# How big a battery to disconnect from Grid>



## michigantesla (Mar 9, 2019)

I am feeling upset with the power company so am wondering how big a battery I would need to be "comfortable" disconnecting.

I have 2 DYI solar systems installed (one where I work and one at home). These work great, are grid tied, and have 5 year payback. But now that I am looking to build a new house the power company is no longer offering net-metering. Instead they will only buy power back at about half the rate we pay for power - which is about $.15kwh here.

Luckily my installed systems are grandfathered at true net metering for 10 years.

If I put in a new system on my new home I will likely go with battery storage since that would save more then selling power back at half price.

So that got me thinking just how much storage would I need to be comfortable actually disconnecting from the grid entirely. I charge my M3 from home and use about 1mwh total per month. In the summer it is easy to produce this much per month with solar. In the winter not so much. For example, in December I only produced 300kwh as it was pretty cloudy and snowy. For me I would like enough battery to cover 2 months of use to feel comfortable. In the event of something catastrophic happening (solar system burns up, nuclear winter) I would have time to get an alternate power source (generator, reconnect to grid).

So for me I think I would need a *2mwh battery*. This would be *horribly expensive* today. But some day...maybe it will be cheap enough that we really don't need the grid. Power companies should think about and plan for that day as it really may not be that far away.


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## garsh (Apr 4, 2016)

michigantesla said:


> So for me I think I would need a *2mwh battery*. This would be *horribly expensive* today.


Instead of figuring out how much battery you need to disconnect from the grid, perhaps consider how much battery you need in order to store your excess solar generation so that you never have to sell energy back to the power company?


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## gary in NY (Dec 2, 2018)

Wow, that would be one hell of battery. About 160 powerwalls, or 20 Model S or Xs (if they had VTG). 

Winter is tough here too. Without net metering, I would definitely have a powerwall or two (or 3 or 4). But as you say, maybe someday the prices will be affordable.


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## msjulie (Feb 6, 2018)

Not including the Model 3 or a couple higher draw items in the house, 2 11 kWh LG batteries can run a big piece of our house for a while. Add in the solar panels (big-ish system) and our all electric house in sunny-ish California had a true-up of -$362 (we came out ahead). Solar runs most lights, etc during the day and there's still good excess back to the grid.

The solar + batteries are sitting behind a 30amp breaker for critical load failover

We're not trying to be off grid so much as handle the downtime of the grid (PG&E afterall) and keep the bill low/predictable.

fwiw


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## JasonF (Oct 26, 2018)

The big flaw with getting a battery that size is you will never, ever be able to fully charge it!

In Florida, home generators are often sized by maximum daily wattage consumption, and the fuel tank on how long you think you might be without power in a hurricane. Some people pick a standard of 3/4 days, some pick 6/7 days.

That's probably what you should do for a battery array - find out what your daily maximum wattage consumption is, and how many sunless days you want it to be able to run it for at a time _without charging _(ex: a 3-day long rainstorm/snowstorm). Then make sure your solar panels supply enough surplus to charge the batteries during the day in short enough time so you don't constantly fight a diminishing charge in the batteries.


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## Bigriver (Jan 26, 2018)

@michigantesla, I too average around 1000 kWh of energy used each month, my December production was a dismal 350 kWh, and I too live with 4 seasons. The question of how big a battery would have to be to go off grid is a pretty easy question for me to answer as I have always viewed net metering with the grid as one big battery to handle the seasonal solar production variation. The excess sent to the grid and the use of that excess is something I track.

The month of March is generally a break even month, where my generation pretty much keeps up with my energy usage. Then from April to October, we produce more solar than we use, and November through February we use that excess. Here is a plot showing my banked kWh through the year, which peaks around 3200 kWh. I also show the design basis which peaks at only 2200 kWh, which is lower because we tend to generate a little more and use a little less than planned. At the end of the year, I have around 1500 kWh extra that gets sold back to the utility.









So if I went off my design basis scenario, I would need a 2.2 MWh battery, if wanting to store all my actual excess, I would need a 3.2 MWh battery, or if I just focused on needing to deal with the November through February period it could be around 1.7 MWh (3.2 - 1.5).

Of course at this time this is just a theoretical discussion as the cost of a battery of this magnitude is crazy for residential applications. What I have wished I could have is a supplemental personal wind turbine. The annual/seasonal wind curve is almost the mirror image of the solar production curve.

@msjulie and @Jason F, I think both of you live in the land of perpetual sun 🌞. I think the OP question is more difficult in the land of 4 seasons because the solar array is always going to massively under-produce in the winter relative to energy needs. There is a whole season that solar cannot come close to meeting the need.


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## msjulie (Feb 6, 2018)

@Bigriver I wish perpetual sun but yeah certainly as compared to Pittsburg - I was born in Phila and spent many years in dreary east coast weather so I get that.

Even with the weather advantage, days get shorter here in winter which does reduce sun capture totals; someday I hope for super efficient panels and reliable utility companies (or neighborhood storage?) so batteries save even more 'free' energy.


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## garsh (Apr 4, 2016)

msjulie said:


> I wish perpetual sun but yeah certainly as compared to Pittsburg


It's Pittsburgh, not Pittsburg (which _is_ perpetually sunny).
</pet peeve>


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## msjulie (Feb 6, 2018)

@garsh Mia Cupla!


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## Ed Woodrick (May 26, 2018)

michigantesla said:


> I am feeling upset with the power company so am wondering how big a battery I would need to be "comfortable" disconnecting.


Disconnecting from the grid entirely doesn't make sense for most houses. But it can be done for houses explicitly design to be disconnected from the grid, but don't expect the standard modern amenities of life if you go that way.

In Michigan, you have to worry about heat. I'll assume that you may go with natural gas or oil for that, but if it is electric, it will be expensive in battery storage, VERY expensive.
If you plan on running any air conditioner in the summer, that will be expensive as well.
Same goes for charging an EV. 
If any of the above applies, then you aren't going off-grid.

If you go off-grid, then you basically have to have an alternative power source to provide power when the sun doesn't shine. And I expect in the winter, that could mean a week or two or maybe even more, that's a lot of expensive battery storage. But if you have a good sized whole home sized generator, then it could be an option. And of course enough fuel for probably a month of running. Don't forget that you have to change oil in these things pretty often if running for weeks.

So size your battery to handle your daily, sunshine load, and maybe an extra day or two. If you have one of the big loads mentioned above, you'll have to decide if they can be counted or if you can run the generator to provide the current.

Everything except for those three loads are viable for solar/battery and cost effective.

** There can be other large loads, water or pool pumps, space heaters, etc that are big power hogs, they should be watched for.


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## JasonF (Oct 26, 2018)

Bigriver said:


> @msjulie and @Jason F, I think both of you live in the land of perpetual sun 🌞. I think the OP question is more difficult in the land of 4 seasons because the solar array is always going to massively under-produce in the winter relative to energy needs. There is a whole season that solar cannot come close to meeting the need.


In mid summer in Florida it's quite possible to have several days of rain in a row. It might still be warm, but there's no sun for that!


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## orekart (Nov 15, 2018)

Honestly it's way more economical to get a second EV and have one charging w/ the excess. 10kW nameplate of solar PV should yield about 30kWh/day. If the 10kW PV array is producing anything you should see at least 1kWh/day even in a freakin' solar eclipse with panels covered in snow. Minimum you should have 2/3rd battery bank relative to the total energy used for any given day; If you want a starting point you could perhaps have 1/3rd only just to get something useful for the occasional EV charging need.

Solar PV is cheap and if not limited by PV then you'll need to size the energy storage to accept charging from the PV solar controllers - figure 0.2C rate: 35kWh battery bank is something like 690Ah so 138A 0.2C rate. Charge controllers are maybe 5kW or 6kW 6s3p PV each 100A charging capability.

Storing energy in an electrochemical battery bank doesn't make the most sense financially. When I need to charge the EV in winter I fire up the wood stove and turn off the electric heat. I'm only rocking a 31.5kWh battery bank (25kWh usable for 80% DoD) and have designed the system to double that battery bank in the future at an additional $30k USD cost.

No grid where I live though... local power company wants $90k USD just to erect power poles through one mile of right-of-way. For that amount I'll have an EV (or two) and a starter battery bank.


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## Bigriver (Jan 26, 2018)

msjulie said:


> Even with the weather advantage, days get shorter here in winter which does reduce sun capture totals;


Yes, indeed. I do know we all have to contend with much shorter days during the winter and that is a huge factor in solar production for all. I think, tho, from the few use vs production curves I've seen for people in warmer climates is that the energy needs better follow the solar production - higher in the summer and lower in the winter. Mine is not like that.



JasonF said:


> In mid summer in Florida it's quite possible to have several days of rain in a row. It might still be warm, but there's no sun for that!


We actually have rain sometimes in the summer too. 😏 But a cloudy day in the summer is as good as a sunny day in the winter. And not once have I had snow covering my panels in the summer. Look at how dismal December was: generation was around 5 kWh/day (or below) about 2/3 of the time. There is simply no way for solar to provide for my needs during any portion of the month of December. It has to be some alternate source.


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## msjulie (Feb 6, 2018)

It is interesting to see the production curves by time of year - ours went live end of 2019


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## GeoJohn23 (Oct 16, 2018)

There are residential wind turbines available. I don’t have any experience with how well any of them work, but that might help as the OP mentioned it was windy when not sunny there.

Additionally, while I know Elon has a beef with Hydrogen Power Cells (and given where that tech is today vs batteries, it seems wise to have gone with batteries so far); but, I think Power Cells may someday make much more sense for something like house storage if not also for vehicles. This is a bit like physical hard drive data storage media vs solid state.... many times it’s been claimed that we we are the end of storage density for physical media only to have another leap made in that tech while challenges with solid state continued.... this could end up being the same with chemical batteries vs hydrogen since the batteries do seem to keep getting faster, cheaper, and less prone to degradation over time...


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## miimura (Jan 3, 2018)

garsh said:


> Instead of figuring out how much battery you need to disconnect from the grid, perhaps consider how much battery you need in order to store your excess solar generation so that you never have to sell energy back to the power company?


This is the right answer unless you're facing a situation where the utility wants to charge you 10's of thousands of dollars for a new grid connection for a new house. Truly off-grid is a completely different situation.

The OP said his power is $0.15/kWh and the utility will only buy back at half that, unless I misunderstood. Compared to California, that is still pretty cheap power. So, the idea is to install enough solar to produce approximately your annual kWh consumption. Then, battery sizing would be approximately 2 Summer days of generation or 5 Winter days of net consumption, whichever is higher. However, the basic problem is that solar doesn't meet your daily needs during all 4 seasons.


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## Needsdecaf (Dec 27, 2018)

Alex ***** of Alex on Autos happens to be one of my favorite car reviewers on YouTube. He's also a serious / serial DIY guy. Last year he decided to build a panel / battery array to combat the frequent outages in No Cal. He's got some good info here.

Even with his tiny (about 800 SF) house in a temperate climate with only two people, his panel and batteries aren't quite big enough for 100% off the grid. And that's with no EV.


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