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RVelectricity – Just Ask Mike (J.A.M.): How many batteries equal a propane tank of energy?

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. Today I compare the energy from a tank of propane to different types of batteries.


 


Dear Mike,
I’m curious just how much battery power is needed to equal a 20-lb. tank of propane? For example, my 3-way fridge can run for a few weeks on propane mode, but when I try to power it from my batteries and inverter, it won’t even last a day before the battery is dead. Can you explain why this happens? —JP

Dear JP,
Ah, yes, it’s the energy density thing. And while modern lithium batteries can store a lot of power, they pale by comparison with a tank of propane. Let’s compare by first calculating how many kWh (Kilowatt Hours) of energy is in a gallon of propane.

Do the numbers

You can easily look up that the available energy in 1 gallon of propane is equal to 27 kWh (Kilowatt Hours) of electricity. Now, this is assuming a 100% conversion efficiency, but we’ll ignore that variable for now.

A 20-lb. tank of propane is right around 4.8 gallons, since each gallon of propane weighs 4.1 lbs. Because they never fill these tanks all the way to the top, let’s assume a maximum of 4.5 gallons in a 20-lb. tank. To find the total energy we just do the multiplication and find that 4.5 gallons x 27 kWh = 121.5 kWh in a 20-lb. tank of propane.

What about battery energy?

This is even easier to calculate. We already know how many amp-hrs of storage each battery has. And you also should know that lead-acid batteries (flooded or AGM) should only be discharged down to 50% SoC (State of Charge) or battery life will suffer and you’ll be buying a new battery (or batteries) next season.

Let’s use a common 100 amp-hr (or Ah) battery as an example. If it’s an AGM battery, you should only use 50 amp-hrs of its capacity. So 100 Ah x 12 volts = 1,200 watt-hrs (Wh) of energy. But since it shouldn’t be discharged below 50%, that’s 600 Wh of available energy. Here’s what the equation looks like: 100 Ah x 12 volts x 50% = 600 Wh.


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What about lithium batteries?

It’s the same calculation of 100 Ah x 12 volts x 100% = 1,200 Wh of available energy. As I’ve written many times, a 100 amp-hr lithium battery has twice the available storage of a lead-acid battery. Plus, they charge faster and weigh a lot less than a lead-acid battery. So a win-win if you have the budget for them.

Final calculations

We already calculated there’s around 120 kWh of energy in a 20-lb. tank of propane, which is around 120,000 watt-hrs for a 100 amp-hr AGM or flooded lead-acid battery

If we divide propane energy by battery energy, then you need 120,000 Ah / 600 Ah = 200 lead-acid batteries of 100 Ah capacity each to equal the energy in a single 20-lb. tank of propane.

Similarly, for lithium batteries, we see that 120,000 Ah / 1,200 Ah = 100 lithium batteries of 100 amp-hrs each to equal one 20-lb. tank of propane energy.

Wow, that’s a while lotta energy!

While it will be a long time before there’s a battery no bigger than a propane tank that can store 120,000 Wh of energy, all is not lost. I’m studying fuel cells that do a direct conversion of propane to electricity. They come in up to 1,500 watt modules that can be stacked in parallel.

If these fuel cells work out as planned, you could have the best of both worlds. Since the conversion efficiency is close to 90% compared to the 30% conversion efficiency of a generator running on propane, you could power an RV with an air conditioner for nearly a week on a single tank of propane with a fuel cell. Stand by, as I’m working on getting a demo fuel cell to experiment with.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there….

Send your questions to me at my new RVelectricity forum here.

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

You don’t want to miss Mike’s webcasts on his YouTube channel.

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Steve Hericks
1 month ago

A minor but important detail. The energy available in a battery is equal to capacity (in Ah) x nominal voltage. The nominal voltage of a battery is defined as the voltage at 50% capacity. A LA battery is 12.0V, a LFP battery is 12.8V so the capacity (energy) in a 100Ah LFP battery is 2.8V x 100Ah = 1280Wh. For a LA battery, your calculation is correct. The reason there are so many errors in energy calculations from capacity in amp-hours is that all energy calculations need to use the nominal voltage for the specific battery chemistry.

Mike Sokol
1 month ago
Reply to  Steve Hericks

You are correct, but I’m trying to keep the concept as simple as possible. This is what I call a SWAG (Scientific Wild A## Guess). When I was calibrating nuclear missile guidance systems back in the ‘80s we had to be accurate down to Parts Per Million (PPM). Good times…

HappyCamper7424
1 month ago

Really looking forward to your fuel cell experiments. Guess they will be pretty expensive though

Bob M
1 month ago

Hope you’re feeling better Mike. The trouble with going electric is the electric companies are known to find ways for us to pay more for electric. Watch for surcharges to upgrade the electric service. Electric cars will be more expensive than ICE cars to drive. My wife bought a new electric stove and I hate it. Difficult to regulate the burners temperature. Old electric stove top worked great. If something happens to her, the electric stove is gone and a gas stove will replace it.

Mike Sokol
1 month ago
Reply to  Bob M

Interestingly, I’m proposing an experiment to determine if more or less propane is used to boil water directly over a flame compared to a propane fuel cell generating electricity that’s powering an induction cooktop boiling the same amount of water.

Steve
1 month ago

Don’t know if anyone else read the article but governments want to ban all gas and diesel modes of transportation. Doubt they will succeed, but what if? No ones paying attention to the cost and damage being done to create electric alternatives.

Spike
1 month ago
Reply to  Mike Sokol

Very interesting comparison. Thanks. Also hope the fuel cell technology can help to ease the transition that’s coming.

As the character John Hammond in Jurassic Park said: “Creation is an act of sheer will!”

Tom M
1 month ago
Reply to  Spike

Or profit.

Gary W Mayberry
1 month ago
Reply to  Steve

We need a viable way to recycle all spent electric car batteries before we even think about this, Also, my two vehicles that are 2008 and 2009 models. On fixed income, I can’t afford to replace them now, and thanks they are low mileage and very good condition, but what if they were electric. Would they still be operative at their age and 70 and 80 thousand miles? Costing me very little operating upkeep or costing $10,000.00+ for new batteries 2x’s already?

Mike Sokol
1 month ago

I believe there’s a big push to recycle all EV batteries. Interestingly, Tesla has been finding it difficult to get used Tesla batteries turned in for recycling since they’re still working great for solar energy storage. As I learn more I’ll report on it.

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