive, Bergman relates that during the filming of Prison (1949) both film and electricity were being rationed. What was the cause of this?
I don't know of the particular circumstances of Ingmar Bergman, but I do know about rationing in Sweden at that time. Although for Sweden's part, World War Two ended in 1945, war rationing continued until 1951. The goods rationed included many that a film crew or studio would directly or indirectly need to produce, shoot and edit a film. As for 1949, many things that were previously rationed no longer were, including car tires, laundry detergent, meat, soap and sugar.
General: During the War there was a shortage of imported goods and the government in Sweden (as in other countries) took over the supply chain of key goods in order to make sure the population would get by as best as possible. In 1939, Statens livsmedelskommission (lit. The Government's Food Supply Commission) began handing out rations stamps/cards, which first applied to coffee, sugar and tea in the following year. Other goods like beans, bread, candles, cocoa, cream, eggs, electricity, film, flour, fuel (petroleum-based, wood, gas), grains, peas, salt, shoes, spices, textiles and tobacco soon followed. With the end of the War in 1945, rationing was steadily ended, finally with coffee in 1951.
Contemporary: For anyone who can read Swedish, has some time on their hands and wants to get some input on how this might work today, I can recommend this: Ransonering och prisreglering i krig och fred (Rationing and price regulation in times of war and peace) from 2009, published by the Swedish government.
Rationing was continued after the war as an economic measure. As inflation had been held down by the issue of war bonds and intruduction of a sales tax, there was a fear that free consumption would lead to inreased imports, a trade balance deficit and a large inflation.
Sweden used hydroelectric powerplants (dams) and coal powerplants to produce elecity, and coal was one of the commodities that had to be imported.
As /u/mousike writes, rationing was gradually abolished as trade resumed and exports of iron ore, lumber, wood pulp, ball bearings, steel, industrial goods and paper picked up.