Understanding Round Trip Efficiency & Powerwall Overhead When Installing a Powerwall
Round trip efficiency and Powerwall Overhead are two important considerations when installing a powerwall. The round trip efficiency of a powerwall is the amount of energy lost when discharging and charging the battery. Generally, it is around 90%, meaning that if you discharge 5 kWh from the battery, your house or grid will only get 4.75 kWh (5 kWh x .95%). Additionally, powerwalls consume energy themselves, referred to as overhead. This is due to the batteries naturally losing charge over time plus the cost of communication to the outside world by the powerwall and the cost of running the active cooling/heating system internal to the powerwall. This can be up to 500 watts per day per powerwall.
Data from existing powerwall users suggests that if they are left at 100% backup and not cycled, they use about 0.5 kWh per day. If they are cycled down to 20% and back to 100%, this cost roughly doubles to 1 kWh per day. This is consistent with an annual overhead of 383 kWh/year.
For those on a grid-tied system, it may not be necessary to cycle the powerwalls in order to feel the effects of 1 kWh/day. Instead, they can be used for maintaining solar production during a grid outage during the day, as a backup for any time the grid is down, and to stay off the grid during peak TOU rate periods if out of banked kWh with their electric utility.
Finally, if net metering is available which gives retail price for excess energy generated by solar, then Powerwalls may cost money due to energy being lost when discharging and charging the batteries, as well as going through their inverters to match frequency to the grid. It may be best to keep them at 100% backup in this case.
In conclusion, round trip efficiency and powerwall overhead are two important considerations when installing a powerwall. Generally, the round trip efficiency is around 90%, while the powerwall overhead can be up to 500 watts per day per powerwall. Data from existing users suggests that if they are left at 100% backup and not cycled, they use about 0.5 kWh per day. If they are cycled down to 20% and back to 100%, this cost roughly doubles to 1 kWh per day. For those on a grid-tied system, it may not be necessary to cycle the powerwalls in order to feel the effects of 1 kWh/day. Finally, if net metering is available, it may be best to keep them at 100% backup due to energy being lost when discharging and charging the batteries.
What is the round trip efficiency of Powerwall 2?
The Powerwall 2 has a round trip efficiency of 90%, meaning that when you discharge 5 kWh from the battery, your house or grid will only receive 4.75 kWh (5 kWh x 0.95%). Likewise, if you charge the battery with 5 kWh from your solar system, the battery will receive 4.75 kWh.
What is the powerwall overhead?
The powerwall consumes around 400-500 watts per day, referred to as overhead. This is due to the batteries naturally losing charge over time plus the cost of communication to the outside world by the powerwall and the cost of running its active cooling/heating system.
How much energy does it take to cycle down and up the powerwalls?
If you leave them at 100% backup and don't cycle them, they use about 0.5 kWh per day. If you cycle them down to 20% and back to 100%, the cost roughly doubles to 1 kWh per day.
Is there any benefit in using Powerwalls if I have net metering?
As far as can be told, if you have net metering that gives you retail price for the excess energy generated by your solar, then Powerwalls will cost you money. It may be best to keep them at 100% backup, because it costs energy to discharge and charge the batteries to run the house and absorb solar.
What is the best way to use Powerwalls if I don't have net metering?
If you don't have net metering, then it may be beneficial to use the Powerwalls to store energy during peak hours and discharge it during off-peak hours. This will help you take advantage of time-of-use rates, which can save you money. Additionally, you can use the Powerwalls to store excess solar energy that would otherwise be wasted.
How much does a Powerwall cost?
The cost of a Powerwall 2 is around $6,500 USD for the 13.5 kWh version. This includes installation costs, but not taxes or other fees. It also does not include any additional equipment such as an inverter or solar panels.
Is there any way to reduce the cost of a Powerwall?
Yes, there are several ways to reduce the cost of a Powerwall. You can purchase used batteries, look for deals on new batteries, or buy in bulk if you plan on installing multiple Powerwalls. Additionally, some utilities offer incentives for installing renewable energy systems such as Powerwalls.
What is the warranty on a Powerwall?
The Powerwall 2 comes with a 10-year, 100% capacity and parts warranty. This means that if the battery fails to hold its full capacity or any of its parts fail within 10 years, Tesla will replace it free of charge.