Inverter question for Off-Grid Tiny House

[oceanairbrush]

Greeting Forum, my first post!

I have a 12x30' shed i am converting to a guest house and it is already wired with a breaker box set up to be connected to my main house. I would like to add solar panels, a battery bank and an inverter to supply all power to the house and hook it up to the breaker box to distribute it throughout. What kind of inverter would I need to do this. I would like something between 3-5000 watts which is overkill but since it will be running a small a/c unit during the day i would rather have it loafing and running cooler than stressing at it's peak.

I am looking at 6 285 watt panels and I currently have 8 75amp/hour AGM batteries so i would think I have multiple options for configurations. The only thing I haven't figured out yet is the inverter. What do i need to wire directly into the breaker box that will support varying loads on each leg?

[mapmaker]

I am looking at 6 285 watt panels and I currently have 8 75amp/hour AGM batteries so i would think I have multiple options for configurations. The only thing I haven't figured out yet is the inverter.

Welcome to the forum,

Is this a 48 volt system? What is the voltage of those 75 ah batteries? I'm trying to figure out whether you have enough battery to do what you want.

--mapmaker

[ChrisOlson]

The only thing I haven't figured out yet is the inverter. What do i need to wire directly into the breaker box that will support varying loads on each leg?

If the system is split-phase (120/240V) then you need one of these:

http://www.schneider-electric.com/products/ww/en/7000-solar-off-grid-and-back-up/7010-inverters-chargers/7654-conext-xw/

And this is how they wire up internally - on the left you have the AC output with L1, N and L2. In the middle you have AC1 input for grid connection (if you have grid). On the right you have the AC2 input for generator (if you have generator). You just run #4 THHN from the AC output to your power panel.

[hill5673]

Greeting Forum, my first post!

I have a 12x30' shed i am converting to a guest house and it is already wired with a breaker box set up to be connected to my main house. I would like to add solar panels, a battery bank and an inverter to supply all power to the house and hook it up to the breaker box to distribute it throughout. What kind of inverter would I need to do this. I would like something between 3-5000 watts which is overkill but since it will be running a small a/c unit during the day i would rather have it loafing and running cooler than stressing at it's peak.

I am looking at 6 285 watt panels and I currently have 8 75amp/hour AGM batteries so i would think I have multiple options for configurations. The only thing I haven't figured out yet is the inverter. What do i need to wire directly into the breaker box that will support varying loads on each leg?

Is this going to be tied to the grid through the main house? Whole different animal.

[oceanairbrush]

the batteries are 12 volt

the entire system will be independent from the grid strictly off grid there will be no house connections no connections of any sort to the grid.

[mapmaker]

the batteries are 12 volt

Eight 75 ah 12 volt batteries configured as two strings in parallel is a 150 ah 48 volt bank. That's 7200 watthours and you don't want to discharge below 50%, so you've got 3600 wh to maneuver in.

That's a fairly small battery capacity to be running air conditioning and whatever else you have in mind. (you haven't mentioned much about your loads) If you are only running your loads while the sun is shining, you might be able to get away with it.

--mapmaker

[Sunking]

Don't mean to rain on your parade but if the hous eis already connected to the grid it would be very foolish to take anything off-grid. Essentially what you are saying or asking for is I want a very limited supply of electricity, spend a couple weeks each year without power, and pay 10 times more for the power the rest of my life. Is that what you really want because that is what you are asking for.

[Shockah]

Greeting Forum, my first post!

I have a 12x30' shed i am converting to a guest house and it is already wired with a breaker box set up to be connected to my main house. I would like to add solar panels, a battery bank and an inverter to supply all power to the house and hook it up to the breaker box to distribute it throughout. What kind of inverter would I need to do this. I would like something between 3-5000 watts which is overkill but since it will be running a small a/c unit during the day i would rather have it loafing and running cooler than stressing at it's peak.

I am looking at 6 285 watt panels and I currently have 8 75amp/hour AGM batteries so i would think I have multiple options for configurations. The only thing I haven't figured out yet is the inverter. What do i need to wire directly into the breaker box that will support varying loads on each leg?

Plug that A/C into the main house with a Kill-A-Watt and get an actual reading of watt consumption. Yes, your 8 batteries will be stressing at their peak.

[oceanairbrush]

The AC is an 8000BTU window unit drawing 6.1 amp with thermostat control. So about 700watts if I did the calculation correctly? The building is spray foam insulated to minimize the running time.

I agree that the battery bank is small but, I have designed everything in here to be efficient as possible, the insulation, the AC, LED television, LED lighting running off 12v. I have tried to keep as much off the inverter as possible to minimize conversion losses. I need the battery bank to run TV, fridge and A/C and limited other items during the night. I would however like the option of being able to use an efficient washer dryer combo during the day during peak sun hours. Water heating is solar as is the heating system it's self. Water pressure is provided by a 12v rv pump which maintains a steady 50psi and cuts out at 60psi at 2.1 gpm. I am located in FL so we get lots of sun.
Oh, water pump and LED lighting have their own separate batteries and solar panels. I have tons of those little 7ah agm's you get in computer UPS's.

I am not even opposed to puting a back up generator on it with an auto-start as an alternative for heavy loads. ( I have a 4kw onan in the garage). But i don't want it running more than an hour or two per day in the worst of conditions.

[ChrisOlson]

The AC is an 8000BTU window unit drawing 6.1 amp with thermostat control. So about 700watts if I did the calculation correctly? The building is spray foam insulated to minimize the running time.

I agree that the battery bank is small but, I have designed everything in here to be efficient as possible, the insulation, the AC, LED television, LED lighting running off 12v. I have tried to keep as much off the inverter as possible to minimize conversion losses. I need the battery bank to run TV, fridge and A/C and limited other items during the night.

You don't have enough load for a big inverter. You need to load an inverter to about 20% of it's continuous output capacity to get peak efficiency from it. Also doesn't sound like you need a split-phase system because there's no 240V loads? Everything is 120V?

Those batteries you got will run the AC for about 3-4 hours in the evening after the sun goes down and they'll be sacked. You need way better quality, and way more capacity for your batteries to do what you want. We have a 1.5 ton central AC unit for our off-grid home here. We can power it fine during the day on solar powre. But batteries are too expensive to attempt to power an AC unit at night in an off-grid installation. We use a diesel prime generator to power the AC at night instead of cycling the hell out of our batteries. When it is hot here our AC unit ALONE uses 30 kWh/day. And I will guarantee you that our 24.5 SEER central unit is 3.5x more efficient than that window unit. The best thing to do with a window AC unit off-grid is to drop it off at the recycler, sell it at a garage sale, give it away - or something similar - whatever it takes to get rid of it. Then go buy a mini-split for a small installation, or a high-efficiency central unit for a larger home. Then you got a fighting chance of being able to power it off-grid. A window AC unit is like throwing good money after bad.

[oceanairbrush]

I was unaware of the 20% rule on inverters, this is the first I have heard it mentioned.
As for 220v loads, the washer dryer combo would have a 220v dryer. although I suppose it would be just as easy to source a 110v one.
I looked at mini-splits and they seems to be drawing similar amperage to the window units at this BTU size. So where is the advantage?

[ChrisOlson]

Here is an example efficiency curve for a typical inverter. This happens to be for a Xantrex 2524, but all inverters are similar. Notice it takes minimum 400-500 watt load on this little inverter to reach peak efficiency. And notice how low the efficiency is at very light loading. This is why you do not want to over-size the inverter.



Here - read this and don't fool yourself on those window AC units. The mini-splits start at 13 SEER (Seasonal Energy Efficiency Ratio) and they make them as high as 27 SEER. Window units are exempt from SEER ratings, but if it was applied there are none above 10. If you are off-grid and need AC you buy The Best There Is in equipment, otherwise you'll be kicking yourself because you'll find you can't power it. Take it from me with many, many years of off-grid experience - off-grid power is EXPENSIVE compared to utility power. You start running AC, even for a small off-grid home, and you need the most efficient unit money can buy. The hardware in the mini-splits is lots more advanced with variable freq drive compressor and fans, etc than you get in a window unit.

http://www.slate.com/articles/health_and_science/the_green_lantern/2011/05/global_cooling.html

On your dryer, also don't pick just any dryer. There are good ones and not so good. We have a high-efficiency Maytag cross-flow dryer but we never use it in the summer. We have a different dryer for summer time called a "clothes line". You throw a clothes dryer into the equation, along with A/C and now you are talking SERIOUS power consumption. You're going to need one hell of a lot more than 6 285W solar panels, and you'll need a Conext XW6048 just to handle the starting surge from the A/C compressor while the clothes dryer is running. A/C compressors normally pull 3x running amps for starting, and 4-5x running amps at locked rotor (pressure on the head).

Dude you are digging yourself a hole here you will not climb out of. You can throw in a big Schneider Conext/Xantrex inverter like we got and power it all, but big inverters take big food to make 'em run. And inverter food is expensive.

[Sunking]

The AC is an 8000BTU window unit drawing 6.1 amp with thermostat control. So about 700watts if I did the calculation correctly?

That is not efficient with a SEER of 8000 BTU / 730 watts = 10.9 which is extremely low or inefficient. If you were to use say a 24 volt air conditioner made for solar and telecom there is several units with variable output of 7000 to 14,000 BTU's and only use 300 and 650 watts respectively or SEER's of 23 and 21. If you want to spend the big bucks you can get SEER's of 26 in DC models.

Secondly 12 volt systems are very inefficient.

[ChrisOlson]

That is not efficient with a SEER of 8000 BTU / 730 watts = 10.9 which is extremely low or inefficient.

Sunking, I believe that equation yields the EER for A/C units, not the SEER. But still your message is valid - it is an energy hog.

Incidentally, just for some trivia, the EER of A/C units is a paradox. It is an imperial measure of heat (BTU/hr) divided by a metric power consumption figure (watts). That one has always given me a bit of a chuckle.

[Sunking]

Sunking, I believe that equation yields the EER for A/C units, not the SEER. But still your message is valid - it is an energy hog.

Incidentally, just for some trivia, the EER of A/C units is a paradox. It is an imperial measure of heat (BTU/hr) divided by a metric power consumption figure (watts). That one has always given me a bit of a chuckle.

Yep sorry about that but SEER and EER use the same formula, with the exception SEER is a average over over Season.