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dgpcolorado
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What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 1:41 pm

Thought this was fun:

As most here know, the Tesla "Gigafactory" is intended to produce batteries for stationary applications in addition to those used in the cars. Wired Magazine decided to take a look at What Size Battery Would You Need to Power Your House? For those of us with grid-tied solar, a battery backup is something that many of us have thought about. With a reliable — and affordable — battery system we could go off-grid.

The thought exercise in the article postulates a household usage of 2000 watts; that works out to 48 kWh/day. Seems way high to me since my household usage, exclusive of LEAF, is about 1/10th that. But I'm on the extreme end of conservation when it comes to electricity usage.

Anyway, by going with the highest density Li-ion battery, they come up with a battery volume of 0.52 m³ to generate 2000 W for a week. I'd need considerably less. But I don't agree with the assumption that a battery for a stationary application would need to have high density, since size and weight are relatively unimportant — unlike with an EV — and cost and materials usage would be the main concern. Nevertheless, it does appear that a battery to power a house for a week wouldn't have to be all that big, at least in physical size. But that 48 kWh for a week works out to 336 kWh, which is a pretty expensive battery if it is Li-ion. (Not that you'd really need one to last a whole week IMO.)

But, you ask, will it have TMS? ;)
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2k1Toaster
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 2:44 pm

dgpcolorado wrote:Thought this was fun:

As most here know, the Tesla "Gigafactory" is intended to produce batteries for stationary applications in addition to those used in the cars. Wired Magazine decided to take a look at What Size Battery Would You Need to Power Your House? For those of us with grid-tied solar, a battery backup is something that many of us have thought about. With a reliable — and affordable — battery system we could go off-grid.

The thought exercise in the article postulates a household usage of 2000 watts; that works out to 48 kWh/day. Seems way high to me since my household usage, exclusive of LEAF, is about 1/10th that. But I'm on the extreme end of conservation when it comes to electricity usage.

Anyway, by going with the highest density Li-ion battery, they come up with a battery volume of 0.52 m³ to generate 2000 W for a week. I'd need considerably less. But I don't agree with the assumption that a battery for a stationary application would need to have high density, since size and weight are relatively unimportant — unlike with an EV — and cost and materials usage would be the main concern. Nevertheless, it does appear that a battery to power a house for a week wouldn't have to be all that big, at least in physical size. But that 48 kWh for a week works out to 336 kWh, which is a pretty expensive battery if it is Li-ion. (Not that you'd really need one to last a whole week IMO.)

But, you ask, will it have TMS? ;)


The grid if available, is the best battery you can have. Let the utilities store energy in water dams or giant underground batteries. It is not economical or a good idea otherwise.

However, I use about 80KWh a day (not including EV charging), so about 1.68 times his numbers so my awesome Tesla battery would only be 0.876m^3.

Of course this all assumes 100% efficiency in energy conversions. If the charger is 90% efficient and the inverter is 90% you are already at 1.08m^3.

But of course you also don't want 100% charge on your battery and you don't want 0%, so adopt the 90/10 and now you can only use 80% of the pack. So size up accordingly, now at 1.35m^3.

And are we assuming the battery doesn't degrade? Because designing for use now means that over time it won't be enough. Since we can call 80% capacity dead, scale up so that 80% meets today's needs right? Now at 1.68m^3.

I can buy 1.2KWh cells of LiFePO4 for $130 a piece, so my 154KWh/day or 1078KWh/week pack would be 900 batteries and cost $117K today. I would get about maybe 3K cycles? So just about 8 years. A cost of $14.6K per year for batteries alone.

Alternatively, I pay the permit & inspection fees once to have grid tied solar and no batteries required.
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 3:21 pm

2k1Toaster wrote:The grid if available, is the best battery you can have. Let the utilities store energy in water dams or giant underground batteries. It is not economical or a good idea otherwise.

A counterpoint, if I may.

Your judgment that the "...grid...is the best battery..." could be right or wrong depending on what's important to you and/or your starting assumptions.

You seem to be looking at the economics of storage and deciding that the grid is less expensive. It might be worthwhile to look at exactly what factors "economics" includes and ignores. Thermodynamics is one really important factor that economics doesn't consider. Climate change, poor air quality, acid rain, loss of biodiversity, the down-stream effects of dams, and poisonous fish that kids and pregnant mothers can't eat are others.

Including at least some of the externalities traditional economics ignores is enough to dramatically change the judgement on using the grid.

Not an attack or a judgement, just a quick view of what I've been trying to wrap my mind around as I've been trying to cut the cords.

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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 3:38 pm

AndyH wrote:
2k1Toaster wrote:The grid if available, is the best battery you can have. Let the utilities store energy in water dams or giant underground batteries. It is not economical or a good idea otherwise.

A counterpoint, if I may.

Your judgment that the "...grid...is the best battery..." could be right or wrong depending on what's important to you and/or your starting assumptions.

You seem to be looking at the economics of storage and deciding that the grid is less expensive. It might be worthwhile to look at exactly what factors "economics" includes and ignores. Thermodynamics is one really important factor that economics doesn't consider. Climate change, poor air quality, acid rain, loss of biodiversity, the down-stream effects of dams, and poisonous fish that kids and pregnant mothers can't eat are others.

Including at least some of the externalities traditional economics ignores is enough to dramatically change the judgement on using the grid.

Not an attack or a judgement, just a quick view of what I've been trying to wrap my mind around as I've been trying to cut the cords.


Cost is not the only option. In fact I am generally the first to point out that money isn't everything and gladly pay more for something with less of an impact.

However, individual battery solutions don't seem like the way to go. Everyone storing their own inefficient batteries that degrade and wear out versus the government or utility companies storing power in a large industrial way. For instance it is not feasible for you or I to build a dam and pump water upstream with excess solar production, but a utility can.

My assumptions come from the point of view that the grid is not a bad idea and very stable. I have maybe an hour or less per year where the grid goes down. Last year it was down for a grand total of 2 hours because a dump truck ran into a ground-level neighbourhood distribution box after the guy who parked it didn't apply the parking brake. Whoops! 2 hours out of a year is 0.0228% downtime, or 99.9772% reliability. Why distribute all these inefficiencies to individual areas to cover a problem that doesn't exist.

In the Armageddon scenario, I already have a way of power the house without the grid (solar) and something tells me that will be the least of my worries. And who would want to draw out their existence longer in a world without the internet!!!!! (j/k) But seriously, in Armageddon where the grid goes down indefinitely, I will have better things to worry about, and I probably won't be running 80KWh a day. My 11KW of panels will be more than I could use, until the zombies rip them from my roof.

If you live in sub-Saharan Africa where you might be lucky to get 2 hours a day of grid power if it is on at all that day or where the "stable" parts still experience 4-5 power cuts of greater than 3 hours (PER WEEK!) then obviously personal storage looks a little better. But here in the western world, it is not something we have to worry about and should have the guts, brains, and money to throw money at the problem collectively.
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 3:41 pm

The prospects of significant number of folks going off grid gives the utility companies nightmares. And the day may come when it's economically feasible even for non-eccentrics. When that day approaches don't be surprised to discover there are suddenly laws or punitive fees against self-sufficient electrical systems.
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 4:25 pm

And are people buying these to get power for a week, or to buy power at off-peak rates and then consume the power during peak rates? You need drastically less capacity to power your home for 18 hours (of which you'll only be awake and active for 6) than powering the home 24/7 for a week.

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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 4:34 pm

I have maybe an hour or less per year where the grid goes down.


Now think about people who experiences dozens of outages a year, some of them in Winter, and the grid looks less like the ideal way to store power.

I'm considering two battery UPS systems for our house, as a short-term alternative to running the Honda-clone generator. I figure that since I'm the only one here who really needs constant heat in Winter, I'll make my room the center of the upstairs UPS Zone, and run an electric heater (these are what really run up the consumption, but if the power is Clean, then the heat is as well) as well as a light and the PC, from a 100-150AH AGM battery. There would be a second, similar or smaller-sized battery downstairs, to run the fridge in Summer or a smaller heater in one room in Winter. The sump pump is already on a UPS. Since our affordable choices for heating here are fuel oil or hydro-power (from a 100+ year old dam), we have an electric-intensive house. I'd love to have a ground-source heat pump system, but we can't afford one.
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 5:29 pm

There's a guy on TMC who got a fully intact 85kw pack and he's using it for grid storage.. he's pulled it all apart, etc.. look it up there and his story..

If you want the guts of the goodies... page 6 here...
http://www.teslamotorsclub.com/showthread.php/34531-Plan-Off-grid-solar-with-a-Model-S-battery-pack-at-the-heart/page6

If you want this guy's history, what he's doing (very complicated solar/off-grid setup) then goto the first page here...
http://www.teslamotorsclub.com/showthread.php/34531-Plan-Off-grid-solar-with-a-Model-S-battery-pack-at-the-heart

Page 11,12 and up is also neat
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Re: What Size Battery Would You Need to Power Your House?

Fri Feb 13, 2015 6:40 pm

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Re: What Size Battery Would You Need to Power Your House?

Sat Feb 14, 2015 12:02 am

LeftieBiker wrote:
I have maybe an hour or less per year where the grid goes down.


Now think about people who experiences dozens of outages a year, some of them in Winter, and the grid looks less like the ideal way to store power.

I'm considering two battery UPS systems for our house, as a short-term alternative to running the Honda-clone generator. I figure that since I'm the only one here who really needs constant heat in Winter, I'll make my room the center of the upstairs UPS Zone, and run an electric heater (these are what really run up the consumption, but if the power is Clean, then the heat is as well) as well as a light and the PC, from a 100-150AH AGM battery. There would be a second, similar or smaller-sized battery downstairs, to run the fridge in Summer or a smaller heater in one room in Winter. The sump pump is already on a UPS. Since our affordable choices for heating here are fuel oil or hydro-power (from a 100+ year old dam), we have an electric-intensive house. I'd love to have a ground-source heat pump system, but we can't afford one.


Ok, "dozens", so lets say 24 times where it goes out for 2 hours at a time? That's still 99.5% reliability, batteries are still useless. Use the grid. Your house doesn't instantly use heat. Your food doesn't instantly spoil. You have a generator for the times it is out for multiple days.

I have about 500 pounds of batteries that run my server rack. They are expensive and only last about 2-3 years before I have over 70 batteries to replace. Why? Because on the internet, 0.0228% is too high of a failure rate. If your website drops out for a couple hours, that could be thousands in lost revenue, and much more in bad PR, bad customer experiences, etc. But your average home, it doesn't make a lick of difference.
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