How much manure a plant needs
A household biogas plant is a sealed tank, called a digester, that turns fresh cattle manure into biogas, a gas burned for cooking, and a leftover material called slurry that can go on fields as fertilizer. It only keeps producing gas if it is fed roughly the same amount of manure every day. A 1996 training manual written for the Food and Agriculture Organization states that the smallest tank commonly built, 4 cubic metres, needs about 24 kilograms of fresh dung a day. A cow gives about 10 kilograms of dung a day, so feeding the smallest tank takes at least 3 cattle, a figure found by dividing those 2 numbers. A 2025 survey of 2,559 Nepal households with biogas plants found the same threshold from actual use. Households whose plants still worked owned an average of 2.22 cattle or buffalo. Households that had abandoned their plants owned an average of 1.02.
These cattle counts are calculated, not stated directly by the source. Each is the dung a tank needs each day, divided by 10 kilograms, the dung 1 cow gives, using the ratios in source 1.
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| 4 cubic metres | 3 |
| 6 cubic metres | 4 |
| 8 cubic metres | 5 |
| 10 cubic metres | 6 |
What happens when the herd or a part is missing
Nepal has installed 450,000 household biogas plants, and the same 2025 survey found 54% had been abandoned by the time researchers visited each site in person. The single most common cause, found in 59% of abandoned cases and confirmed by researchers looking at the plant itself rather than only asking the household, was a broken mixer, valve or gas pipe. A herd that had shrunk below what the tank needed caused 14% of abandonments, and several smaller causes made up the rest, shown below. Only 78,406 households, 17.4% of every plant ever installed, told the 2021 national census they cook with biogas.
These percentages are shares of the plants already found abandoned, not shares of every plant ever built. They add to 100.
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| A broken mixer, valve or gas pipe | 59 |
| A herd that had shrunk below what the plant needed | 14 |
| Gas production stopped for no stated reason | 7 |
| Household members moved away | 7 |
| The plant was demolished for other construction | 5 |
| The household switched to bottled gas | 4 |
| Earthquake damage from 2015 was never repaired | 3 |
| The household left farming entirely | 1 |
Without a consistent supply of feedstock and local capacity to maintain the system, gas production can decline and communities may lose confidence in the technology.
Bornface Siliya Kenan Zulu, a renewable energy engineer based in Zambia. Source 5.
Where a plant does keep running, the time it saves is real but far smaller than older programme surveys claimed. A 2025 study measured 88 hours a year in the low lying Terai region and 128 hours a year in the mountains, closer to 15 to 21 minutes a day than the hours a day older, programme run surveys reported.
A bigger tank costs more
The same 1996 manual priced complete installations by tank size, in Nepal rupees convertible at the rate the manual itself states, 56 rupees to 1 US dollar. In 1996 dollars, a 4 cubic metre tank cost about 279 dollars installed, rising to about 430 dollars for a 10 cubic metre tank. These are 30 year old prices, not current costs, and no more recent figure could be confirmed for this article.
These dollar figures are calculated from the rupee costs and the exchange rate the source itself states for 1996, 56 rupees to 1 US dollar. They are not adjusted for inflation and are not current prices.
Show the numbers
| 4 cubic metres | 279 |
| 6 cubic metres | 322 |
| 8 cubic metres | 380 |
| 10 cubic metres | 430 |
A matched study of 123 South Ethiopian households that had adopted biogas and 123 that had not found the same pattern outside Nepal, adopters owning more livestock than nonadopters.
The other half of the tank, fertilizer
A biogas plant also produces slurry, the material left over once the gas has been drawn off, and a 2 year Ethiopian field trial published in 2026 tested it as fertilizer for bread wheat. Slurry alone did not beat chemical fertilizer alone. A quarter dose of chemical fertilizer combined with three quarters slurry produced the highest wheat yield of any treatment tried, 2.8 to 4.4 tonnes a hectare across the 2 years of the trial, a gain of 40 to 60% over chemical fertilizer by itself. For a household that already keeps enough cattle to feed a tank and enough spare parts to keep it running, the same tank produces both its cooking gas and a fertilizer that beats a bag of chemical fertilizer on its own.