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Electric heater calculations

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I need a way to get some heat into my little 64 square foot shed where my batteries live. I'll do something permanent over the summer, but for now I just need something that works. I tried a Mr. Buddy propane heater, and my building is airtight, so the oxygen sensor shuts it off after a few minutes. I picked up this little heater to try. Am I figuring this right?:

250 watts / 110 volts = 2.73 amps

2.73 amps * 8ish hours of run time = 21.84 amp hours.

I have 2x200ah batteries in parallel, so 400ah- 200ah (safe dod) - 21.84=178.16 amp hours remaining.

Did I figure that correctly?20191126_095806.jpg
 
New Mexico Will said:
I need a way to get some heat into my little 64 square foot shed where my batteries live. I'll do something permanent over the summer, but for now I just need something that works. I tried a Mr. Buddy propane heater, and my building is airtight, so the oxygen sensor shuts it off after a few minutes. I picked up this little heater to try. Am I figuring this right?:

250 watts / 110 volts = 2.73 amps

2.73 amps * 8ish hours of run time = 21.84 amp hours.

I have 2x200ah batteries in parallel, so 400ah- 200ah (safe dod) - 21.84=178.16 amp hours remaining.

Did I figure that correctly?View attachment 2539
Click to expand...
Amp hours are voltage based, 1Ah of 12v isn’t the same as 1Ah at any other voltage.
Compute it in Wh, much better for the brain to calculate.
 
New Mexico Will said:
I need a way to get some heat into my little 64 square foot shed where my batteries live. I'll do something permanent over the summer, but for now I just need something that works. I tried a Mr. Buddy propane heater, and my building is airtight, so the oxygen sensor shuts it off after a few minutes. I picked up this little heater to try. Am I figuring this right?:

250 watts / 110 volts = 2.73 amps

2.73 amps * 8ish hours of run time = 21.84 amp hours.

I have 2x200ah batteries in parallel, so 400ah- 200ah (safe dod) - 21.84=178.16 amp hours remaining.

Did I figure that correctly?View attachment 2539
Click to expand...
250Watts x 8 hours is 2000Wh
Divide by 12v gets you 166.666666 Ah
 
The 2000Wh that the heater will consume in 8 hours of runtime...
Actually, the inverter has losses, so likely 2100Wh.

Has to come out of the 12v battery.
So you need the 12 capacity with 2100Wh usage.

That gives you the AH of the battery voltage.

Remember...

Watts are watts, there is no conversion that alters the watts.
So the battery needs to have 2100 Wh removed from it.
At 12v, that's 167+ Ah of battery.
 
So if my batteries were in series at 24 volts it would require half? Oh, I just realized how stupid that was. Thanks for your help
 
Yup, just to recap.
1) 250W AC heater. <- Make sure you inverter can handle this load.

2) To calculate "how much battery" you need to calculate two things, capacity and continuous current.

Capacity
250W x 8hrs = 2000Wh. Divide by 70% (0.7) to account for inverter inefficiency and so you don't discharge your batteries completely. This ends up being 2857Wh, rounding up to 2900-3000Wh would give you a bit more cushion.

If you're using 12V batteries, you'll need 3000Wh / 12V = 250Ah of capacity. You're right that if you use 24V batteries, you'll need half the Ah, so 125Ah.

Continuous Current
250W from 12V batteries will need about 20A of continuous current, for 250Ah batteries, this should be no problem. It's half for 24V systems, but is also no problem,
 
So now that you're through the math, maybe it's time to look at alternative way to keep the batteries warm. You could put them on a thermostatically controlled heat mat. Will did a video on that. Or if they're just AGM or FLA you can keep them reasonably charged and just let them be cold.
 
If it's JUST batteries, a lot of insulation in a box, and a heating pad under the batteries (heat rises).
An insulated box captures charge/discharge heat you don't have to pay for.
 
Great ideas. I've been looking for a heating blanket, but all the ones I see have auto-shutoff. My building is extremely well insulated, so another box probably wouldn't do a whole lot.

My concern stemmed from the fact that I was worried about my batteries "freezing". They are GEL batteries (lead acid). A little more research shows that "freezing" of my batteries should not occur until 5 degrees farenheit, assuming I have at least 50% charge on them, which I try very hard not go below. So, perhaps I am worried about nothing.

My tiny house is 20 feet from my solar shed. I think I'm going to run some 2" PVC between the two underground and move air to the solar shed with a small inline duct fan, which consumes 0.3 amps. Much better. Of course, I'll have to run a return line so the air will circulate between the two. From what I understand (google), off gassing is not a concern with proper charging.

This would be in preparation for going LiFePo4 when budget allows.

I'd of course welcome your opinions.
 
krby said:
Yup, just to recap.
1) 250W AC heater. <- Make sure you inverter can handle this load.

2) To calculate "how much battery" you need to calculate two things, capacity and continuous current.

Capacity
250W x 8hrs = 2000Wh. Divide by 70% (0.7) to account for inverter inefficiency and so you don't discharge your batteries completely. This ends up being 2857Wh, rounding up to 2900-3000Wh would give you a bit more cushion.

If you're using 12V batteries, you'll need 3000Wh / 12V = 250Ah of capacity. You're right that if you use 24V batteries, you'll need half the Ah, so 125Ah.

Continuous Current
250W from 12V batteries will need about 20A of continuous current, for 250Ah batteries, this should be no problem. It's half for 24V systems, but is also no problem,
Click to expand...

Ready for dumb question? The 20A is only flowing from batteries to inverter, correct? Once it leaves the inverter at 120v, it's much less.
 
I'd put a min max thermometer in there and monitor how low it goes. I thought you were in southern NM so you shouldn't see 5 degrees F?
 
PatBlack said:
I'd put a min max thermometer in there and monitor how low it goes. I thought you were in southern NM so you shouldn't see 5 degrees F?
Click to expand...

WILDLY unlikely. Central NewMexico, high desert. Winter temps regularly get into the teens. About 10 years ago we were -15 for a few days, so that's what I'm planning for.

I have a min/max thermometer in there, and with temps in the mid 20s, it doesnt go below 42 overnight. But, like I said, I'm planning for colder and lithium.
 
New Mexico Will said:
Great ideas. I've been looking for a heating blanket, but all the ones I see have auto-shutoff. My building is extremely well insulated, so another box probably wouldn't do a whole lot.
Click to expand...
An insulated battery box WOULD make a world of difference. The internal heat generated in the battery would stay in the battery instead of trying to heat your shed.
 
Also FYI regarding radiant gas heaters with O2 sensors, at high altitudes the O2 sensors are more sensitive to low oxygen.
 
New Mexico Will said:
Ready for dumb question? The 20A is only flowing from batteries to inverter, correct? Once it leaves the inverter at 120v, it's much less.
Click to expand...

Not a dumb question at all. You are correct. The flow looks like this:
250W from batteries to inverter
12V * 20.833A = 250W to inverter
250W from inverter to heater
120V * 2.083A = 250W from inverter

This doesn't account for any losses from cables, connections, or the inverter. The real-world current from the batteries will probably be more like 23-25A. The thing to remember when doing this sorts of calculations, is that watts (and watt-hours) are the common unit of measurement. If the load you are trying to power needs 1000W at some voltage, you need to pull at least 1000W from your batteries regardless of the voltage of the batteries the voltage of the load, regardless of AC or DC.
 
Mars said:
Also FYI regarding radiant gas heaters with O2 sensors, at high altitudes the O2 sensors are more sensitive to low oxygen.
Click to expand...

My primary tiny house heat is a propane ventless fireplace-works great. It's just that my shed is airtight.
 
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