In his introduction to Funeral in Berlin, Len Deighton mentions that East and West Berlin shared their water, sewage, gas and electricity networks. It makes sense that you can't just suddenly split these networks (especially water and sewage) into two, but I'm interested to know if this continued until unification and how this worked in practice. How did charging work?

He appears to be mistaken. The separation of East and West Berlin's underground utility networks (electricity, water, town gas/natural gas, and sewage) began during the period of the Berlin Blockade (June 1948-May 1949) amid heightened tensions between the United States and the Soviet Union. This process was mostly completed by the early 1950s, save for a minority of the sewage network as, presumably, separating, blocking, or re-routing a certain number of the large underground sewage tunnels that happened to pass in between the two halves of the city was logistically difficult. Although West Berlin invested in its own sewage treatment plants, most of the sewage that West Berlin produced was actually disposed of by sewage farms located in the German Democratic Republic (East Germany) just outside the boundaries of the city, and East German authorities never disrupted this arrangement. During the Berlin Blockade, however, sewage was disposed of by dumping it into the Spree and Havel Rivers for want of sufficient electricity to run the pumps to the sewage farms.

After the 1971 Berlin Agreement, which reduced tensions, East Berlin and East Germany formally agreed to process a certain proportion of solid waste originating within West Berlin; before, this waste had been disposed of in the city itself by incineration.

During the Berlin Blockade, the separation of utilities was not yet completed, and electricity to West Berlin was cut off by Soviet authorities as the generating plants were still in their zone. Fuel such as coal, in addition to food and other supplies, had to be flown in to keep West Berlin's lights on as all road and rail links had been severed. After the end of the blockade in May 1949, East German power stations began to supply West Berlin with electricity again, but authorities in West Berlin had committed by that point to becoming independent from the East in this regard. A new power station, funded with Marshall Plan aid, was opened in December 1949. On 21 September 1950, the last power lines to East Berlin were turned off, and the West Berlin power station assumed the entire electrical load for the half-city.

The city of Berlin took over the task of municipal water supply from individual consumers' wells in the 1870s after the unification of the German states, and shifted drawing water from a point on the Spree River at Stralau to plants above and below the city. Berlin is located in an urstromtal, or glacial valley, and West Berlin chiefly relied on groundwater processed through a water treatment plant. The rapid consumption of this water, as it was estimated that only ten to fifteen percent of runoff actually re-entered the water table, began to become a problem in the 1960s. Efforts to restore the level of groundwater through natural sources, such as the utilization of groundwater that infiltrated into the area of West Berlin from outside the boundaries of the city, and the conservation of water, were deemed successful by the 1980s.

By the 1950s, West Berlin was independent in the production of town gas (before the widespread usage of natural gas, gases derived from coal or oil were commonly used for for domestic heating), but in 1985 secured a spur of the Soviet natural gas line that supplied East Germany.

The land border between East and West Berlin was essentially sealed in May 1952, leading to a transformation of the public transit networks in each part. The remaining sewer tunnels between East and West Berlin proved to be an attractive route for escapees, many assisted by a secret organization of West Berlin university students known as the Girrmann Group. It is estimated that as many as 300 people used the sewers to escape East Berlin between 1961 and 1989, but by the early 1960s with the Berlin Wall being constructed, the prospect of escaping through the sewers became "increasingly dangerous;" the sewer entrances were patrolled by the East German secret police (Stasi). Telephone communication for ordinary citizens was cut between the two Berlins, and was not completely re-established until the mid-1970s.

Even then the two Berlins were not totally isolated from one another, but the extent of remaining contacts was very limited: postal service, a teletype connection between the police forces, telephone connections between the transit systems and the fire departments. Western subway lines that traveled under Eastern territory without stopping, a train system in the West operated by the East, Eastern water serving a few corners of West Berlin, and sewage flowing freely under the Wall wherever gravity so dictated. The result was a need for a few discreet technical discussions and an occasional skirmish over fees or sovereignty. The thaw in East-West relations in 1970 led to a resumption of telephone service, a few more opportunities for people to cross the wall, and negotiations to share supplies of electricity, gas, and garbage.

Sources:

Bainbridge, John. "Die Mauer: The Early Days of the Berlin Wall." The New Yorker, October 27, 1962.

Elkins, Dorothy, Thomas H. Elkins, and Burkhard Hofmeister. Berlin: The Spatial Structure of a Divided City. London: Methuen Publishing, 1988.

Ladd, Brian. The Ghosts of Berlin: Confronting German History in the Urban Landscape. Chicago: University of Chicago Press, 2008.

Merritt, Richard L. "Infrastructural Changes in Berlin. Annals of the Association of American Geographers 63, No. 1 (March 1973): 58-70.

Robinson, G. W. S. "West Berlin: The Geography of an Exclave." Geographical Review 43, No. 4 (October 1953): 540-557.

Smith, Jean Edward. The Defense of Berlin. Baltimore: Johns Hopkins University Press, 1963.

Taylor, Frederick. The Berlin Wall: A World Divided, 1961-1989. London: Harper Perennial, 2006.

Turner, Henry A. The Two Germanies Since 1945: East and West. New Haven: Yale University Press, 1987.