AC-Coupled Solar System Sizing

DC-coupled solar (connected directly to Powerwall 3) is strongly preferred over AC-coupled solar for the following reasons:

Less equipment required for DC-coupled solar, resulting in reduced system cost
Increased efficiency for DC-coupled solar
Low energy management during a grid outage

In some scenarios, it is difficult or not feasible to avoid systems with AC-coupled solar. Most commonly, this occurs when Powerwall 3 is installed on a system with existing AC-coupled solar. As shown below, solar can be installed alongside Powerwall 3 solar, or with Powerwall 3 as storage only.

Figure 1. Powerwall 3 with AC and DC Coupled Solar

Figure 2. Powerwall 3 with AC Coupled Solar (Storage Only)

For systems with AC-coupled solar only, a maximum of 7.68 kW AC per Powerwall is allowed in the backup circuit (the smaller of AC inverter rating or DC system size¹).

¹The 7.68 kW PV to Powerwall ratio was put in place to protect the Powerwall system from excessive PV power during a grid outage. 7.68 kW is used because it is a common solar inverter size, allowing more PV systems to be fully backed up without needing to be split apart, and inverters don’t always produce their maximum power. This ratio does not prevent all issues; Powerwall’s maximum charge rate is 5 kW under ideal conditions (notably operating temperature). If there is more than 5 kW kW of excess PV per Powerwall, the system will frequency shift to try to reduce PV power, and may have to shut PV production down completely.

Note

The AC-coupled PV to Powerwall ratio and the maximum DC solar system sizing are independent of each other. See Powerwall 3 DC System Sizing for information about sizing the Powerwall 3 DC system.

Note

Powerwall 3 Expansion units do not impact the PV to Powerwall ratio; the ratio is still 7.68 kW regardless of whether Expansion units are installed.

Note

When using Tesla Solar Inverters, the number of inverters is not impacted by the PV to Powerwall ratio (the maximum number of Tesla Solar Inverters in the backup circuit is 200A, or 5 Solar Inverters).

CAUTION

Exceeding the PV to Powerwall ratio may result in high fault current during a grid outage, which creates a serious risk of damage to Powerwall and/or the customer's home loads. If a Powerwall is damaged by high fault current due to excess PV during an outage, it will be out of warranty.

Options to avoid exceeding the ratio:

Increase number of Powerwalls on site
Utilize Tesla Brand Solar Device for all PV on site to avoid PV to Powerwall ratio
Split PV Inverter Point of Interconnection, In / Out of backup (confirm with local AHJ that this practice is accepted)
Downsize PV to meet ratio
Shed part of the PV system using grid dependent relays/contactors


Powerwall Qty	Maximum continuous output of solar array (kW)	Examples	Explanation
1	7.68	10 kW DC solar on 7.68 kW AC inverter	Allows solar over 7.68 kW DC because the output will be capped to 7.68 kW AC by the (overdriven) inverter. The Powerwall will not see more than 7.68 kW solar.
1	7.68	7.68 kW DC solar on 10 kW AC inverter	While the inverter is capable of 10 kW AC, the 7.68 kW DC solar array will not produce more than 7.68 kW AC solar.
2	15.36	20 kW DC solar on (2) 7.68 kW AC inverters	Allows solar over 15.36 kW DC because the total output will be capped to 15.36 kW AC by the (overdriven) inverters.
2	15.36	15.36 kW DC solar split between (1) 6 kW AC inverter and (1) 10 kW AC inverter	The 16 kW AC combined output of the inverters can exceed 15.36 kW because the 15.36 kW DC solar array will not produce more than 15.36 kW AC solar. Note that the solar does not need to be evenly split in the ratio; instead, consider the total output.

Note

The 7.68 kW PV to Powerwall ratio can be applied to any Powerwall unit in the system, including Powerwall+ or Powerwall 3. For example, a 7.68 kW third party solar inverter can be backed up with a single Powerwall+ or Powerwall 3, as the ratio rule does not apply to the solar inverter portion of the Powerwall+ or Powerwall 3.