Why we feel the PowerWall is ideal for Off-Grid

David and Laura · May 2015

We assert that the 7 kW-hr daily-cycling PowerWall model is both economical and desirable for off-grid solar.

First a little background: The PowerWall sits between your high-voltage solar array and your inverter (or charger-controller). During the day, part of the solar energy captured is used to charge the PowerWall. Once the sun goes down, the PowerWall continues to supply power to your inverter, as if the sun was still up, until the PowerWall battery is nearly discharged. Since Lithium chemistry batteries can support a far higher depth of discharge, the 7 kW-hr is roughly equivalent to a 12 kW-hr lead-acid battery array.

Here’s why we feel it makes sense for off-gird use:

With a PowerWall added to a traditional off-gird system, the PowerWall takes care of ordinary daily cycling. This means that your lead-acid battery bank will rarely be cycled, and thus will last anywhere from two to three times longer. This is a significant cost savings over time, and more than pays for the PowerWall.
Expanding an existing lead-acid battery bank is difficult, because you can’t normally mix new and old batteries in a single bank, and almost no systems support multiple independent battery banks. By adding a PowerWall, that battery is drained first, before starting to discharge the lead-acid battery. This makes it easy and economical to expand battery capacity.

I feel that many have missed these advantages because they have been thinking about the PowerWall replacing the lead-acid battery bank. We feel that it doesn’t replace it, rather it complements it.

Thoughts? Counterarguments? Does our logic make sense?

SolarPowered · May 2015

Finally a power wall forum discussion that is practical and doesn't talk about, grid tied use. Now we are going some where with this.

westbranch · May 2015

Dave, not that I dont think that a Li based battery is not a good thing, your proposal of using the Power Wall unit as a part of a 2 battery type installation is interesting, though IIRC 'Northerner' proposed the opposite, since the Li battery does not need to be fully recharged s does an FLA.

I looked into the RedFlow battery ( a~48V battery) and they suggested that a good application of their flow battery would be to store more energy in a LA chemistry battery and a flow chemistry battery and use the FB to recharge the lead based battery if it needed to be boosted as the Flow type was most efficient if fully depleted, after charging.

I believe there are better value for $$ spent than the Tesla product. My vision of it is to recharge your Tesla car or as a substitute for 'Peaker " plants...

SolarPowered · May 2015

The only practicality I see from lithium batteries is if the high demand for a high duty cycle discharge "when immediately needed".
Power Wall fails at this because the safety features added to the battery to prevent thermal run away, only allows the 7kWh to discharge a maximum of 3.3kWh and this is limited for roughly 1 minute before the safety mechanisms to the BMS respond and then only allow a continuous cycle of 2kWh. This is designed for initial load start ups such as compressors, condensers, and pumps.

A standard lead acid bank of 4 120Ah batteries will continuously discharge 4kW for roughly a half hour cycle before it plateaus to 2kW, and then becomes diminishing in power after 1 hour.

The BMS safety features on the Tesla Power wall to prevent high discharge still make LiFePO4 better suited than the power wall regardless of the initial purchased price of the investment. Limiting the discharge rate doesn't make the battery any more efficient than lead acid unless it has a potential greater discharge than lead acid.

Both charge and discharge rates have to be taken into consideration to determine overall all efficiency, and a battery that uses a BMS to regulate 50% of 90% of its initial discharge for needed supply and demand, does not make it any more efficient than lead acid, or even NiMh. For the price you can obtain a lithium based solar charge controller for use with NiHm, you are better off building your own NiHm battery bak and still yield a greater discharge than what the Power Wall offers.

mike95490 · May 2015

or you can parallel 2 or more Powerwalls to get the Ah you need.

SolarPowered · May 2015

mike95490 wrote: »

or you can parallel 2 or more Powerwalls to get the Ah you need.

O.K, so then some one spends $7000 to equal the same discharge that LiFePO4 will offer at that price?

This is why LiFePO4 still beats power wall as far as discharge. LiFePO4 BMS is also designed to work with standard charge controllers in 12v, 24v,36v, and 48v inverter systems.

This is also why we don't see a lot of people hacking Toyota Prius 202v 6Ah NiMh batteries, the voltage mismatch is more expensive to work with. You can purchase Prius NiMh for under $1500 refurbished.

Components needed for a minimum power wall system.
-MPPT voltage
-380V DC/ 240V AC inverter
-420v string array

Looking at possibly another $12000 in equipment?

$3500 + $12,000= $15,500
That's not a cheap rig.

Power Wall requires a completely seperated array, which is going to cost more money, since it would need a minimum 380v~420v string solar array.

I don't think people have thoroughly thought through all the engineering on the power wall.

It's unorthodox to standard off grid, so everything for system components cost more than that of the Power Wall.

David and Laura · May 2015

In our case, where we have a SMA SunnyBoy inverter and an array that is already 300 - 400 volts, adding a PowerWall is straightforward and does not require any additional equipment.

But we understand that many off-grid systems are based on lower voltage arrays, which as SolarPowered pointed out, increases the costs.

When a PowerWall is used to either a) reduce cycling on your lead-acid battery bank, or b) add battery capacity to an existing lead-acid battery bank, the lead-acid battery bank provides the surge power and provides for higher power loads. Thus, for those use cases, the limited power rates of the PowerWall is not as much of a drawback.

Dave Angelini · May 2015

I be glad to hear David and Laura's findings after they buy one and live with it for 5 years. It is just too easy with 1,100 AH 2V cells at around $300 each. The 15 minutes a month to check water is not that hard no matter how busy you are. Most Offgrid folks I know are definitely not that busy and they are happy about that!

SolarPowered · May 2015

David & Laura wrote: »

In our case, where we have a SMA SunnyBoy inverter and an array that is already 300 - 400 volts, adding a PowerWall is straightforward and does not require any additional equipment.

But we understand that many off-grid systems are based on lower voltage arrays, which as SolarPowered pointed out, increases the costs.

When a PowerWall is used to either a) reduce cycling on your lead-acid battery bank, or b) add battery capacity to an existing lead-acid battery bank, the lead-acid battery bank provides the surge power and provides for higher power loads. Thus, for those use cases, the limited power rates of the PowerWall is not as much of a drawback.

So now you have to examine a complete off grid solar system efficiency.

Lead Acid systems are roughly 62% efficient on total stored harvest.

Lithium systems are roughly 69% efficient on total stored harvest.

So your system is Hybrid since using both integrated into one system.

69 + 62 = 131
131 / 2 = 65.5% integrated harvest efficiency

So you really have to consider the invested price impact that as a total system it will never perform to a true lithium based system for the price.

The hindered inability to fully discharge 90% of capacity in 1 hour elapsed time is what you have to truly consider. If that marginal 3.5% gain in stored harvest efficiency is worth the additional 65% in cost, when you could just supply another lead acid battery bank to offset that load.

Doesn't make sense from a performance or economical standpoint. The value truly comes from it's maximum discharge, not the added capacity at it valuation.

zoneblue · May 2015

Sounds a bit strong to me. Sounds like its more likely to at least work with your AC coupled setup. But most of us use low voltage controllers for a reason, at the current time theyre cost effective. Thats not to say that low voltage is the right thing, you understand. Until that gap closes between what we want and what we get, imstill seeing more potential in reconfigured tesla modules.

SolarPowered · May 2015

zoneblue wrote: »

Sounds a bit strong to me. Sounds like its more likely to at least work with your AC coupled setup. But most of us use low voltage controllers for a reason, at the current time theyre cost effective. Thats not to say that low voltage is the right thing, you understand. Until that gap closes between what we want and what we get, imstill seeing more potential in reconfigured tesla modules.

Low Voltage will always be the "in thing".
AMPS are the multiplying factor to equal watts and on a conversion efficient level thus far, micro inverters have proven there is more efficiency of converting amps to volts on an efficiency conversion scale when it comes to cycling per duty cycle of watt output.

Higher Voltages run hotter than lower voltages, the higher the voltage ramp up the more voltage drop that is applied to the converter, not to mention the conductors to the system. Any anticipated generated heat depriciates watt output efficiency so there for in conversion there are much more high scale losses rather than for converters converting amps to volts.

Now I believe, TESLA's theory is that a 2% voltage drop is less than, a 2% drop in amps. If a system loses 2% in amps VS the higher voltage that is multiplied, there is less watts lost in their calculated output.This is actually reverse engineering going on here, and here is where everyone is fooled. There is 2% lost on harvest voltage input, and a 2% loss on voltage output. Thats a 4% voltage loss across the board.
This is more of a I fooled the consumer.

The TESLA batteries limitation of efficient operation is 92%. No where near a PSW inverter which maintains a constant conversion to deliver watts at 95%. Its also no where near LiFePO4 which maintains 95% efficiency. No matter what, the BMS restricts optimal 98.5% efficiency on any type of Lithium battery.

So this is why 48V is tried and true for harvest and discharge efficiency.

As far as harvesting is concerned, 12V far surpasses 36v and 48v, however discharging anything over 2kWh becomes an in efficient task for 12V batteries, and the size conductors required to deliver watts.

westbranch · May 2015

SolarPowered wrote: »

TESLA's theory is that a 2% voltage drop is less than, a 2% drop in amps. If a system loses 2% in amps VS the higher voltage that is multiplied, there is less watts lost in their calculated output.

Solar Powered, I am confused, specifically this part

SolarPowered wrote: »

a 2% voltage drop is less than, a 2% drop in amps

If Volts are 120 and 10 Amps, 1200W, 2% of 120 is 2.4v and 2% of 10 is 2. so if the loss is in V then we now have (117.6V x 10)1176 W, if it is amps that are lost we have (120 x 9.8 ) 1176W....??? where is the difference, is it in the transmission over long distances on undersized wire???

SolarPowered · May 2015

Actually I hurt my own brain with that statement, I was probably to wordy. Yes transmission for voltage is a key factor, temperature is the unknown variable in any given circumstance so what could be "assumed" at 2% based from a calculation for voltage isn't the same 2% calculation for amps. There are more conversion losses from voltage, at any given moment, than amps.

I should of stated that there are more variables associated with voltage losses, VS amp losses.

You could hypothetically span a conductor as long as you want without getting amp losses, as long as the conductor is sized proportionally to its Ohm impedance according to ohm law.

Voltage suffers from long distances, and heat, that conductor would have to exceed wire sizes, that amps typically do not.

Using 2% as a hypothetical was incorrect, I started typing ahead of myself, as it is more dependent on variables that cause a drop in efficiency.

Voltage will suffer more variables causing voltage drop and greater conversion losses, VS AMPS, to achieve watts.

No matter the condition on the wire sizing, or wire run, there is also temperature, not only ambient temperature, but battery temperature so there will always be losses to voltage on the input and output of the battery, I call it 2% in, 2% out. AMPS there is a loss because of battery internal resistance, but I don't believe it is anywhere near the level of losses voltage suffers. This is why lower voltage, higher amps in conversion is better, and converting amps to volts is better. Up conversion rather than down conversion.

David and Laura · May 2015

SolarPowered wrote: »

Doesn't make sense from a performance or economical standpoint. The value truly comes from it's maximum discharge, not the added capacity at it valuation.

An excellent analysis of the complete lifecycle economics can be found here:

http://www.lowtechmagazine.com/2015/05/sustainability-off-grid-solar-power.html

David and Laura · May 2015

Dave Angelini wrote: »

I be glad to hear David and Laura's findings after they buy one and live with it for 5 years. It is just too easy with 1,100 AH 2V cells at around $300 each. The 15 minutes a month to check water is not that hard no matter how busy you are. Most Offgrid folks I know are definitely not that busy and they are happy about that!

We agree that any proofs will ultimately be in the performance of the system. Being an early adopter is also fraught with a multitude of risks.

We’ve signed up with Tesla, but aren’t holding our breath. We will be surprised if they are ready to ship in Canada by mid 2016.

We wish we could get lead-acid batteries that inexpensively. For us, lead-acid batteries are far more expensive, and based on our comfort having worked with remote telecommunication installations in the past, we personally chose to go with AGM batteries.

zoneblue wrote: »

Sounds a bit strong to me. Sounds like its more likely to at least work with your AC coupled setup. But most of us use low voltage controllers for a reason, at the current time theyre cost effective. Thats not to say that low voltage is the right thing, you understand. Until that gap closes between what we want and what we get, imstill seeing more potential in reconfigured tesla modules.

We agree that the economics aren’t as good (and may be quite poor if you have to replace perfectly good equipment) if you don’t already have a high-voltage array/inverter.

Dave Angelini · May 2015

http://www.solar-electric.com/batteries-meters-accessories/batteries/suprdecyba/2voltbatteries1/rolls-surrette-s-1450.html

AGM is quite a bit more expensive but a battery made in your country from the store here is $350 and available at distributors in Canada for $300 if you have a trailer, truck, or boat. I just had Canadian clients do this. I can see why you are interested but if you want the supposed ease of maintenance/safety of an AGM and then are willing to cross your fingers on the Lithium BMS,
there seems to be a inconsistency in you thought process. I do wish you good luck.

SolarPowered · May 2015

My biggest problem with TESLA lithiums, not lithiums in general, is that TESLA knows they are using a dangerous chemistry that runs hot, is prone for thermal runaway.

My biggest concern is BMS failure. If the BMS becomes defective huge risk of fire.

Now you can have a company that boasts a 10year warranty on the battery, I think that's great, it's a step in the right direction,

I do believe with the use of the BMS the battery internals will infact last 10 years, but then we should all be questioning how fail safe is the BMS when if it does fail, how will it stop a lithium battery from from thermal runaway in bulk charge, or discharge. Just sounds scary to put something like that in my home and say, oh let's just suspend the battery in water as a preventative measure.

I have one word...... INSURANCE

JoshK · May 2015

SolarPowered wrote: »

My biggest concern is BMS failure. If the BMS becomes defective huge risk of fire.

There are 62 million US home owners trusting their natural gas furnace's control system, and it kills some of us.

And I don't know how homes are built elsewhere, but here it's common for a garage to be built with fireproof materials. Sheet-rock walls & ceilings, metal doors and windows, and concrete foundations. Vehicles bursting into flames is actually pretty common. You say WHAT?! It's usually organic debris pressed against the catalytic converter that is the source.

Mountain Don · May 2015

SolarPowered wrote: »

My biggest concern is BMS failure.

That would be mine too. Even with LFP, though there it's suppesedly only going to damage the batteries, not cause a fire.

David and Laura · May 2015

There seems to be two threads of argument here:

1) That any product like the PowerWall is not economical:
1a) Too expensive compared to alternatives (no effective savings)
1b) Insufficient peak power discharge

2) That the PowerWall is specifically flawed, but other similar products could be successful:
2a) Choice of voltage
2b) Choice of battery chemistry
2c) Unproven in the market

Dave Angelini wrote: »

AGM is quite a bit more expensive but a battery made in your country from the store here is $350 and available at distributors in Canada for $300 if you have a trailer, truck, or boat. I just had Canadian clients do this. I can see why you are interested but if you want the supposed ease of maintenance/safety of an AGM and then are willing to cross your fingers on the Lithium BMS, there seems to be a inconsistency in you thought process. I do wish you good luck.

Ultimately, our interest is in economics, since we see an opportunity to extend out the lifetime of our lead-acid battery bank by eliminating most daily cycling, and gain some extra storage capacity as part of the deal.

At some point, it is less expensive to just replace the batteries, but that can be calculated:

Breakeven Point = ((Battery Bank Cost / Ordinary Lifetime) - (Battery Bank Cost / Extended Lifetime)) * (Extended Lifetime - Ordinary Lifetime)

For example, let’s assume a 1124 AH bank made from S1450s, using the link: (24 * $350 = $6000), and that we can double the lifetime from 10 to 20 years by purchasing two PowerWalls (two because you need to assume the lifespan is 10 years, thus you need to purchase two over 20 years).

Breakeven Point = (($6000/10) - ($6000/20)) * (20 - 10)
Breakeven Point = ($600 - $300) * 10
Breakeven Point = ($3000)

So, for that particular battery bank, you would need to be able to purchase a PowerWall for $1500 to break even.

Note that this doesn’t take into account ignoring shipping costs, installation costs, disposal costs, etc), which would make the Breakeven Point higher.

However, the numbers are quite different for an AGM bank. Thus, given an expensive battery bank, and that we already have a high-voltage array/inverter, the PowerWall more than pays for itself.

SolarPowered wrote: »

My biggest problem with TESLA lithiums, not lithiums in general, is that TESLA knows they are using a dangerous chemistry that runs hot, is prone for thermal runaway. My biggest concern is BMS failure. If the BMS becomes defective huge risk of fire.

Personally, we would never put off-grid electrical equipment inside or beside a house. It should ideally be located in a separate shed or building. In our case, a PowerWall would be installed outdoors, hanging on the back of our array where our solar inverters are located.

inetdog · May 2015

David & Laura wrote: »

Personally, we would never put off-grid electrical equipment inside or beside a house. It should ideally be located in a separate shed or building. In our case, a PowerWall would be installed outdoors, hanging on the back of our array where our solar inverters are located.

I have not seen a discussion yet on what fluid the Tesla battery bank uses for cooling. But putting the pack outside in the open might cause the battery to have a high equivalent self discharge trying to keep the cell temperature high enough. I understand that the pack as used in the vehicle supports both heating and cooling as necessary.

PS: your "&" expressed as "&amp:" does not seem to work well in the context of a Quote, so I had to edit it above. :cool:

SolarPowered · May 2015

inetdog wrote: »

I have not seen a discussion yet on what fluid the Tesla battery bank uses for cooling. But putting the pack outside in the open might cause the battery to have a high equivalent self discharge trying to keep the cell temperature high enough. I understand that the pack as used in the vehicle supports both heating and cooling as necessary.

I'm willing to bet it is really simple, more than likely 50% water, 50% engine coolant "antifreeze" so it won't freeze over, over heat, or become corrosive.

David and Laura · May 2015

Thank you for fixing the quote.

Since we’re sure the question will be asked, the location where we would install this is a) out of the rain and b) in the shade. Our location rarely gets temperatures below zero.

The rated operating temperature range for the PowerWall is: -20C (-4F) to 43C (110F)

High ambient temperatures combined with internally generated heat may cause this range to be exceeded. I can see this being an issue on a hot sunny day.

We have not seen if there are any deratings applied by the system when the edges of this operating temperature are reached.

TrevorDav · January 2016

If someone was building a new house, could they construct their solar system in a way that would make the powerwall more economical? What kind of system would that look like? I'm new to solar and have picked up a lot of the terms but am still learning.

Thanks!

BB. · January 2016

In general, you need to "know your loads"--Wattage, hours per day of each load, AC mains available vs pure off grid, seasonal issues (heating/cooling), etc.

And, conservation is your first place to spend time and money. It is almost always cheaper to conserve power than to generate it.

Other issues involve where the house is on the property, how it is oriented to sun, seasonal vs full time, where it will be built (sunny climate?), heating/cooling/water pumping/etc.

A good place to start is the HomePower magazine. They have lots of interesting articles, and an online subscriber archive. Don't always agree with what they suggest, but it can get you moving in the right direction and avoid some common mistakes.

Once you have answered many of the above questions, we can go into system design.

Just to give you an idea, assuming heating/cooking/hot water is from fuel/other sources, a good aim point is ~3.3 kWH per day (100 kWH per month). That can give you a "near normal" life style (albeit with series conservation/power management). You can go larger, but the costs get scary and it can exceed the DIY capabilities of most people. And remember, after you install it, you have to replace batteries (typically) every 5-7 years, electronics every ~10+ years, and weekly inspection of your battery system (hydrometer, monthly water level checks, cleaning of batteries, etc.). Such a system is probably in the $10,000 to $30,000 range (up front parts and installation costs).

Just to give you an idea--All costs in, utility power costs around $0.10 to $0.30 per kWHr... Off grid battery power is almost 10x that cost ($1-$2+ per kWH is common, a few people here have gotten down to $0.50 per kWH, with lots of effort).

Some folks "love" micro managing their power systems--And others only want to do the minimum (or even contract an outside service company). But--In any case, this is much different than writing the monthly check to the power company.

Please start your own thread to discuss your system/power needs. I really suggest discussing loads/conservation/power needs first. Then size the system (straight forward math), then lastly start picking hardware.

-Bill

oil pan 4 · January 2016

Makes sense.

Until you look at the price tag and the fact that almost no one can get one.

oil pan 4 · January 2016

TrevorDav said:

If someone was building a new house, could they construct their solar system in a way that would make the powerwall more economical? What kind of system would that look like? I'm new to solar and have picked up a lot of the terms but am still learning.

Thanks!

Ummm, no. Its still over priced and under powered.

I guess if you built the house around it you could make room and get 2 of them installed and make it twice as uneconomical.

Why we feel the PowerWall is ideal for Off-Grid

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