Apparent excess heat is observed in light water electrolytic cells containing a variety of nickel cathodes, a platinum anode, and an electrolyte of K2CO3 in H2O. High-accuracy calorimetric measurements show apparent excess heat in the range of 15 to 37% of input power if a 100% Faraday efficiency is assumed for H2 and O2 gas release. The H2 and O2 gases released during electrolysis are recombined in a vessel external to the cell, and the quantity of recombined H2O is compared with the quantity of H2O expected from 100% efficient electrolysis. The measured Faraday efficiency is shown to be significantly <100%, and conventional chemistry can account for the entire amount of observed apparent excess heat to within an accuracy of better than 0.5%.