Dear Readers, I found this question on the Keystone Cougar Owners Group. Let’s go over the power and energy math since it’s a great way to understand how much solar and battery you need for different boondocking challenges. First, let’s determine power requirements…
This is a great debunk analyse and for those that still are thinking of trying this an even more rigorous view includes the following addition considerations.
1. Cold weather is synonomouse with low sun angles and thus 300 watthours a day is probably wildly optimistic, more like 150 WH for panel mounted flat on an RV room.
2. The duty cycle for these tank heaters are typically on at 45 degrees and off at 60 degrees (tank temperature) which essentially means a 100 % duty cycle in all practical purposes, so there you go.
It would take a massive solar system to keep up with this demand... Maybe adding as auto start to a generator and a big charger would be an answer, but unattended would not be a very great idea. Better would be to minimize water use, use a composting toilet and haul the water bock and forth.
Getting a bigger propane supply and insulating the tanks and running heat ducting to them would be a better solution, and even that would take quite a bit of power to run a furnace blower more often. I assume that they want to keep a minimum heat for the rest of the rig using a propane furnace and that takes a 6 amp 12 volt blower going anyway, not a trivial power requirement itself. THANKS for having these discussions...
Many great points discussed here. It takes a significant amount of power for tank heaters (and often there are multiple tank heaters, or at least separate gray and black tank heaters), and energy consumption can quickly exceed available battery capacity.
While the original question is about a stationary configuration, there is also the tank & drain line concerns when driving in freezing temperatures.
Looking at my thermometer this morning (20°F), that ΔT makes a 50% estimated duty cycle of tank heaters look generous. At some point, the ΔT overwhelms energy input even with a 100% heater duty cycle, and the only feasible option is to increase the insulation and reduce the outward energy flow.
Don't forget that while the tank pads are thermostatically controlled, the pipe and elbow heaters are generally not and run 24/7 in all temps when the switch is on, although at a lower total wattage due to their smaller size.
I kinda echo the other previous commenter. Those tank pads - which I, too, use - turn on when the temp drops below 45 degrees F, and remain on (unless you flip the switch off...) until the temp rises above 45 degrees. We do need to remember that those tanks won't instantly freeze - it takes quite a while for that mass of water, etc. to reach a low enough temperature to freeze up. As mentioned, this is a good use of a generator! Warm the tanks up in the evening/morning, unless you are in significantly cold weather, they should hold for the day. My 2 cents! ST
Propane has the energy density to do this.
Here are my calculations of how many batteries equal the energy of a 20 lb tank of propane, just in case you want to win a bar bet!
1 Gallon of Propane = ~27 kWh (Kilowatt Hours) of electrical energy
So 4.7 Gallons of Propane = 127 kWh of electrical energy
A 100Ah battery x 12.5 volts = 1.25 kWh of energy
So 127kWh / 1.25kWh = 101 Lithium batteries that can be discharged to 0% SoC
Or 202 Lead Acid batteries that should only be discharged to 50% SoC
How much does all of this weigh?
One 100Ah Lithium battery weighs around 30 lbs.
So 101 Lithium batteries would weigh 30 x 101 = 3,030 lbs.
One 100Ah Deep Cycle Lead-Acid battery weighs around 70 lbs.
So 202 Lead-Acid batteries would weigh 70 x 202 = 14,140 lbs.
A 5-gallon propane tank with the same amount of energy weighs 35.8 lbs.
This is a great debunk analyse and for those that still are thinking of trying this an even more rigorous view includes the following addition considerations.
1. Cold weather is synonomouse with low sun angles and thus 300 watthours a day is probably wildly optimistic, more like 150 WH for panel mounted flat on an RV room.
2. The duty cycle for these tank heaters are typically on at 45 degrees and off at 60 degrees (tank temperature) which essentially means a 100 % duty cycle in all practical purposes, so there you go.
It would take a massive solar system to keep up with this demand... Maybe adding as auto start to a generator and a big charger would be an answer, but unattended would not be a very great idea. Better would be to minimize water use, use a composting toilet and haul the water bock and forth.
All good points. Electric heaters in general take a LOT of energy, so powering them from solar/batteries alone isn’t practical.
Getting a bigger propane supply and insulating the tanks and running heat ducting to them would be a better solution, and even that would take quite a bit of power to run a furnace blower more often. I assume that they want to keep a minimum heat for the rest of the rig using a propane furnace and that takes a 6 amp 12 volt blower going anyway, not a trivial power requirement itself. THANKS for having these discussions...
And all this is for one tank heater, so times two or three for two or three tanks.
Many great points discussed here. It takes a significant amount of power for tank heaters (and often there are multiple tank heaters, or at least separate gray and black tank heaters), and energy consumption can quickly exceed available battery capacity.
While the original question is about a stationary configuration, there is also the tank & drain line concerns when driving in freezing temperatures.
Looking at my thermometer this morning (20°F), that ΔT makes a 50% estimated duty cycle of tank heaters look generous. At some point, the ΔT overwhelms energy input even with a 100% heater duty cycle, and the only feasible option is to increase the insulation and reduce the outward energy flow.
Don't forget that while the tank pads are thermostatically controlled, the pipe and elbow heaters are generally not and run 24/7 in all temps when the switch is on, although at a lower total wattage due to their smaller size.
I kinda echo the other previous commenter. Those tank pads - which I, too, use - turn on when the temp drops below 45 degrees F, and remain on (unless you flip the switch off...) until the temp rises above 45 degrees. We do need to remember that those tanks won't instantly freeze - it takes quite a while for that mass of water, etc. to reach a low enough temperature to freeze up. As mentioned, this is a good use of a generator! Warm the tanks up in the evening/morning, unless you are in significantly cold weather, they should hold for the day. My 2 cents! ST