The efficiency of these two types of systems depends on two factors that can vary significantly: the efficiency of the combustion process and the efficiency of the power plant generating electricity. However, efficient wood-and-oil boilers provide better fuel-to-heat efficiency than most power plants, as of 2015.
The annual fuel utilization efficiency rating, of AFUE, is used to score individual system based on efficiency, and electrical boilers almost always rate at 100 perfect, meaning they are more efficient at turning the electricity they use into heat. However, this rating can be deceptive as power plants, which must convert heat into electricity, are less efficient than systems that simply distribute the heat generated from fuel. As a result, their overall efficiency is often rated poorly depending on the fuel they use.
Dual-fuel systems have another advantage over electrical boilers: versatility. Homeowners can use oil heating during much of the year but also have the ability to fall back on wood heating if their oil supplies are interrupted or if prices become prohibitively expensive.
Older oil- and food-fired systems operate far less efficiently than regulations in 2015 typically permit, and many systems lose energy due to suboptimal combustion and venting. Wood systems, in particular, need to release particulate matter and combustion byproducts, and heat is often lost in the process.