Solar + 2x PWs; what's my ideal setting?

[sk00]

Don't think my system produces enough during the winter to make TBS effectively save me money, so I have my system on Self-Powered mode.

Trying to figure out what my battery reserve level should be. Think I've read somewhere on this forum that the process of charging and discharging the the batteries is inefficient -- ie, 10kwh of charge translates to only 9kwh of discharge (or something like that). So then would it be best to keep the reserve at 100% and have any excess generation go to the grid? Also, is constant charging and discharging harmful to battery life, so it's better to keep reserve at 100% from that regard as well?

Sorry if these have been answered elsewhere, and thanks in advance for your response.

 

[jjrandorin]

There isnt any such thing as what anyones specific reserve level should be. It depends on how much solar your specific house gets, your goals, what your home run rate on electricity is, etc etc. So, no such thing as some blanket recommendation for reserve.

Powerwalls are around 90% efficient, so 10kW in = 9kW out after the back and forth. Whether that matters or not depends on what your goals are.

The batteries are ment to be cycled. The battery warranty is 10 years, unlimited cycles if charged by solar, 70% capacity. Many people report here little capacity loss after a couple of years. A few have had some more capacity loss, but there is no consensus on why that might be.

TL ; DR on what I am saying above:

There isnt anything as a recommended reserve, just like the setup for everyones home it will be different.
Cycling the batteries is the intended use.
Whether you want to let them sit at 100% or use them is up to you.

 

[sk00]

There isnt any such thing as what anyones specific reserve level should be. It depends on how much solar your specific house gets, your goals, what your home run rate on electricity is, etc etc. So, no such thing as some blanket recommendation for reserve.

Powerwalls are around 90% efficient, so 10kW in = 9kW out after the back and forth. Whether that matters or not depends on what your goals are.

The batteries are ment to be cycled. The battery warranty is 10 years, unlimited cycles if charged by solar, 70% capacity. Many people report here little capacity loss after a couple of years. A few have had some more capacity loss, but there is no consensus on why that might be.

TL ; DR on what I am saying above:

There isnt anything as a recommended reserve, just like the setup for everyones home it will be different.
Cycling the batteries is the intended use.
Whether you want to let them sit at 100% or use them is up to you

Thanks.

I guess my goal is to maximize cost savings in Self-Powered mode (constant pricing, so no need to load shift). In that case fair to say that 100pct reserve would be the way to go given the 10pct efficiency loss in charging/discharging the batteries? Unless selling into the grid has similar or worse efficiency loss, though I assume that's not the case.

Your response got me thinking -- in Self-Powered mode (constant pricing), why would anyone have the reserve set below 100pct? Other than bragging rights for the ability to go off-grid, not sure if there's any practical benefit.

 

[jjrandorin]

If you are looking at no need to load shift, and dont have any goals of using less power from the grid in general, and are not charged non bypassable charges for pulling power from the grid, then 100% reserve will net you 10% more net metering credit (roughly) than using your powerwalls.

In that case, you might consider setting your reserve high (like 90%) or, alternatively, picking a random day to run your batteries down and charge them back up (so they do not sit at 100% for super long periods of time). With that being said, 100% in the app isnt 100% of the powerwall capacity in general, since there is some overhead to prevent exactly that (sitting at full battery for a very long time).

 

[ptdusn]

@sk00, As an example, we have a 9.45kW system with 2 Powerwalls. I have our system on Self-powered mode and only have had to set the backup percentage to 100% to fully charge the Powerwalls due to cloudy days. For the last week, with sunny days, the Powerwalls have charged to 100%. We have run off the solar during the day and then the Powerwalls at night, with the charge level ending up at 35-40%.
As an Electronics Technician, I have studied lithium-ion batteries and their properties. The average lifespan is approximately 5000-6000 charge cycles before loss of capacity. So, let's say you were charging and discharging your Powerwalls every day, that would translate to 14-16 years before any loss of capacity.
As jj said, the batteries actually keep 5% of charge to be able to restart your system if they discharge and go into standby during the night.

 

[EVRider-FL]

I have constant pricing here in FL, but I keep my Powerwalls at 20% to minimize grid use, mainly to use cleaner energy. If there’s a chance if an outage, I’ll set the reserve to 100%.

 

[RKCRLR]

@sk00, As an example, we have a 9.45kW system with 2 Powerwalls. I have our system on Self-powered mode and only have had to set the backup percentage to 100% to fully charge the Powerwalls due to cloudy days. For the last week, with sunny days, the Powerwalls have charged to 100%. We have run off the solar during the day and then the Powerwalls at night, with the charge level ending up at 35-40%.
As an Electronics Technician, I have studied lithium-ion batteries and their properties. The average lifespan is approximately 5000-6000 charge cycles before loss of capacity. So, let's say you were charging and discharging your Powerwalls every day, that would translate to 14-16 years before any loss of capacity.
As jj said, the batteries actually keep 5% of charge to be able to restart your system if they discharge and go into standby during the night.

Tesla warranties Powerwalls for 37.8 MWh of aggregate throughput when used in a non solar self consumption/non-backup mode. If you divide that by 13.5 kWh energy capacity per Powerwall you get exactly 2800 cycles so that is likely the minimum design life of a Powerwall.

 

[jjrandorin]

Tesla warranties Powerwalls for 37.8 MWh of aggregate throughput when used in a non solar self consumption/non-backup mode. If you divide that by 13.5 kWh energy capacity per Powerwall you get exactly 2800 cycles so that is likely the minimum design life of a Powerwall.

Minimum design life of a powerwall to maintain 70% of its capacity, since that is what they are warrantying.

 

[RKCRLR]

Minimum design life of a powerwall to maintain 70% of its capacity, since that is what they are warrantying.

So if you want your Powerwalls to last 20 years years you should limit your discharges to an ~38% (of new capacity) daily average.
This assumes the numbers are right and it doesn't fail due to old age first.

 

[sk00]

Thanks all. Super helpful.