Net Negative Grid Interactive System

[birddogtrinkets]

Hello all,

I've done a TON of research and I need some help.

I'm buying a new home. It's a small 1,000 sq ft home in the country side. The panels are going to face true south and the latitude is about 43 degrees so I plan on having a ground mount system angled at about the same degree.

It currently has a 200 amp grid tied main service with a 30 amp sub panel designed for a generator. I don't want to have to rely on a generator. I want to be completely independent of the grid if need be.

I would like to set up a net-negative solar system that will service all the home needs and sell power back to the grid through net metering. I'm planning on having all major appliances be electric - washer, dryer, induction range, microwave, dishwasher, refrigerator, chest freezer, and heat pump water heater. The furnace is currently LP and I'm planning on getting a Central Boiler installed so I can be completely fossil fuel free. I'm going to add an electric vehicle, as soon as my pre-order for the F150 Lightning or Silverado go through.

I'm estimating my currently the usage around 600 kwh per month. I would like to sell the rest of the surplus back to the grid. I priced out the panels on Signature Solar and a 16 kw solar array would run around $8,000-9,000. I would like to have a grid tied inverter/charger that's capable of powering my needs and a battery bank to rely on during dark days and the nighttime, with the array selling most of the excess back to the grid during the day after charging the batteries. I've contacted Solar Wholesale and they don't do battery tied systems. I have my feelers out to Unbound Solar and am waiting to hear back.

What are my options here? It seems like I can get the wiring, panels, batteries, and mounting systems easily but getting a UL certified inverter/charger is going to be the hardest part.

Thank you for your help!

 

[BentleyJ]

Actually there are several very good choices for UL 1741 SA inverters available that will do exactly what you describe. Off the top of my head.
Outback
Schneider
Sol-Ark
SMA??

 

[birddogtrinkets]

Thank you for replying! So the Sol-Ark only does 4500 VA per leg, so it would not work for my situation correct? Leaving me with Outback or Schneider, maybe Victron? Also, is the SMA integrated inverter charge or do I need a seperate module?

 

[Mr-Sandman]

What you are proposing is a fairly large setup and investment.

I don't see you mention Air Conditioning. I do see a mention of a heat pump, but for hot water.... Air Condition an Heating is the number 1 load in any house. Just starting a large heat pump or AC unit can be a challenge for most inverters, much less having a big enough battery bank to hold that load for any length of time. 600kwh a month is pretty low, so maybe you will be OK if that's a realistic estimate. If you haven't you should verify that you can get net metering and make sure selling is an option. Not all utilities offer it.

As far as equipment at 16kw, you are way past a single inverter setup. You will need multiple inverters. Schneider XW PRO 6848 is a hybrid battery inverter that can also sell to the grid. It's UL listed.. Solar Ark makes a couple models that do that and are popular and mega expensive. Outback has a solution for that as well. Whichever one you choose, you will need more than one for 16kw of solar. Probably 3 inverters actually. So there's $10k to $20k in inverters depending on which brand you go with.

Battery's is your next huge expense. If 600kwh per month is right, that's 20kwh a day on average. Since your grid tie, if it's only for emergency use, you could get 20kwh of battery's and make it 24 hours or longer depending on sun. Just depends on how long you think the grid will be down, for how many batteries you buy. I can tell you you will need 600ah mininum for 3 inverters though, just for the amps they pull. 600ah of Lifepo4 batteries will be another $10k+.

 

[Mr-Sandman]

NOW what I did and would suggest for you, since you do have a grid connection and assuming you can get net metering, is a split system. Hook 12kw of your panels into (2) Sunny Boy 6.0 grid tie inverters. Install 4kw of your panels into a Schneider MPPT 100a 600v charge controller, hooked to a single Schneider XW Pro 6848, with (4) 100ah 48v batteries of your choice. That will give you (1) day of battery backup or longer if the sun shines and should easily make you Net Zero or Net Negative. All (3) inverters will be selling to the grid once the batteries are full.

 

[birddogtrinkets]

What you are proposing is a fairly large setup and investment.

I don't see you mention Air Conditioning. I do see a mention of a heat pump, but for hot water.... Air Condition an Heating is the number 1 load in any house. Just starting a large heat pump or AC unit can be a challenge for most inverters, much less having a big enough battery bank to hold that load for any length of time. 600kwh a month is pretty low, so maybe you will be OK if that's a realistic estimate. If you haven't you should verify that you can get net metering and make sure selling is an option. Not all utilities offer it.

As far as equipment at 16kw, you are way past a single inverter setup. You will need multiple inverters. Schneider XW PRO 6848 is a hybrid battery inverter that can also sell to the grid. It's UL listed.. Solar Ark makes a couple models that do that and are popular and mega expensive. Outback has a solution for that as well. Whichever one you choose, you will need more than one for 16kw of solar. Probably 3 inverters actually. So there's $10k to $20k in inverters depending on which brand you go with.

Battery's is your next huge expense. If 600kwh per month is right, that's 20kwh a day on average. Since your grid tie, if it's only for emergency use, you could get 20kwh of battery's and make it 24 hours or longer depending on sun. Just depends on how long you think the grid will be down, for how many batteries you buy. I can tell you you will need 600ah mininum for 3 inverters though, just for the amps they pull. 600ah of Lifepo4 batteries will be another $10k+.

I already checked. It's a small local utility. I can sell back at .01269 per kwh. Central air is a great point that I didn't mention in my post. I will probably use it little, if at all, since 60% of my living space is going to be in a full concrete basement. I'm willing to make the investment, as I'm looking to go fossil fuel free and I know it will pay off - even if it's 10+ years. $10-20k seems high but I'm definitely not an expert at this point. I'm willing to delay complete implementation until better equipment comes out if necessary. Sol-Ark seems OUT because the AC legs can't handle enough VA to power electric appliances.

 

[birddogtrinkets]

NOW what I did and would suggest for you, since you do have a grid connection and assuming you can get net metering, is a split system. Hook 12kw of your panels into (2) Sunny Boy 6.0 grid tie inverters. Install 4kw of your panels into a Schneider MPPT 100a 600v charge controller, hooked to a single Schneider XW Pro 6848, with (4) 100ah 48v batteries of your choice. That will give you (1) day of battery backup or longer if the sun shines and should easily make you Net Zero or Net Negative. All (3) inverters will be selling to the grid once the batteries are full.

So it's like a split system - am I going to be able to pull enough AC amperage through the system to keep the whole house running without any downtime or grid pull?

 

[birddogtrinkets]

What you are proposing is a fairly large setup and investment.

I don't see you mention Air Conditioning. I do see a mention of a heat pump, but for hot water.... Air Condition an Heating is the number 1 load in any house. Just starting a large heat pump or AC unit can be a challenge for most inverters, much less having a big enough battery bank to hold that load for any length of time. 600kwh a month is pretty low, so maybe you will be OK if that's a realistic estimate. If you haven't you should verify that you can get net metering and make sure selling is an option. Not all utilities offer it.

As far as equipment at 16kw, you are way past a single inverter setup. You will need multiple inverters. Schneider XW PRO 6848 is a hybrid battery inverter that can also sell to the grid. It's UL listed.. Solar Ark makes a couple models that do that and are popular and mega expensive. Outback has a solution for that as well. Whichever one you choose, you will need more than one for 16kw of solar. Probably 3 inverters actually. So there's $10k to $20k in inverters depending on which brand you go with.

Battery's is your next huge expense. If 600kwh per month is right, that's 20kwh a day on average. Since your grid tie, if it's only for emergency use, you could get 20kwh of battery's and make it 24 hours or longer depending on sun. Just depends on how long you think the grid will be down, for how many batteries you buy. I can tell you you will need 600ah mininum for 3 inverters though, just for the amps they pull. 600ah of Lifepo4 batteries will be another $10k+.

Also - I looked at MPP Solar 6048 and 6548 - the idle load is like 135w per panel. Is Scheinder the same way because that is a huge draw for no load...

 

[Mr-Sandman]

So it's like a split system - am I going to be able to pull enough AC amperage through the system to keep the whole house running without any downtime or grid pull?

Sure, if you are grid tied and the gird is up, you are only limited to your main breaker size (200 amp service). In this configuration you batteries aren't used unless the grid is down. Then you are limited to the output of your battery inverter. The Hybrid battery inverters will have a critical loads panel usually "not the main panel" that it powers during a grid failure. You don't want large loads, like Air Conditioning, and electric hot water on the battery inverter. Your Grid Tied inverters will not help during an outage.

For a Grid Tie system to make sense and be fair to you, you will want a 1:1 Net Metering agreement, where you are credited equally for power pushed, as power drawn. Not talking about the end of billing cycle avoided cost price, but rather the day to day way of calculating the "NET usage". Push a 1kw, you should be able to pull that 1kw back. If that's not the case with your utility Net Metering agreement, I would second think doing the Grid tie inverter setup.

 

[birddogtrinkets]

The other question becomes where the crossover point is. Maybe it makes more sense to stick to a 2 inverter system and max it out?

 

[Mr-Sandman]

Also - I looked at MPP Solar 6048 and 6548 - the idle load is like 135w per panel. Is Scheinder the same way because that is a huge draw for no load...

Ya they are BIG inverters, so they have some idle draw. Not a big deal with the size of battery bank and solar you will have on those.

 

[birddogtrinkets]

Sure, if you are grid tied and the gird is up, you are only limited to your main breaker size (200 amp service). In this configuration you batteries aren't used unless the grid is down. Then you are limited to the output of your battery inverter. The Hybrid battery inverters will have a critical loads panel usually "not the main panel" that it powers during a grid failure. You don't want large loads, like Air Conditioning, and electric hot water on the battery inverter. Your Grid Tied inverters will not help during an outage.

For a Grid Tie system to make sense and be fair to you, you will want a 1:1 Net Metering agreement, where you are credited equally for power pushed, as power drawn. Not talking about the end of billing cycle avoided cost price, but rather the day to day way of calculating the "NET usage". Push a 1kw, you should be able to pull that 1kw back. If that's not the case with your utility Net Metering agreement, I would second think doing the Grid tie inverter setup.

So is it not possible or prohibitively expensive to have a set up that relies entirely on the battery system at that point? Like, I don't need the grid at all with having the inverters power everything? It is 1:1, like I said, it's a small utility so I'm not sure they have implemented a lot of rules "protecting" themselves from individual solar arrays.

 

[Mr-Sandman]

The other question becomes where the crossover point is. Maybe it makes more sense to stick to a 2 inverter system and max it out?

For a system to work it's best, there's a LOT of engineering and things to consider on a system the size you are talking about. Could be money well spent getting professional engineering help, as the system you are talking about is touching $30k.

So is it not possible or prohibitively expensive to have a set up that relies entirely on the battery system at that point? Like, I don't need the grid at all with having the inverters power everything?

If Net metering isn't a good option, then you can simply invest in a battery based Hybrid system. You an still do load shaving and use your batteries to utilize your solar generation, using the Grid as a backup will allow you to run a smaller system, for low sun days, etc.. Of course this cost more to do, it's a lot more expensive to store your own power "batteries" than push it to the grid and then pull it back later "1:1 Net Metering".

There's a LOT to consider with what you are proposing.

 

[Hedges]

I already checked. It's a small local utility. I can sell back at .01269 per kwh.

??
Can you confirm that price isn't a typo?

Even if you meant $0.1269/kWh, batteries aren't cost effective. Batteries are useful for backup during a power outage, but cost more than utility rates.
Put in enough PV to cover some loads, or all loads. Maybe put in more PV than you need, to reduce how much power you draw from grid (especially if you don't get 1:1 credit).

Panels aimed SE and SW will produce a bit less total kWh/day, but will more closely match consumption. Reduced peak output more likely to fit within export limits (utility rules, or "120%" rule on PV breakers in main panel.)

 

[TheDan]

I already checked. It's a small local utility. I can sell back at .01269 per kwh. Central air is a great point that I didn't mention in my post. I will probably use it little, if at all, since 60% of my living space is going to be in a full concrete basement. I'm willing to make the investment, as I'm looking to go fossil fuel free and I know it will pay off - even if it's 10+ years. $10-20k seems high but I'm definitely not an expert at this point. I'm willing to delay complete implementation until better equipment comes out if necessary. Sol-Ark seems OUT because the AC legs can't handle enough VA to power electric appliances.

I would suggest putting all of this in a spreadsheet and calculate your ROI so you know exactly how much you will save AND, I think more importantly, how long it will take to get the money back. With your low consumption it may take over ten or 15 years to just recoup the initial cost and add it battery maintenance/replacement and it could become a poor investment.

If you are not concerned with saving money and are just want to get grid independent then that is a different story but it's still good to look at the numbers.

 

[nilus]

I designed a system for similar use case, though settled on only backing up critical loads. The main difficulty in the setup you describe is battery power capability. Most batteries and inverters top out around 5kW in backup mode. That's only ~20 amps @ 240V.

Another caveat - if your jurisdiction follows 2017 NEC / IFC / IBC or later, there is a 20kWh limit on batteries, after which a number of new rules kick in.

For my system, I designed for 2 batteries for 10kW power and just below 20kWh capacity. Components are 2x SolarEdge SE6000H and 2x LG 10h prime batteries with 13kW solar input. For 16kW array 2x SE7600H would fit well.