3 dung from farm animals, limiting its use. For the foreseeable future, LPG will likely remain the most widely available gaseous fuel as well as the most common alternative to biomass for cooking in developing countries. Unless heavily subsidized, kerosene, LPG, and natural gas are typically more expensive than solid fuels, particularly biomass which can be acquired at no financial cost through collection by household members in many parts of the developing world. Where the entry of these household members into the labor force is difficult, the opportunity cost of the time spent collecting can be very low, making biomass collection a financially attractive choice. Cash income, necessary for regular fuel purchase, can also be irregular, particularly in rural areas. It is not surprising that many low-income rural households choose biomass as their main cooking fuel and rarely use LPG even when it is available. Urban households face different circumstances  incomes tend to be higher and more regular, LPG is likely to be more widely and conveniently available, and biomass may no longer be free but has to be purchased  and use LPG more than their rural counterparts.

When trends on fuel use across different income groups are examined, as incomes rise, there is both a _substitution_ effect—shifting some households away from biomass to LPG and other cleaner-burning commercial fuels—and an _income_ effect for those choosing to use biomass, in which consumption initially rises with income. The income effect may initially dominate at lower income levels and the total use of biomass for the group may increase, while at higher income levels the cumulative effect of households switching away from biomass eventually leads to an overall reduction in its use when averaged across all households (Barnes, Krutilla, and Hyde 2005; Bacon, Bhattacharya, and Kojima 2010). These observations are described as fuel stacking, in which households use several fuels, adding extra fuels as incomes increase (Masera, Saatkamp, and Kammen 2000).

Bacon, Bhattacharya, and Kojima (2010) reported that, out of six countries for which information on the primary cooking fuel was available, more than half of the top two urban quintiles in two countries and more than half of the top two rural quintiles in five countries named biomass as their primary cooking fuel. These findings indicate that availability and affordability, although very important, may not be the sole determinants of fuel choice and use, and that the use of solid fuels for household energy is not always a result of poverty but occurs over a broad income range in developing countries. Gaining a better understanding of the factors that determine household fuel use patterns is important for setting appropriate policies to shift households away from biomass to LPG and other cleaner-burning options.

The foregoing discussion raises two separate questions: (1) What factors influence the household’s decision to use a gaseous fuel or electricity (where it is used for cooking and heating) to meet household energy needs? (2) For those households that have decided to use cleaner forms of household energy, what determines the amount of their use? National household expenditure surveys provide a large amount of information on households as well as their total expenditures and patterns of expenditure on energy sources. To carry out a comparative analysis of different fuels, fuel use would need to be standardized on a common basis—for example, gigajoules of _usable_ energy delivered by each fuel—because bundles of firewood and kilograms (kg) of LPG are not directly comparable nor can they be added for those using both fuels. Further, a number of surveys do not ask for information on the quantities of dung or collected firewood consumed. Calculating the amount of usable energy requires not only the energy