What could you reasonably run with 2 6v batts tied together and 370watts of panels?

Bass-O-Matic · June 2016

I made a spreadsheet with my usage for my little Trailer. Not much. Some LED Lights, a small DC Fan, occasional water pump. But I have room on my roof and in my trailer for 2 6volt batts tied to 12v and 370 watts of solar.

I feel like I could run more if I put in an inverter.

I have an AC mini fridge that draws Power: 85 W/1.3 A. If I add an inverter... could I reasonably run this 24/7?
What about the 5000 BTU ac? 500 to 650 watts I saw a guy run one off 250 watts solar panel I guess also connected to a batt? That doesn't seem right.
How big an inverter would I need to run the AC and the Fridge? Or just one or the other? I came up with 300w for the fridge.
How long could I run the AC off the batts? or is this totally nuts.
Would the batts get hot and explode?
Why do people use temp sensors on batts? Do I need one?

All decent guesses welcome! If I could just run the little mini fridge that would be great. I realize the AC is a stretch.

Thanks so much!

DConlyGuy · June 2016

just to give you example a 5000 btu air conditioner runs around 500 watts, if you use a 12 volt inverter that close to 40 amps and hour to run it. if your 2 battery's are like mine they are rated at 235 amp hours minus 50% so you dont kill your battery's you can use about 115 amp hours, so you can run the air conditioner for only 2 hours

oil pan 4 · June 2016

The panels need to be large enough to power the air conditioner and charge the battery all the way up during the day.
So no, a 370w of solar will likely not be nearly enough. Ideally the battery only charges during the day and the panels can handle all the loads and charge the battery all the way up.
How big is the battery?
That panel is likely just big enough to power the fridge by its self, assuming you have enough battery power.
To power an air conditioner you need more like 1.5 to 2kw of panels minimum and a lot of batteries.

Johann · June 2016

Two 6 volt batteries mean nothing and can not be used by itself to determine how much capacity such batteries have to run anything.
What is the amp hour rating on them? Are they deepcycle or marine or starter batteries?
What is your other power consumption in watts hours besides the intended AC and/or fridge.

Bass-O-Matic · June 2016

Thanks folks! Yea, AC sounds crazy as I suspected. I have not bought the batteries yet. I was going to go with Costco Golf carts. Somewhere I wrote down 225AH... it's been awhile.

So, yea, forget the AC.

Is there any hope for the fridge? Power: 85 W/1.3 A.

Let me put it another way. If I wanted to run that fridge 24/7 what would you suggest? I'll add the extras later....

oil pan 4 · June 2016

That 200+ amp hour batteries and a big panel should be able to power a fridge all the time and have some additional power.

vtmaps · June 2016

Bass-O-Matic said:
Is there any hope for the fridge? Power: 85 W/1.3 A.

Small mini-fridges (most, but not all) are notorious for being inefficient. Let's say the duty cycle of your fridge is 40%. That means it is drawing 85 watts 40% of the time. That's 816 watthours per day.

A pair of golfcart batteries stores 225 ah X 12 volts = 2700 watthours. The less of that you use each day, the longer the battery will last.

You can probably run the fridge on those batteries for a day without ruining them... but it depends on the duty cycle which depends on ambient temp and how often you open and close the door and how much warm food you put in it.

--vtMaps

Bass-O-Matic · June 2016

Thanks again! I think I'll put in inverter on the fridge with the understanding that I'll only use batteries for a few hours at a time... when the ice block runs out... only in an emergency.... if food is actually going to rot... I'll even drink warm beer...

how big an inverter? I heard 3x the watts. so 300? Then I heard more from some other source.

Bass-O-Matic · June 2016

oh, and do I need pure sine for a fridge? I would think not but what do I know.

Thom · June 2016

You need to get a watt meter and do a power assement . Do it right the first time. It will save you money .

Thom · June 2016

You need to get a watt meter and do a power assement . Do it right the first time. It will

vtmaps · June 2016

Fridges run smoother, cooler and more efficiently with pure sine. As for the size of the inverter... maybe 1000 watts... no guarantees.

Don't forget that a large inverter has tare (self consumption) losses. My inverter uses 20 watts just being turned on (not bad for a 3500 watt inverter). Over a day's time that's 480 watthours... just a bit less than my fridge uses in the summer.

In your system, a conventional fridge becomes a defining load. That one load forces a number of design decisions that you wouldn't otherwise make. One possibility is to get a variable speed compressor fridge. They come in AC and DC models... the DC ones need no inverter. The AC ones do not have high startup surges and will work with a small inverter.

Variable speed compressor fridges are more efficient... it's better to run a compressor at 1/3 speed all the time, than to run it at full speed for 1/3 of the time.

--vtMaps

Bass-O-Matic · June 2016

I'll get a watt meter. Great suggestions. We can check them out from the library here! I would consider a more energy efficient fridge except I am going for some design considerations. the one I have has a glass door with stainless steel. It's really a beverage cooler but I'm modding it to be colder. I'm sure it will NOT be efficient. Putting it on solar is just an emergency use measure to impress my wife.

DConlyGuy · June 2016

2 things you may want to look into dc fridges and the other is looking into making your system at least 24 volts. it will help running them bigger appliances

oil pan 4 · June 2016

A kill-a-watt meter from a hardware store will tell you howm much power stuff uses.
Get a pure sine wave inverter. Those cheap modified sine inverters are garbage.
Normally we size battery systems to power the load for 2 to 4 days.

scrubjaysnest · July 2016

Saw a guy last year in AL, claimed to run his 13.5kbtu A/C for about 3 hours. Don't know about his battery bank but the solar was two 6 X 8 feet arrays. Say 8 panels per array and he could tilt them.
With the new camper we just bought my two 100 watt panels take about 6 hours of sun to keep up with the fans, two led lights, and the water pump. Battery bank is unknown beyond it's 2 - grp 27's labeled D27DC - 160. Best guess is they may be 95 Ah each.

Bass-O-Matic · November 2017

Hey! Me again! I finally built the trailer. I got a killawat and the only thing I care about running is a 2.6 CU inch Magic chef Mini fridge rated at 65 watts. Killawat says it's drawing only 300 watts at startup and 50 watts when running. Can that be right? If so, I ended up with a measly 170 watts of solar... mounted flat and two 6volt Costco golf cart batts wired to make 12v. So,,, can I get a 300min 600max watt pure sinewave inverter and run the fridge? if so, any recomendations? I don't trust amazon. Thanks!

Photowhit · November 2017

I didn't think Kill-A-Watt measure startup, is this the new version? If it does only draw 300 watts on start up you should be able to run it on 600 watts pure sine inverter. BUT understand that with 170 watts of panels you will only produce about 75% during the summer with sun directly over head and less during the winter when sun is low on the horizon.

Figure you are producing 120 watts and you have average 4 hours (that is going to be generous this time of year with flat mounted panels unless you are on the equator. You can produce 4 hour x 120 watts for 500 watt hours and your fridge uses 50 watts per hour while running. Add your inverter using 10-12 watts an hour and if you are running off battery you will lose 20% storing the energy. You aren't left with much to run the fridge.

Bass-O-Matic · November 2017

Oh. That doesn't sound too good. Well, For a weekend campout what do you think would happen if I put a small block of dry ice in the freezer tray of that Fridge? Crazy? I wonder if it would be TOO cold for the metal/plastic? I wonder if it would raise the CO2 levels in the camper to you know... bad?

Bass-O-Matic · November 2017

Hmmm... well, with many caveats. http://www.dryicecorp.com/uncategorized/how-to-use-dry-ice-during-a-power-outage/

Well, if I had a 600 watt pure sine wave ( I assume you mean minimum with a max of 1200 or so ) I could at least run the fridge for a few hours while we traveled to our next AC Plug eh?

myocardia · November 2017

Well, sort of. You will get a maximum of 2.0 hours run time, before you start seriously damaging your batteries. After about 3 hours, you've done major damage to them. Note, as long as you do your driving between 11am and 1pm during the summer/late spring/early fall*, you will use almost zero of your battery power, so you would be able to go for 4.0 hours, without harming your batteries. Honestly though, it would make a ton more sense to just remove the food from the fridge, and put it into one of these, carried inside your air conditioned vehicle:
https://www.amazon.com/Wagan-EL6224-12V-Cooler-Warmer/dp/B00NHBYOA6/ref=sr_1_2?ie=UTF8&qid=1510206647&sr=8-2&keywords=12V+cooler+for+car

* And only on completely cloud-free days, of course.

BB. · November 2017

Kill-a-Watt meters do not usually do a good job catching the startup surge. If your fridge only draws ~50 Watts while running, that is much less than the ~100-120 Watts typical--So the 300 Watt surge is a possibility (typically around 5x running current for surge).

How much energy the fridge uses in a day (Watt*Hours or kWatt*Hours) is still in question... A warm room with already cool food--typically run around 50% duty cycle. Or:

50 Watts * 24 hours per day * 0.50 duty cycle = 600 Watt*Hours per day = 0.6 kWH per day

That actually seems pretty low for energy consumption... I would have guessed closer to the ~800 WH per day range.

If this is the correct refrigerator:

https://www.energystar.gov/productfinder/product/certified-residential-refrigerators/details/2213650

Annual Energy Use (kWh/yr): 215
215,000 WH per year / 365 days per year = 589 WH per day
589 WH per day / 24 hours per day = 25 Watt average load

The manual says "This unit is not designed to be installed in an RV or used with an inverter."....

2x 6 volt @ 200 AH "golf cart" batteries will reasonably supply:

12 volts * 200 AH * 0.50 max discharge * 0.85 inverter eff = 1,020 WH max

Your two batteries would supply this fridge for almost 2 days of "no sun"--Not bad.

Say you mount your solar panels flat to the roof, 170 Watts, in the Austin Tx area:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Austin
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.83	3.41	4.40	5.25	5.62	6.36
Jul	Aug	Sep	Oct	Nov	Dec
6.56	5.94	4.97	4.05	3.07	2.63

If you assume at least 4.0 hours of sun per day:

170 Watts * 0.52 off grid AC system eff * 4.0 hours of sun per day = 354 WH per average "October day"

Or, working the other way, your 170 Watt panel would need "hours of sun" per day:

589 WH per day * 1/0.52 off grid system eff * 170 Watt panels = 6.7 hours per day "break even" with 170 Watt panel

Or, if you assume Feb-October camping, 3.41 hours of sun (Feb):

589 WH per day * 1/0.75 "fudge factor" * 1/0.52 off grid AC system eff * 1/3.41 hours of sun = 443 Watt "recommended" solar array

Obviously, tilting panels towards sun, where you will be camping (farther north or south), warm weather=longer fridge run time, etc... A bunch of factors that can effect your power generation and needs.

-Bill

Bass-O-Matic · September 2019

Wow. Thanks to both for the very extensive responses.

What could you reasonably run with 2 6v batts tied together and 370watts of panels?

Comments

AustinAverage Solar Insolation figures

Austin
Average Solar Insolation figures