How it's calculated
Energy in bottle (kWh) = Gas (kg) × 13.9 kWh/kg (propane)
Hours = Energy ÷ (Appliance kW × Average use)
Propane holds about 13.9 kWh per kg (about 21,600 BTU per lb), using the gross figures appliance ratings are based on. Butane holds about 13.7 kWh per kg. Appliances list their gas rating as kW, BTU/h or g/h.
Typical appliance ratings
| Appliance | Typical rating |
|---|---|
| Single hob burner | 1.5–2 kW (110–150 g/h) |
| Absorption fridge on gas | 0.15–0.25 kW (11–18 g/h), runs constantly |
| Caravan blown-air gas heater | 1.8–3.7 kW |
| Combi water/space heater | 2–6 kW |
| Portable propane heater (US) | 4,000–18,000 BTU/h |
| Barbecue (US) | 30,000–40,000 BTU/h |
Worked example
A 13 kg propane bottle holds about 180 kWh. A 3 kW heater averaging 50% uses 1.5 kW, so the bottle lasts about 120 hours, or roughly 20 days at 6 hours a day.
In very cold weather, a nearly empty bottle may not supply enough pressure for full output, so plan to swap before it's completely empty.
Frequently asked questions
How long will a 13kg propane bottle last?
On a 3 kW heater at full power, about 60 hours. On a fridge using 15 g/h, around five weeks.
How long does a 20 lb propane tank last?
A 20 lb tank holds about 432,000 BTU. A 10,000 BTU/h heater on full runs about 43 hours; a 40,000 BTU/h grill about 11 hours.