Generac Power Cell (PWRcell) review

The Generac brand is best known for their fossil fuel-powered generators, which range from small portable generators and home backup units, all the way up to huge industrial-sized generators.

Generac has also jumped into the home battery market. This makes sense: batteries paired with solar panels fulfill a need that had previously only been met by natural gas or propane-powered generators. Even though the initial cost is higher, many consumers are choosing the clean energy alterative. And unlike generators, batteries and solar panels are eligible for tax credits.

Generac’s PWRcell product line consists of several components, including a battery cabinet, stackable batteries, an inverter, an automatic transfer switch, and a load management device that lets you turn on and off individual circuits in your home.

The ability to configure the battery cabinet with anywhere between 3 and 6 batteries is a standout feature of the Generac PWRcell and allows more flexible configurations than other batteries (like the Tesla Powerwall).

Generac PWRcell batteries - key features and specifications

There are two main components of the PWRcell battery system: the PWRcell Battery Cabinet, and the PWRcell Battery.

Here are the key specifications and features:

• Up to 18 kWh of storage per cabinet
• Up to 6.7 kW of power output per cabinet
• Modes for backup power, self-supply, time-of-use, zero-import and export limiting
• 96.5% round-trip efficiency
• Lithium nickel manganese cobalt (NMC) chemistry
• Can be DC- or AC-coupled
• Battery price: about $2,000 per battery
• Cabinet price: about $3,000 (batteries not included)
• 10 year warranty
• Outdoor rated
• Total installed cost for a 9 kWh system (including inverter): about $18,000

The battery cabinet can be configured with a minimum of three and a maximum of six batteries. Each battery has 3 kilowatt-hours (kWh) of capacity, so that means the cabinet can be configured with between 9 kWh and 18 kWh of total storage.

The PWRcell is typically installed in a DC-coupled configuration in combination with the PWRcell Inverter. A single inverter can be coupled to two battery cabinets, which means that the PWRcell inverter can support up to 36 kWh of storage. Alternately, the PWRcell system can also be used in an AC-coupled configuration. (More DC- vs AC-coupling later.)

Here are the specs for the PWRcell Battery:

The batteries are not used standalone, but are installed into the PWRcell Battery Cabinet in 3, 4, 5, or 6 battery configurations. Combining batteries this way increases not just the capacity, but the power output and peak starting current.

Battery configurations for the PWRcell Battery Cabinet

Peak starting current is important for appliances with large motors, such as an air conditioner or well pump. While running, motors draw a steady current, but when they first start up they demand an initial big spike of electricity. A battery might not be able to satisfy that startup current, which is why the rated starting current of the PWRcell might be an important specification for you.

Here are the possible configurations of the Battery Cabinet:

Depending on the number of batteries installed, spacers may be used to occupy unused slots in the cabinet.

How is the PWRcell configured?

There are three required components in the PWRcell system:

• a PWRcell Battery Cabinet
• a minimum of three batteries
• a PWRcell inverter

The system is outdoor rated, but an indoor installation is preferred in most climates. Lithium-ion batteries have a reduced lifespan when subjected to temperature extremes. As noted in the specifications above, the optimal temperature range for PWRcell batteries is between 41°F and 104°F. That range is exceeded in many climates in the US, so storing the system indoors will be best for most sites.

Except for the transfer switch, all of the components are connected on the customer’s side of the electrical panel.

PWRcell operating modes

Like most home storage batteries, the PWRcell is a “smart” system that has different operation modes that you can control from an app. This matches the functionality found in competing products such as the Tesla Powerwall 2 and the Enphase IQ Battery.

The PWRcell modes include:

• Time-of-use mode. This mode uses stored electricity to prevent your home from using grid electricity during expensive utility peak hours. This mode will generally be the best for customers who have TOU plans and full net metering, as many California homeowners do.
• Self-supply. In this mode, the PWRcell will try to consume as much solar electricity as possible by using to power your home and charge your battery before exporting any excess to the grid. This mode is useful if you don’t have full net metering from your utility company.
• Priority backup mode. This mode will keep your battery fully charged using either solar or grid electricity. You would use this if you have frequent blackouts or have critical equipment in your home.
• Clean backup mode. This mode is similar to priority backup mode, except that it will use only solar electricity to charge the battery.
• Sell mode. This maximizes the export of your solar electricity to the grid.

Can the PWRcell power your house during a blackout?

In the event of a blackout, your solar array will shut down automatically.

With battery setups from other manufacturers, your home won’t automatically switch over to battery power in a blackout without the help of an additional component. This is known as grid isolation or “islanding” mode because your home operates independent of the electric grid.

For example, the Tesla Powerwall depends on the Backup Gateway to switch into islanding mode. Enphase - which makes the popular IQ battery - offers the IQ System Controller.

The Generac system is different because the PWRcell Inverter - which has the primary responsibility of converting DC power from the solar panels into AC power that a home use - can supply up to 7.6 kilowatts of power in islanding mode without the need for additional components.

A typical 120 volt household outlet can continuously handle about 1.5 kW, so that means the PWRcell inverter can fully power about 5 regular household outlets. That’s good enough for an emergency: it means you can keep your refrigerator, lights, and a few other key appliances running, but it’s not enough to keep most homes fully powered.

If partial power is acceptable for a homeowner, the contractor can install what’s known as either a protected load panel or critical load panel. This is a small electrical subpanel that will power just a few selected circuits in the home. Critical appliances such as a furnace, refrigerator, some lights, and maybe a window air conditioner would be connected to this panel. The rest of the home would be unpowered.

If you want a fully powered home in a blackout and you’ve got enough batteries to supply the power, you will need the PWRcell Automatic Transfer Switch. The ATS switch disconnects your home automatically in the event of a blackout. The inverter will then supply power directly into your main electrical panel. (You wouldn’t have a critical load panel in this setup.) Most likely, you’ll need two inverters to supply enough wattage for all of your home.

However, another solution that Generac offers are Smart Management Modules (SMMs). These modules tie into your electrical panel and allow you to control loads from your smartphone. For example, if you have an air conditioner that you don’t need during a blackout, you can switch it off using an SMM to prevent your battery capacity from being exceeded. More about SMMs later.

PWRcell battery chemistry

The PWRcell is a lithium nickel manganese cobalt (NMC) battery. This is the same type found in electric vehicles and many other popular home storage batteries, including the Tesla Powerwall and the SolarEdge Home Battery.

The other competing chemistry is lithium iron phosphate (LFP), which is used in the Enphase IQ Battery.

NMC batteries have the advantage of high energy density, which means they store more electricity in a smaller package than LFP batteries do. However, they have the disadvantage of lower durability, which means they can withstand fewer charge-discharge cycles than LFP batteries. In addition, NMC batteries have a slightly higher risk of thermal runaway (ie. fire) than LFP batteries, though it should be stated that lithium batteries are generally very safe.

According to research done by the Department of Energy, an NMC battery can be expected to have a cycle life of 1,200. (Cycle life is the number of charge and 80% discharge cycles the battery can provide over its lifespan.) Meanwhile, an LFP battery (like the Enphase) can be expected to have a lifespan of 2,100, which is 75% more.

This is something to consider when comparing the price of NMC to LFP batteries - LFP batteries might cost more, but will generally last longer. However, if you plan to use your battery infrequently for backup and not to avoid daily peak utility rates, the difference in durability probably won’t matter much to you.

DC-coupling and AC-coupling: the PWRcell does both!

There are two ways that a battery can be connected to a solar array. The configuration described above, which is called DC-coupling, has the battery charged using DC power from the solar panels. In the case of the PWRcell, energy from the solar panels flows to the PWRcell inverter, which contains a charge controller that safely delivers the correct amount of DC electricity into the battery. There’s no DC-to-AC conversion, even though the batteries are technically connected to the inverter.

The other method, called AC-coupling, involves a couple conversion steps. First, DC power from the solar panels passes through an inverter and is converted into AC power. That AC power is then converted back into DC power before being fed into the battery. The Tesla Powerwall is an AC-coupled battery, and has an inverter and charge controller onboard.

The advantage of DC-coupling is that it’s more efficient because the DC-to-AC conversion steps are eliminated. As a result, the PWRcell battery has a round-trip efficiency of 96.5%. That’s notably better than the Tesla Powerwall 2, which has an efficiency of only 90%.

AC-coupling does have an advantage of being more compatible. You can charge an AC-coupled battery from any power source, including a generator or a solar inverter from a different manufacturer.

The PWRcell can actually function in an AC-coupled configuration. It requires the latest version of the PWRcell Automatic Transfer Switch, and a PWRcell Inverter, which functions as the charge controller for the battery. The requirement of the ATS and inverter probably means that AC-coupling the PWRcell will be a more expensive proposition than other AC-coupled batteries, but it does give you the option if you have an existing solar array and really like the PWRcell battery setup.

Load management with the PWRcell

If you are configuring a PWRcell battery system to back up a whole house, even the combined output of two inverters (15.2 kW total) might not be enough to fully power everything, especially if you’ve worked to electrify your home.

To prevent that from happening without using the more restrictive option of installing a critical loads panel, you can use Generac’s PWRmanager system to control individual circuits in your home. The PWRmanager consists of a central control unit and small control modules that connect to individual circuits at your electrical panel.

Once everything is connected, the WiFi-enabled PWRmanager lets you control up to twelve 120v circuits or six 240v circuits from a smartphone app:

This is useful because you can monitor the state of the battery and easily reduce the power load if you’re draining your battery too fast. It’s more flexible than a critical load panel because if you have more than 12 circuits in your home, you can easily reconfigure the control units at the panel if you decide that the initial configuration wasn’t ideal.

The PWRmanager is an optional component that many installations won’t use, but it offers great flexibility in how you power your home while off-the-grid.

Bottom line: flexible battery storage with some limitations

The Generac PWRcell is a home battery system with a lot of flexibility. It offers a complete solution that includes configurable battery capacity, power load management, and islanding with the PWRcell inverter. The default DC-coupled configuration gives you high efficiency so that you waste less of your solar electricity.

But possibly the best feature of the Generac is availability. Many popular batteries on the market have limited availability. I’ve heard from some installers this year that a six month wait time for the Tesla Powerwall isn’t uncommon. The Generac, on the other hand, has good availability. That availability, combined with the complete set of features the system offers, are good reasons for you to consider the PWRcell for your project.