This is a very common question among Powerwall owners. Unfortunately, there isn't a one size fits all answer to this question since there are many variables that will determine how long a battery backup system will last during an outage, namely:
- How much power is being used by the home
- How much solar, if any, is being produced
- How many batteries are installed (total battery capacity)
While we can't give a definitive answer, we can offer a method to estimate your off-grid run time so you can tailor your settings to meet your needs. Each Powerwall has a usable capacity of 13.5 kWh. What does that mean? If you have one Powerwall installed (without solar) and you're home is using 1 kW, you would be able to run your home for 13.5 hours.
We can use this as a baseline to determine how long your system might last during an outage (if your batteries were full when the outage started).
- Select the row corresponding to the number of Powerwalls you have installed
- Select the column that is closest to your home's average power consumption
- The cell where these two meet is an estimate of how long your system will provide backup power (without solar)
- You have 2 Powerwalls
- You typically use .5 kW
- Your off-grid runtime is estimated at 54 hours (with no solar)
- You have 3 Powerwalls
- You typically use 1.5 kW
- Your off-grid runtime is estimated at 27 hours (with no solar)
|Home Usage (in kW)|
|# Powerwalls||.25 kW||.5 kW||1 kW||1.5 kW||2 kW||4 kW||5 kW|
|1 (13.5 kWh)||54 hrs||27 hrs||14 hrs||9 hrs||7 hrs||3 hrs||3 hrs|
|2 (27.0 kWh)||108 hrs||54 hrs||27 hrs||18 hrs||14 hrs||7 hrs||5 hrs|
|3 (40.5 kWh)||162 hrs||81 hrs||41 hrs||27 hrs||20 hrs||10 hrs||8 hrs|
|4 (54.0 kWh)||216 hrs||108 hrs||54 hrs||36 hrs||27 hrs||14 hrs||11 hrs|
|5 (67.5 kWh)||270 hrs||135 hrs||68 hrs||45 hrs||34 hrs||17 hrs||14 hrs|
|6 (81.0 kWh)||324 hrs||162 hrs||81 hrs||54 hrs||41 hrs||20 hrs||16 hrs|
Keep in mind that this is an estimate and is designed to give you a rough idea of how long your system can run off grid without solar.
If you have solar, your solar can power your home and recharge your batteries during the day which can greatly increase your runtime (to the point that you could run off-grid indefinitely!). With that in mind, if you have solar, you might consider how much power you use overnight to determine a goal for off-grid runtime.
You can also use a similar methodology to determine your reserve for outages. For example, if you want to make sure that your system can run off-grid indefinitely, you should probably set your reserve so that you can run for around 12 hours off-grid. This will provide enough energy to power your home through the night from sun down to sun up, when solar can start replenishing the battery system and help power your home.
- If you have 2 powerwalls (27 kWh) and your home typically uses 1 kW, that means you would need 12 kWh to power your home for 12 hours (1 kW x 12 hrs = 12 kWh), which is 40% of your total battery capacity (12 kWh/27 kWh = .40 = 40%)
- If you have 2 powerwalls (27 kWh) and your home typically uses 1.5 kW, that means you would need 18 kWh to power your home for 12 hours (1.5 kW x 12 hrs = 18 kWh), which is 67% of your total battery capacity (12 kWh/27 kWh = .67 = 67%)
- The formula to calculate a reserve is:
(runtime goal x average consumption) ÷ (# of Powerwalls x 13.5)
- Runtime goal is in hours
- Average consumption is in kW
- runtime goal = 10 hours
- average consumption = 1.4 kW
- # Powerwalls = 3
(10 hours x 1.4 kW) ÷ (3 x 13.5 kWh)
= 14 kWh ÷ 40.5 kWh
Keep in mind that your Powerwall system will power down when it reaches 10% (if your are running off-grid) to keep some energy in reserve to restart the solar in the morning, so you should add 10% to your calculation to ensure you have sufficient energy in reserve for an outage. You can read more about that here.