Converting 3,500 watt grid tie to off grid

tonybluegoat · September 2018

I started with the idea of installing a 2nd solar array with a battery bank as a solar backup generator for my small, think TINY, 200 square foot house in Texas. Now I'm thinking of just converting my existing grid tie system to an off grid. I know it's a silly thing to do, but I might do it anyway. Sort of an off-grid system using available grid power as my back-up.

(rules and regulations not withstanding)

3,500 watt array connected to my Sunnyboy 3000 TL-US.
On full sun days it delivers about 20 kw of electricity through the SB3000. So I have a max charge rate of 20 kw per day.
The system runs at about 385 volts currently. I have a Midnight solar combiner box at the panels. So I could technically break the array apart from being one giant 385 volt array to 4 smaller arrays. Lower volts higher amps. I'm using 6 gauge wire over 100' run. The DC Amps might overload it?

The array can charge up to 20 kw per day. The apartment uses 10 kw per day or less... But let's say 10 kw per day is the budget.

Question 1. Can I run on EIGHT 200 ah batteries (12 volt). That's 19,200 watts of storage capacity... Twice my budget.

Question 2. I don't think there is any way to keep the SB300 TL-US in this system. Right? It requires grid 240 volt input in order to operate.

Question 3. The transition... budget conscious but not "shoot myself in the foot" budget conscious.

8 batteries at $375 each = $3,000

Charge controller that handles higher voltage? These are grid tie panels so I don't think I can get a clean 24 or 48 volts out of them.

And then the inverter. 4000 watt with surge for the window ac unit and small fridge. I'm thinking Magnum, or is Ames good enough? There's a big price difference. How long do grid tie inverters last? I know it depends on quality... what's the line? Is it Magnum?

What else?

Thanks.

bill von novak · September 2018

tonybluegoat said:

Question 1. Can I run on EIGHT 200 ah batteries (12 volt). That's 19,200 watts of storage capacity... Twice my budget.

Question 2. I don't think there is any way to keep the SB300 TL-US in this system. Right? It requires grid 240 volt input in order to operate.

Question 3. The transition... budget conscious but not "shoot myself in the foot" budget conscious.

8 batteries at $375 each = $3,000

Charge controller that handles higher voltage? These are grid tie panels so I don't think I can get a clean 24 or 48 volts out of them.

OK. If you are truly off-grid you need at least 3 days of autonomy and a generator. So that's a ~60kwhr battery bank. You'd be at 1200 amp hour 2 volt cells, to make a 48V battery. Perhaps 24 S-1660's would work. So that's about $10K.

To support that size battery bank, you will need to charge at least C/13. Which means another 3000 watts of solar.

You can keep the SB3000 in the system if you do an AC coupled system. They are complex and hard to set up, and you will need a 240V inverter like the Radian. They are about $4K. The Radian 8048 will power pretty much anything you throw at it.

You can get a controller that handles up to 600 volts. Both Xantrex and Morningstar make one. You'll need two. They are about $1500 each.

So you're looking at a total of around $27K for the basics. Look at replacing batteries once every 10 years if you are careful.

Photowhit · September 2018

Panels will last a long time and are a non-factor, you could go with a high voltage charge controller, so you wouldn't have to rewire your array ($1000) Likely a 4000 watt inverter is over kill for a window air conditioner. I can crank mine on a 1100 watt Exeltech inverter (minimal size 5200 BTU).

I don't know your energy use of solar insolation but at 10Kwhs you might be under sized at 3500 watt array. Usually you will want about 3x the size array off grid as one that would off set your energy use in a grid tied system! This has to do with system losses 9about 50%) and needing to have a system large enough to completely charge your batteries a couple times a week and storage for cloudy days.

Doing it with the grid as back up sounds like a lesson in spending money frivolously. Perhaps a serious prepper expecting a complete collapse? Costs $1000 CC, $1500+ inverter, $3000 battery bank that will need replacing from time to time...

tonybluegoat · September 2018

Photowhit said:

Panels will last a long time and are a non-factor, you could go with a high voltage charge controller, so you wouldn't have to rewire your array ($1000) Likely a 4000 watt inverter is over kill for a window air conditioner. I can crank mine on a 1100 watt Exeltech inverter (minimal size 5200 BTU).

I don't know your energy use of solar insolation but at 10Kwhs you might be under sized at 3500 watt array. Usually you will want about 3x the size array off grid as one that would off set your energy use in a grid tied system! This has to do with system losses 9about 50%) and needing to have a system large enough to completely charge your batteries a couple times a week and storage for cloudy days.

Doing it with the grid as back up sounds like a lesson in spending money frivolously. Perhaps a serious prepper expecting a complete collapse? Costs $1000 CC, $1500+ inverter, $3000 battery bank that will need replacing from time to time...

You figured it out. I'm king of the preppers. This is an "Ark" build. It's a small, high efficiency apartment on my farm that requires minimal energy. I already run it on the 3500 array for the most part. And I have it connected to the grid for a backup. In the "Stand-alone" configuration with off-grid power I would simply have a main breaker that will allow me to run it off the batteries and solar or directly off the grid.

I'm trying to do the transition for under $5,000. $3,800 would be the sweet spot for this prep. Some people spend money on cigarettes or buy a used ATV or a boat. I spend my fun money on preps. I've got them all. Food for a year, EMP proof truck, farm, food on the hoof and lots more. This is my next fun project. I can collect the parts I need over the next 6 months and have everything read to switch over in the Spring.

$1000 high voltage charge controller... CHECK
1100 watt Exeltech inverter has enough surge amps to start a small ac. Do you have a small fridge on your's? By small I mean small apartment size, not tiny. That's it plus a ceiling fan and some LED light fixtures. I can switch the hot water over from the electric hot water heater to a propane one. I'll look into that.

Good info

tonybluegoat · September 2018

bill von novak said:

OK. If you are truly off-grid you need at least 3 days of autonomy and a generator. So that's a ~60kwhr battery bank. You'd be at 1200 amp hour 2 volt cells, to make a 48V battery. Perhaps 24 S-1660's would work. So that's about $10K.

To support that size battery bank, you will need to charge at least C/13. Which means another 3000 watts of solar.

You can keep the SB3000 in the system if you do an AC coupled system. They are complex and hard to set up, and you will need a 240V inverter like the Radian. They are about $4K. The Radian 8048 will power pretty much anything you throw at it.

You can get a controller that handles up to 600 volts. Both Xantrex and Morningstar make one. You'll need two. They are about $1500 each.

So you're looking at a total of around $27K for the basics. Look at replacing batteries once every 10 years if you are careful.

I'm not truly off grid. I'm throwing money away. So it sounds like my basic math is right. Everybody throws money away. How much does the average middle aged person spend on a new car? $28-$35K I drive a $3,000 prius with 350,000 miles on it. How much do people spend on an average mid-life plus mortgage? $2,000 per month? My farm is paid off. Etc. I live very inexpensively so I can afford to do crazy things.

If we know that I have full grid backup power then there's no need to worry about the 3 day thing. Also if we assume that if the grid is down I don't mind going a day or two or three on no power whatsoever, then that's gives us some flexiblity.

With a budget... I would like to throw away $5,000... not $25,000.... So 20k of battery on a 20k per day solar array (the grid tie one). No coupled system. 10 kw budget but I can use much less if I want to stretch it.

Really what I'm looking for from an AC standpoint is just a cold room to sleep in (East Texas) at night. The rest of the day I don't mind being hot a sweaty. During a long disaster I can get very energy picky.

Some guys buy a bass boat on credit. I want to buy an off-grid lifeboat for cash. I have the apartment built. 200 square feet.

Thanks for the info. If you have an adjusted plan based on new info. I'm all ears.

Here's a question... are solar batteries # of cycles and how deeply they are drained dependent. What I'm asking is what I generally DON'T USE THEM. Would they last longer than 10 years or are they like my Prius battery A new one lasts 10 years and then goes out whether you use it or not. It's age dependent, not use dependent.

Photowhit · September 2018

tonybluegoat said:

$1000 high voltage charge controller... CHECK
1100 watt Exeltech inverter has enough surge amps to start a small ac. Do you have a small fridge on your's? By small I mean small apartment size, not tiny. That's it plus a ceiling fan and some LED light fixtures. I can switch the hot water over from the electric hot water heater to a propane one. I'll look into that.

So I actually looked at prices, the Schneider 600 V is around $1200, In a 48 volt system you could effectively use a Midnite Classic SL for about $550 and rewire the array.

I was just trying out the AC on the Exeltech for someone, I don't use it daily, I'm under sized with an 1800 watt Prosine that I regularly run daily life on. It handle everything within it's power limits fine and has for about 12 years and is about 17 years old, though not particularly cheap. I run a microwave and regular (18 year old) fridge. I even run a water heater as an opportunity load, running it's 3600 watt 240 volt inverter on 120 volts (about 900 watts).

Switching the water heater over will save much of your daily load.

tonybluegoat · September 2018

Photowhit said:

So I actually looked at prices, the Schneider 600 V is around $1200, In a 48 volt system you could effectively use a Midnite Classic SL for about $550 and rewire the array.

I was just trying out the AC on the Exeltech for someone, I don't use it daily, I'm under sized with an 1800 watt Prosine that I regularly run daily life on. It handle everything within it's power limits fine and has for about 12 years and is about 17 years old, though not particularly cheap. I run a microwave and regular (18 year old) fridge. I even run a water heater as an opportunity load, running it's 3600 watt 240 volt inverter on 120 volts (about 900 watts).

Switching the water heater over will save much of your daily load.

I'm diggin' it! I'm liking the Sneider 600 V option. It seems pretty straight forward. The Exeltech prices double when you go from 1100 to 2000. I would probably do something different. The Exeltech also may not have as robust a surge power... the surge power on it (according the site) is temperature dependent. It will surge IF... the temp isn't too high. I live in texas and under constant use it might turn into a pain in the rear. I prefer spending a little extra ... since I don't need it at all..

This is all great info.

mike95490 · September 2018

If you have the Prius, you can use IT as a battery when home

https://www.fperkins.com/me/prius-home-generator.php

bill von novak · September 2018

tonybluegoat said:

bill von novak said:

OK. If you are truly off-grid you need at least 3 days of autonomy and a generator. So that's a ~60kwhr battery bank. You'd be at 1200 amp hour 2 volt cells, to make a 48V battery. Perhaps 24 S-1660's would work. So that's about $10K.

To support that size battery bank, you will need to charge at least C/13. Which means another 3000 watts of solar.

You can keep the SB3000 in the system if you do an AC coupled system. They are complex and hard to set up, and you will need a 240V inverter like the Radian. They are about $4K. The Radian 8048 will power pretty much anything you throw at it.

You can get a controller that handles up to 600 volts. Both Xantrex and Morningstar make one. You'll need two. They are about $1500 each.

So you're looking at a total of around $27K for the basics. Look at replacing batteries once every 10 years if you are careful.

I'm not truly off grid. I'm throwing money away. So it sounds like my basic math is right. Everybody throws money away. How much does the average middle aged person spend on a new car? $28-$35K I drive a $3,000 prius with 350,000 miles on it. How much do people spend on an average mid-life plus mortgage? $2,000 per month? My farm is paid off. Etc. I live very inexpensively so I can afford to do crazy things.

If we know that I have full grid backup power then there's no need to worry about the 3 day thing. Also if we assume that if the grid is down I don't mind going a day or two or three on no power whatsoever, then that's gives us some flexiblity.

With a budget... I would like to throw away $5,000... not $25,000.... So 20k of battery on a 20k per day solar array (the grid tie one). No coupled system. 10 kw budget but I can use much less if I want to stretch it.

Really what I'm looking for from an AC standpoint is just a cold room to sleep in (East Texas) at night. The rest of the day I don't mind being hot a sweaty. During a long disaster I can get very energy picky.

Some guys buy a bass boat on credit. I want to buy an off-grid lifeboat for cash. I have the apartment built. 200 square feet.

Thanks for the info. If you have an adjusted plan based on new info. I'm all ears.

Here's a question... are solar batteries # of cycles and how deeply they are drained dependent. What I'm asking is what I generally DON'T USE THEM. Would they last longer than 10 years or are they like my Prius battery A new one lasts 10 years and then goes out whether you use it or not. It's age dependent, not use dependent.

Hey, I'm not saying don't throw money away. Just do a good design.

A few comments:

"If we know that I have full grid backup power then there's no need to worry about the 3 day thing. "

Unless you're a prepper. If you are really planning for the "end of the world" it would definitely suck to kill your batteries a week after the world ended.

"I would like to throw away $5,000... not $25,000"

OK then you are looking at spending most of the money on batteries. Get a small bank (say 8 S-550's) - that will set you back $2600. Plan for 48V. Get a 600V charge controller ($1500 for the Xantrex) and a mid size inverter/charger (like an Outback FX, $1800.) That's $5900. You'll also need things like battery box, wiring, disconnects etc.

"Here's a question... are solar batteries # of cycles and how deeply they are drained dependent. What I'm asking is what I generally DON'T USE THEM."

They will wear out faster if you 'use' them (i.e. cycle them.) They will last longer if you float them forever. An occasional cycle is often considered good to "stir things up" but I haven't seen any hard data on that.

Converting 3,500 watt grid tie to off grid

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