Cooking oil will deliver no more than 40 kJ/g of energy with complete combustion. The greatest density for a cooking oil that I could find was 0.928 g/ml. We thus have 37.12 kJ/ml., or 37,120 kJ/L. This is comparable to the 39,020 kJ/L value used for #2 diesel oil. A restaurant would use a deep fat fryer with about 75 lb oil capacity.
(75 lb)(1 kg/2.2lb)(1,000 g/kg)/(0.928 g/ml)(1 L/1,000 ml) = 36.736 L. According to Cleaver-Brooks, a reputable boiler manufacturer, a 980 KW fire-tube boiler (the smallest they list in their tables) operating at 86% efficiency (the highest efficiency in their tables) consumes 105.98 L/hr. Using these numbers, it would take the waste of 3 deep-fat fryers to fuel such a boiler for 1 hour. Continuous operation of such a boiler would require 486 fryers contents per week, which is the minimum time a restaurant goes between oil changes.
While it is a good, relatively clean idea for the disposal of these wastes, the costs involved would be enormous. An alternate proposal would be to mix it (well strained, of course) with the fuel used by local bus companies, most of which are already government owned.