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How Long Do 45kg Gas Bottles Last?

“How long will my gas bottles last?” is one of the most common questions we hear from NZ home LPG users. 
 
There is no single answer to this question and a bit of explanation is required to get to the answer for your specific situation.
 
To get started, here are some numbers you will need to know for your reference:
  • Energy equivalent conversion: 1kW/hour = 3.6MJ
  • 1kg of NZ LPG = 1.856 litres of LPG (NZ 60:40 propane-butane mix)
  • NZ LPG contains approximately 26MJ per litre
  • A NZ 45kg gas bottle contains about 84 litres of LPG or 2212MJ of energy
Most people know that gas appliances are rated in Mega Joules or MJ/hour.  However, many people believe that this is a measure of output.  In reality, it is a measure of the required gas input.   Output is typically measured in kilowatts.  
The MJ inputs and kW outputs are directly related but are affected by the efficiency of the appliance.  For example, a 25MJ heater might have an output of 6.2kW.  Now, if you do the maths, dividing 25MJ ÷ 3.6MJ, you would expect that the output to be 6.94kW, not 6.2kW.  The difference is that the heater is 89.3% efficient.  No gas appliance is 100% efficient, which is why we have the Star rating system to judge the relative performance of different models.
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So, now that you have the background information, we can get back to answering the original question.  The best way to do this is with examples:

Heater Example: 

A 15MJ 5.8 Star gas heater running at its maximum setting will last approximately 147 hours on a 45kg gas bottle.  We know this because the gas bottle contains 2200MJ. 
The math is as follows: 2212MJ ÷ 15MJ = 147 hours. 
 
However, this same heater, when set on low, consumes 6.0MJ.  So, if you operate it on low only, it will last 367 hours (2212MJ ÷ 6MJ = 368 hours).  As most people would vary the settings, depending on the temperature, the gas bottle will last some duration between 147 and 368 hours. 
So, if you use your heater for four hours every evening, you would expect your gas bottle to last anywhere from 37 to 92 days.

Hot Water Example:

A 16L/min 130MJ 5.0 Star continuous flow hot water heater running at full capacity will last for 17 hours of actual run time.  This equates to about 204 five minute showers. 
However, the water heater can run at a flow rate of as little as 3.5L/min.  When running slower, the unit will self-modulate and consume less gas.  This would increase the run time by roughly 4X or to about 68 hours.  This equates to around 816 five minute showers. 
Assuming you use hot water for other things, as well, totalling 20 minutes per day at maximum, your gas bottle would last about 50 days.

Cooktop Example:

Cooktop burners are an even greater challenge to calculate.  The burners on a particular cooktop could vary from 5MJ for the smallest burner to 9MJ for a medium burner, 12MJ for a large burner and 15MJ for a wok burner.  Add to this the fact that each can be run at less than maximum and you can see the maths challenge.
If you use the medium 9MJ burner as the average, you get 246 hours of cook time.  Not surprisingly, many cooking customers report that a single bottle can last more than a year.

What if you have multiple appliances?

Now the math gets really challenging!  You really can’t add the MJ ratings of all of the appliances together, as they would rarely be used all at the same time and for the same duration.
The easiest method of calculating would be to estimate how long you would use each appliance per day and total their daily consumption.  Using the previous appliance examples:
  1. You use your hot water for 20 minutes per day on maximum.  This equals 43MJ of consumption.
  2. You use your heater, on maximum, for 4 hours every evening.  This equals 60MJ of consumption.
  3. You cook, using one medium 9MJ burner, on maximum, for 45 minutes per day.  This equals 7MJ of consumption
  4. Adding these together, you are using 110MJ per day.  Based on this, you would expect a 45kg gas bottle to last for approximately 20 days.  (2212MJ÷110MJ=20).

For those who can't be bothered with the maths, here is a quick reference chart with examples: 

45kg gas bottle duration chart

Numbers provided are estimates.  Your actual results may vary.

 

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