Propane generators can be connected to a portable 20-pound propane tank or a large in-situ tank that you keep filled at your property.
As fuel may be hard to come by in an emergency or disaster situation, it is sensible to calculate how much propane your generator will use to know how much you need to store.
How much fuel your generator uses will depend on:
- The load on the generator (i.e. how much power you’re drawing from it)
- The generator model and how efficient it is
- How long you’re running it for each day
To calculate how much propane you need to store, first work out how much propane your generator uses per hour. Then multiply this by the number of hours you’re planning to use it for, and then add a bit for contingency.
Total propane = propane use per hour x hours + contingency
Calculating How Much Propane a Generator Uses Per Hour
To work out much propane your generator will use per hour, we need to get into some numbers…
First, we need some basic assumptions about the efficiency of a typical generator for our calculations:
A 2-horsepower generator will normally produce about 1000 watts of electrical output.
To produce 1 horsepower of mechanical energy requires 10,000 BTU per hour of fuel
Propane has a fuel value of 92,000 BTU per gallon
Propane weighs 4.2 pounds per gallon
First, we need to calculate how much horsepower we’re using:
Horsepower = (watts / 1000) x 2
For example, if you had a 3000-watt base load, you would need 6 horsepower.
Next, we need to know how that relates to BTU.
BTU = horsepower x 10,000
In our example, the 3000-watt load would consume fuel which produces 60,000 BTU per hour.
Finally, you can calculate how much propane this equates to.
Gallons of propane = BTU / 92000
Therefore, running a generator with a 3000-watt load for an hour would consume (60,000/92,000) or 0.65 gallons of propane.
Putting It All Together
We can combine these calculations to form one equation:
Propane used per hour (gallons) = (watts/3000) * 0.65
0.65 gallons of propane weighs approximately 2.73 pounds. So a 20-pound propane cylinder will contain 20/2.73 x 0.65 = 4.76 gallons. You could run a 3000-watt load off your generator for around 7 hours.
Bear in mind that these calculations are approximate and don’t take into account the variation in efficiency of individual generators. However, it provides a useful starting point to work out how much propane your generator will use.
Remember to err on the side of caution – it’s better to have some spare fuel left than to run out.
Cylinders vs Tanks
Most portable generators are designed to connect to a 20-pound propane cylinder, which holds 4.7 gallons of propane when full.
As you can see from the calculations above, one cylinder is unlikely to power your house for even a day unless you’re very conservative in your power use.
If you want to use a propane generator to power your home for multiple days, you will need multiple cylinders or an in-situ propane tank.
Bear in mind that connecting to a large tank is not as simple as using a portable cylinder system and you will still need to stop the generator for maintenance. It will also help prolong the life of your generator if you don’t run it twenty-four hours a day.
More Realistic Calculation
In reality, the power you’re drawing from the generator may not be constant. You may have a base load of the refrigerator, freezer and sump pump but then have a higher load in the evening when you’re using the microwave, lights and running the washing machine.
You could account for this increase in your contingency, but it will be more accurate to calculate the amount of propane you’ll use during your base load and peak hours separately, then add the two figures together.
This gives you the following calculation:
Total propane = (propane use per hour base load x hours) + (propane use per hour peak load x hours) + contingency
Here’s an example of how this calculation might work using our top pick propane generator, Champion Power Equipment 100165 and planning to generate power to be self-sufficient for a three-day power outage.
In this example, our base load consists of a refrigerator/ freezer (700 watts), a deep freezer (500 watts), a computer and monitor (800 watts), a security system (500 watts) and a one-third horsepower sump pump (800 watts). Our base load of 3300 watts will run 14 hours a day for three days.
Using our calculator, the propane needed for 3300 output comes out as 0.72 gallons per hour.
Total propane = 0.72 x (14 x 3) = 30.24 gallons
During this 14 hour “base load” we also want to be able to power lights in the house (6 x 60 watts), an electric stove (2100 watts) and a television (500 watts) for three hours during the evening. Once during the power outage, we’ll run both a washing machine load (1150 watts) for two hours and also an electric water heater for an hour (4000 watts) during the daytime.
This gives us three peak loads:
Evening peak load = base load + 2960 watts = 3300 + 2960 = 6260 watts
Additional fuel consumption (over base load) = 2960 /3300 * 0.72 = 0.64 gallons /hr
Washing machine load = base load + 1150 watts = 3300 + 1150 = 4450 watts
Additional fuel consumption (over base load) = 1150 /3300 * 0.72 = 0.25 gallons /hr
Water heater load = base load – computer* + 4000 watts = 3300 – 800 + 4000 = 6500 watts
Additional fuel consumption (over base load) = 3200 /3300 * 0.72 = 0.70 gallons /hr
*We’re assuming the computer would be turned off when the heater is on to stay within the continuous load limit of the generator.
Our calculations for the amount of propane needed for these peak periods across three days are as follows:-
Evening peak load: 0.64 x (3 x 3) = 5.76 gallons propane
Washing machine load: 0.25 x (2 x 1) = 0.5 gallons propane
Water heater load: 0.70 x (1 x 1) = 0.7 gallons propane
Total Propane Required
To calculate your total propane requirements, you simply add the figures together and add a contingency. In this situation, we’ve decided to add a half day’s (7 hours) base load. This allows for extra power draw when appliances are starting up plus any small additional items we may want to use.
Total propane required = base load + evening peak load + washing machine load + water heater load + contingency
Total propane = 30.24 + 5.76 + 0.5 + 0.7 + 5.04 = 42.24 gallons
A Final Note
Even if you’re unable to store enough propane at your home, it’s still worth doing the calculations. The exercise will give you a realistic idea of how much fuel you’ll need to source in an emergency or whether you’ll need to compromise on your use of the generator to make your fuel supply last longer.