To measure a rocket stove’s fuel saving, weigh the wood before and after boiling 5 litres of water on the stove and again on a three-stone open fire, and compare. The overview’s typical figures are 1.5 to 2 kg of wood for the open fire against 0.4 to 0.6 kg for the rocket stove, which works out to a saving of roughly 60 to 80 percent depending on which figures you pair.
Part of Rocket Stove Design
A stove’s fuel saving depends on its wood, pot and cook, so it is measured by boiling a fixed quantity of water on the stove and on an open fire.
Method
Step 1: Weigh the fuel. Weigh a bundle of dry sticks before cooking.
Step 2: Run the standard task. Bring 5 litres of water to a boil. Use the same pot, the same lid, the same starting water temperature and the same dryness of wood for both fires.
Step 3: Weigh what is left. Weigh unburned sticks, and weigh any charcoal pulled out, so that you count only what was consumed.
Step 4: Repeat on the other fire and compare.
Worked Example: The Saving
The overview’s figures: open fire 1.5 to 2 kg, rocket stove 0.4 to 0.6 kg.
- Same-end pairs: 1 − 0.4 ÷ 1.5 = 73%; 1 − 0.6 ÷ 2.0 = 70%.
- Extreme pairs: 1 − 0.6 ÷ 1.5 = 60%; 1 − 0.4 ÷ 2.0 = 80%.
So 60 to 70 percent is a typical value inside a range that the overview’s own figures stretch to 80 percent; your own measurement decides.
Worked Example: Efficiency
Heating 5 L from 20 to 100 °C takes 5 × 4.186 × 80 = 1,674 kJ. Take air-dry wood at about 15 MJ/kg (a typical handbook value, not from the overview).
- Rocket stove, 0.4 to 0.6 kg: 6.0 to 9.0 MJ in, so 1.674 ÷ 9.0 = 19% to 1.674 ÷ 6.0 = 28%.
- Open fire, 1.5 to 2 kg: 22.5 to 30 MJ in, so 1.674 ÷ 30 = 5.6% to 1.674 ÷ 22.5 = 7.4%.
These ignore the heating of the pot and evaporation, so the real useful efficiency is slightly higher.
Common Mistakes
- Comparing wet wood on one fire with dry on another. It ruins the test.
- Ignoring leftover charcoal. It is fuel not used.
- Using different pots. A wide pot takes up more heat.
Safety
Run the test outdoors
Boiling 5 litres of water means a heavy pot of scalding water and a hot stove. Measure outdoors, where smoke and carbon monoxide disperse, and keep children away. See Carbon Monoxide Risks.
What’s Next
- Learn next: Indoor Use & Ventilation — using the stove inside safely
- Combine with: Retained Heat Cooking — saving more fuel after the boil