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.
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.
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….
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.
Mike Sokol wrote this when he was a columnist for RVtravel.com. He has since moved on and is therefore unable to respond to comments.
##RVT1171
Looking forward to the fuel cell conversion part of the story.
Great article! I have a spreadsheet that calculates my propane use rates times per pound cost. We’ve spent $40+ a day running generator, looking forward to efficiency of fuel cells if/when they arrive.
Thank you, Mike and RV Travel! 🙂 Yes, that is a few more batteries than our RV can carry. However, it is a point left only to discussion with us; our RV is all-electric. Thanks again, have a great week, safe travels, and safe stays! 🙂
“They come in up to 1,500-watt modules that can be stacked in parallel.” Where do you get these?
Google it. They exist. Just expensive now, but coming down.
A typical 20 pound (or any size) propane tank is only filled to 80% of it’s capacity which would be 3.84 gallons.173 lead acid batteries worth or 86 Lithium batteries.
OOOOO – put me in line for this! “ studying fuel cells that do a direct conversion of propane to electricity. ” I understand that far better than my solar panels on our new unit (with crummy batteries I believe) …
Battery won’t last a day? That is cutting it close. I would add another one so it lasts overnight. And don’t forget solar cells to recharge over 10 hours in winter. 1200 Watt Hour per battery, you will need about 3-400 watts of solar panels, maybe double for cloudy winter days.