The answer: Quite a bit.
A report from the Center for Sustainable Systems (CSS) at the University of Michigan (ref. 1 below) provides a breakdown of the energy used in the U.S. food system across seven sectors, with a total consumption of 1.02·1016 Btu per year (10.6 exajoules per year). The figure below shows the result, with each sector represented by a different color*. For four of the sectors, I indicate the portion that is related to refrigeration with grey hatching across that sector's color. The CSS report includes how much refrigeration accounts for the energy use of the household sector (40%) and of the food retail sector (50%); I tracked down the comparable value for commercial food service (13%) at the Energy Information Administration and also performed a rough calculation for refrigerated transport trucks (0.4%, that light-pink wedge so small that you can't see the hatching).
Adding up the hatched portions tells us that refrigeration in households, retail, and commercial establishments comprises 14.9% of the energy used in the U.S. food system.
Mike asked specifically about "reefers," the small, usually diesel-fueled engines that cool the contents of a truck trailer or shipping container (also known as transportation refrigeration units, or TRUs). The CSS study doesn't mention the energy consumption from those devices, so I did some digging at the Environmental Protection Agency (ref. 2) to find estimates of the number of TRUs in the U.S. (about 170,000), their average fuel consumption (21 lb/hr) and how many hours they run per year (1,341 hr/yr). With a few spreadsheet operations and unit conversions, I calculated that they require about 3.98·1013 Btu per year, a quantity that is equivalent to about 3% of the energy used in the transportation portion of the food system and 0.4% of the energy in the entire food system.
The 14.9% estimate for refrigeration energy is a good ballpark number, but falls short in two areas: use of refrigeration in the food-processing industry to produce frozen or chilled products, and the significant improvement in the energy efficiency of refrigerators. According to a 2004 report from the Lawrence Berkeley National Laboratory (ref. 3), the electricity consumption of new residential refrigerators dropped by 57% between 1980 and 2004. Because refrigerators last a long time, it takes a while for the new, more efficient refrigerators to have an impact on overall energy conservation. Although I haven't seen documentation about large commercial refrigerators used in restaurants and retail establishment, I would guess that their efficiency has also improved in recent decades. The improvements will continue: after years of stalling, the Department of Energy is writing new regulations that will increase the efficiency of new appliances even more, and many electricity utilities provide financial incentives to their customers to encourage them to buy new, more efficient refrigerators.
*Because of rounding, their sum will not equal 100%
1. "Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System," M.C. Heller and G.A. Keoleian, Center for Sustainable Systems, University of Michigan, publication number CSS00-04, December 2000.
2. "Median Life, Annual Activity, and Load Factor Values for Nonroad engine Emissions Modeling," U.S. EPA, report number EPA420-P-04-005, April 2004 (PDF); "Exhaust and Crankcase Emission Factors for Nonroad Engine Modeling--Compression-Ignition", U.S. EPA, report number EPA420-P-04-009, April 2004 (PDF); "Nonroad Engine Population Estimates" U.S. EPA, report number EPA420-P-05-022, December 2005 (PDF).
3. "Realized and Prospective Impacts of U.S. Energy Efficiency Standards for Residential Appliances: 2004 Update," Stephen Meyers, James McMahon, Michael McNeil, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Repot Number LBNL-56417, May 2005. Available from the