Seeking advice: repurpose small setup for home space heater

[mjgillen]

Hello all, new to this Forum.

I am looking for advice on how to repurpose my recently decommissioned small solar setup to power a space heater for a bedroom. The equipment I currently have is:

1 x 375w solar panel
Morning Star 25a MPPT controller model PS-MPPT-25M
2 x Trojan 27TMX 12V 105AH flooded lead acid battery

This setup has served me well with no issues for 5 years powering a small ham radio setup that drew max 2 amps for 4 hours at any given time. However I am not doing ham radio anymore so the setup has been idle and I would like to put it to good use.

I would like to figure out how to power a small space heater for the master bedroom. I could either power one of my existing 120VAC space heaters or purchase something more 12v friendly. I am also willing to add one or two more solar panels, add more battery storage, move batteries to a different technology/chemistry, etc. Willing to start all over as well. Just want to get something from Mr. Sun. I don't want to tie into the grid.

Budget is $2000

Five years ago I had the notion to create the ham radio setup so I started reading about solar setups and ended up with this setup as my first DIY solar project to get my feet wet. I am pretty happy with the results as the setup has been very minimal maintenance and no issues for the past 5 years. Monthly maintenance has been to top off the water in the batteries and that's it. My original 27TMX lasted about 4 years and then I added another radio and purchased two new 27TMXs for more storage.

I am in the San Diego CA area with plenty of sun and the most expensive energy costs in the country at nearly 50 cents a KWH.

Your advice appreciated.

Michael

 

[sunshine_eggo]

Space heaters are very inefficient and one of the most taxing demands on a power system.

Simple: How many hours do you want to run it for a day?

Multiply that by the wattage for Watt-hours.

That's the minimum battery you need for a single day of operation without charging.

Divide that by 4 - that's the minimum PV you need.

Divide the PV by battery voltage for MPPT current needed.

Probably want to go to 24V for lower DC currents and more PV for a given MPPT.

Example: 1000W space heater for 4 hours/day

4h * 1000W = 4000Wh

24V * 105Ah = 2520Wh of battery, so you'll need more.

4000Wh/4h = 1000W of PV

1000W / 24V = 41.7A of MPPT output.

 

[Partimewages]

The quick answer is any type of space heating or cooling is a major draw on any system. Probably the largest draw you could have in a residential situation.

 

[mjgillen]

Yes I know the usual 120VAC space heaters are 1000w-1500w which is a huge energy suck however they make 12v "car" or "truck cab" heaters that are much less watts 150w and up. I was thinking about getting a 300w one and try it out to see how well it would heat a small bedroom and go from there. If it worked out well then I know I just need to calculate capacity and see what the current batteries can deliver.

I am thinking if I can use the heater even for one hour to heat a room and it does a decent job I would be much better off than no heater at all.

Michael

 

[KITROBASKIN]

Welcome!
Those little 12V heaters don’t do much unless the space is very small and has some insulation (depending on how cold it is outside). They are typically very cheaply made with a limited life if actually used.

Hopefully you will report your research and experimentation. We live modestly off grid. I use a Presto Heat Dish parabolic heater when cleaning up in the morning (Victron Phoenix 24V 1200VA inverter, EPOCH 24V 100Ah LFP battery). Electric blankets work for sleeping. 12V blankets are limited in the size of the blanket/throw heating sections. Space heating even a small bedroom might be possible in your mild climate but you will need more storage capacity for any kind of sustained use.

Your batteries are high quality but if you keep them inside, you will have outgassing to consider.

If you will, please describe why you stopped your ham radio hobby?

 

[mjgillen]

Batteries are kept in large vented space under the main floor so no issues with outgassing.

I got out of ham radio after 8 years because I had accomplished what I wanted. I still have some equipment however not getting on HF anymore. It was very fun for the first few years then it got a little boring. I know there’s much more I could do however I’m happy with my experiences.

The 12v space heaters look like poor quality so I think I’m going to setup a 120v system. So now I’ll start looking at inverters and battery chemistry/technology. Open for suggestions!

It’s too bad the 12v products out there are few and not good quality.

Michael

 

[efficientPV]

Just one 375W panel? Heating is quite simple electronics. I used one of these oil heaters with 60V array with electronics keeping array at power point. If house isn't cold, 400W into this heater wasn't touch safe.

 

[Partimewages]

1 watt = 3.412 btu's. It should be relatively simple to figure your heat load from an internet search.
A inverter will have standby losses for every hour it is powered on.

 

[mjgillen]

Heating is quite simple electronics. I used one of these oil heaters with 60V array with electronics keeping array at power point. If house isn't cold, 400W into this heater wasn't touch safe.

@efficientPV I really like your idea. Can explain a little more how your setup works and how I might be able to duplicate that? It's obvious I don't know all the different ways to heat a house using electricity and was only thinking space heater. Forgive my ignorance.

Michael

 

[Daniel644]

Yes I know the usual 120VAC space heaters are 1000w-1500w which is a huge energy suck however they make 12v "car" or "truck cab" heaters that are much less watts 150w and up. I was thinking about getting a 300w one and try it out to see how well it would heat a small bedroom and go from there. If it worked out well then I know I just need to calculate capacity and see what the current batteries can deliver.

I am thinking if I can use the heater even for one hour to heat a room and it does a decent job I would be much better off than no heater at all.

Michael

it's simple math, resistive heat is 1:1 watts in to watts of heat out regardless the input voltage you need like 3,000 BTU's to heat up an 8'x8'x8' room, a 300 watt "cab heater" is only about a 1,000 BTU's, you can just use something that needs less watts to run because that means less heat, average rooms need a 1,000-1,500 watt space heaters to get any amount of heat into the space, you can't just get a heater that uses less watts, because you NEED ALL THE WATTS (for resistive heating).

If you want a more efficient way to heat (which you need to reduce demand on battery and solar capacity) the answer is a Heat Pump in the form of a Mini-Split installed into that bedroom, there are small 120v models now that could easily be wired to a plug that is plugged into an inverter, but even the cheapest setup (assuming you had the tools and know how to install it) is 600-700 bucks then you still need like double the battery capacity and an inverter powerful enough, probably more expensive then your budget.

what you need to do is start with a space heater and a power meter (like a Kill-a-watt or a smart plug with a power meter) and see just how many kwh's you need per day to actually heat the room to a comfortable temperature, once you have that data then you can share it and we can better analyze your needs and if it's possible in your budget.

 

[mjgillen]

@Daniel644 I understand about the space heater. It's not really a viable solution so I am exploring other "solar heating" discussion on this forum. The oil radiator heater that @efficientPV has a photo of above sounds interesting if I can hack the controls and provide 12v to power it. Although they are also rated for 1500w consumption perhaps I can heat it slowly with fewer watts over a longer period. Not sure. And the more I think about it the more it sounds like another space heater so likely not a viable solution either.

So like I mentioned I am exploring other discussion about solar heating. There's some good info about "solar" heating at https://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm using the sun, not electricity to heat.

So it might be that using electricity as a heat source is just very inefficient and consumes a lot of electricity no matter the source - PV, batteries, the grid.

I was hoping to repurpose my small setup to provide some heat however it appears that the amount of heat I could generate would be so small as to not really make a difference so I will look into what else I can use the energy for. Maybe build some solar collectors and use my PV energy to run some computer fans to move the air. Just thinking out loud.

Anyway, thank you to all that replied. I will continue to search this forum for more info on solar heating and other things I can do with my two RV batteries.

Michael

 

[Partimewages]

Just remember.
Watts are Watts 1 = 3.412 btu's.
Unless you are moving heat (btu's) from one place to another it's still just Watts. You can't make heat only transfer it. Burn propane, gas or wood you didn't make it just transferred it. Same if it's electric watts.
Heat pumps move heat from one place to another. Outside to inside. They use watts to do this so the source is usually the air. Up to 3 to 5 times as efficient as resistance heating. Oil heater is just electric resistance heating oil in a tube.

 

[mjgillen]

It appears that my biggest obstacle is my 12v setup. If I wanted to power a 1500w space heater my inverter would draw 125 amps to generate 1500w at 120v from my 12v batteries (did I get that right?) which is pretty much off the charts. So I want to think outside my existing 12v setup. Let's say I create a 48v setup. At 45v my inverter would draw 31.25 amps which might be doable and at 60v would draw 25 amps.

So I think I will look into creating a 60v system.

Thoughts?
Michael

 

[Partimewages]

125 amps is correct.

 

[Partimewages]

60 volt system is odd ball. Stick with 48

 

[mjgillen]

Thanks! Reading this forum.... I'll get back with a proposal once I feel like I've learned enough...

 

[littleharbor2]

I have a very efficient one ton (12,000 btu) minisplit. $700. Connected to a PowMr 3000/24 all in one, $350. I put 2200 watts of solar on my roof $1,000. I have 4 used 135 ah. 12 volt AGM batteries$??This setup works great to heat my living room for 4 - 5 hrs. in the early morning. It also works great in a/c mode all day running straight off the sun. Total, $2,050 plus whatever you could do the batteries for and my labor to install the system.

 

[mjgillen]

@littleharbor2 Thank you very much. Looking into it now.
Michael

 

[littleharbor2]

@littleharbor2 Thank you very much. Looking into it now.
Michael

The Seer 22 mini split was the deal maker. I had a Panasonic one ton there originally and it pulled almost twice the watts that the Mirage Magnum 22 does. I couldn't run it on my 2000 watt Inverter generator, The magnum runs easily on the rare days I need the genny.

 

[MTM98290]

I have a very efficient one ton (12,000 btu) minisplit. $700. Connected to a PowMr 3000/24 all in one, $350. I put 2200 watts of solar on my roof $1,000. I have 4 used 135 ah. 12 volt AGM batteries$??This setup works great to heat my living room for 4 - 5 hrs. in the early morning. It also works great in a/c mode all day running straight off the sun. Total, $2,050 plus whatever you could do the batteries for and my labor to install the system.

This seems to be the most viable option if you need heat at night and it must be purely electric.

A heat pump makes better use of the watts without outside energy.

If not using a heat pump you must add energy one way or another. You could also power a pellet stove, propane/natural gas stove blower fan with no issue.

Purely electric heat will work fine when the sun is shining. Your batteries won't run a 1500w heater for an hour without going below 50%.

We have a much larger battery and a 2k solar array at our cabin and radiator style oil heater usually set to 600w and I will not use it unless the battery is full and the sun is shining. It is a fine way to use solar excess, but I would not trust it to keep an place warm other than during a sunny day. It is rare that this heater could be used to stay comfortable, it only adds some heat to supplement far from the woodstove. In the summer I could run it at 1200 watts during the day, but we don't use heaters in 80-90 degree weather.

We have a pellet stove in our home and it runs off of a 7.2Kwh bank with a 800w inverter at about 250w constant draw. This system is designed to keep the pellet stove running if we lose power. It gives us heat overnight so that I can wait until morning to hook up generator if the power isn't reconnected before then. This system uses 3.2-3.5 Kwh to accomlish a night of heat at 15,000-30,000btu/hr. Yes It burns pellets too so that must be factored in cost wise.

I give this example to demonstrate the scale of energy needed to make heat.

The most efficient heat pumps would be within your budget. Your battery could possibly run the oil radiator heater for 1.5 hours, it would likely run a small heat pump for 3-4.5 times longer, may be more. The heat pump would be simplest answer in my mind.

Another option it you are looking to tinker and build something. You could heat water with an electric water heater and store the heated water. The water could be heated and stored all day and then pumped to move the heat through the room via a small pump during the night. This allows you to use the daytime sun to build a store of heat but allow it to be used at night. Which is generally the difficulty with solar powered heat. Solar powered AC is easy.