Powerwall efficiency 86%....

[BGbreeder]

I'm getting efficiency of only around 80%.

I have 2 Powerwalls. No solar. They discharge during peak and charge at night. Two batteries are more than enough to meet home energy needs during peak.

I'm calculating it by looking at the monthly data on the Tesla iOS app. I can see how much I drew from the grid and how much Powerwalls discharged in a given month. The difference is the loss, so for example, if loss is 20kWh and PW discharge is 100kWh, efficiency = 100 / (100+20) = 83.3%.

Am I calculating the efficiency correctly? If so, why is it so low? (you can ignore January b/c that's when I got them installed, so Powerwalls had to be charged first)

View attachment 799417

I am not sure that the math is quite fair.

efficiency= home use (B)/net grid use (E), I.e. what the fraction of the power used is relative to how much power pulled from the grid. That brings January up to 87%, and February up to 93%.​

All the best,

BG

 

[dareed1]

@notsure11,
I think you are computing the PW efficiency correctly. However, there are two mechanisms contributing to efficiency loss. The obvious one is the losses in converting electric energy to chemical energy and then back to electric energy. The other loss is the energy required to run the control/monitoring electronics, the cooling/heating, etc. This latter contribution is mostly independent of the energy stored and consumed from the PW.

Take March, where it looks like your PWs supplied 6kWh per day. Based on the anecdotal evidence posted here, the two PWs may draw around 0.5 to 1kWh/day even if they stayed at 100% state of charge. That would correspond to 15 to 30kWh of "vampire" draw for the month.

If the PWs were suppling say 20kWh per day, I guess the efficiency would look much better.

 

[notsure11]

@notsure11,
I think you are computing the PW efficiency correctly. However, there are two mechanisms contributing to efficiency loss. The obvious one is the losses in converting electric energy to chemical energy and then back to electric energy. The other loss is the energy required to run the control/monitoring electronics, the cooling/heating, etc. This latter contribution is mostly independent of the energy stored and consumed from the PW.

Take March, where it looks like your PWs supplied 6kWh per day. Based on the anecdotal evidence posted here, the two PWs may draw around 0.5 to 1kWh/day even if they stayed at 100% state of charge. That would correspond to 15 to 30kWh of "vampire" draw for the month.

If the PWs were suppling say 20kWh per day, I guess the efficiency would look much better.

Thank you. I thought I read somewhere vampire drain per Powerwall is around 0.2-0.25kW (=6 to 7.5kWh per month per Powerwall). But if it’s more like 7.5 to 15 kWh per month per Powerwall, my numbers make sense (they’d be like 90% round trip efficiency). I just didn’t think Powerwalls would draw that much for maintenance. Maybe it’s using a lot to keep warm during the winter.

 

[sorka]

My year over year efficiency is down a full 5% each month when comparing 2021 Jan-April vs 2022 Jan-April

 

[sorka]

My year over year efficiency is down a full 5% each month when comparing 2021 Jan-April vs 2022 Jan-April

OK, so it turns out it's NOT less efficient. The reason, in my case, is that for those same months, in 2022, the amount of power into and out of the powerwalls is less than 2021 by quite a bit. The efficiency decreases because the base overhead of just keeping the poweralls on is a higher percentage of the in/out total.

This is because Tesla as refined their algorithm to more balance between self consumption and outright full on 100% charging to 100% every day before self consuming.

The system has gotten very good at predicting how much I'll actually need from my powerwalls for peak time and it doesn't always charge to 100% every day but instead self consumes more before peak time starts. This ends up actually being more efficient because I'm not over utilizing the powerwalls resulting in more less on the round trip.

 

[Redhill_qik]

Thank you. I thought I read somewhere vampire drain per Powerwall is around 0.2-0.25kW (=6 to 7.5kWh per month per Powerwall). But if it’s more like 7.5 to 15 kWh per month per Powerwall, my numbers make sense (they’d be like 90% round trip efficiency). I just didn’t think Powerwalls would draw that much for maintenance. Maybe it’s using a lot to keep warm during the winter.

The 22.1.1 release in December doubled my daily Powerwall loss from 250Wh to 500Wh. The 22.9.1 release in April return the daily loss to 250Wh.