Home energy storage is not one-size-fits -all, and neither are Fortress Power’s Envy inverters. If you are comparing the Envy True 12kW and Envy Duo 21, there is one key distinction to understand.

The Envy Duo 21 is not a replacement for the Envy True 12kW Both systems remain available, each designed for different system requirements.

Instead, the Duo 21 represents an evolution of the Envy platform. It was designed to support larger solar arrays, higher PV output, and greater system flexibility, while maintaining the same core design philosophy that defines Fortress Power systems.

Key Differences Between Envy 12kW and Envy Duo 21

Both the Envy True 12kW and Duo 21 are hybrid inverters built for high-performance residential energy storage. The primary difference comes down to solar capacity and output potential.

Envy True 12kW: A Proven, Flexible Foundation

The Envy 12kW supports:

• Up to 21 kW of DC-coupled solar
• 18 kW of usable PV output
• A flexible MPPT architecture that accommodates a wide range of residential solar designs

For many homes, this makes the Envy True 12kW a flexible foundation that balances performance today with room to grow.

Envy Duo 21: Built for Larger Solar Arrays

The Envy Duo 21 expands on this foundation with a reconfigured MPPT layout that allows it to support approximately 4 kW more PV capacity than the 12kW. This enables:

• Up to 25 kW of DC-coupled solar
• The ability to convert up to 18 kW of PV power into usable AC output, provided sufficient solar input is available
• Greater flexibility for homes with larger roofs, higher energy consumption, or aggressive electrification plans

This makes the Duo 21 a strong fit for homes planning larger solar installations or anticipating higher future energy demand.

Why PV Capacity Matters More Than MPPT Count in Residential Systems

MPPT count alone does not determine real-world system performance. What matters is how much solar power the system can actually convert into usable energy.

Many residential battery systems advertise higher MPPT counts but limit usable PV throughput. Fortress Power systems are engineered to prioritize usable output, not just headline specifications. Both Envy 12kW and Envy Duo 21 deliver:

• 18 kW of usable PV output
• Balanced AC and DC handling
• System designs that reduce clipping as solar arrays scale

This allows homeowners to design larger solar systems with confidence, without being constrained by artificial throughput limits.

Generator Integration: A Core Advantage of the Envy Platform

One of the most valuable features of Envy inverters is native generator integration without the need for external controllers. With Envy systems:

• Generators connect directly to the inverter
• The inverter manages generator start and stop logic
• Generator power can simultaneously charge batteries and support backed-up loads

That same port can also be configured as:

• An AC-coupled solar input
• A programmable output dedicated to a specific load or circuit

This flexibility is especially useful for loads such as EV chargers. Power can be supplied during an outage only when sufficient battery reserve or solar production is available.

Capacity Is Only Part of the Battery Storage Story

Battery capacity alone does not define system value. This is where battery design and warranty structure become just as important as raw capacity.

eBoost Batteries: Designed for Long-Term Use

Each eBoost battery delivers:

• 15.4 kWh nominal capacity
• 12.8 kWh usable when following the recommended 20 percent minimum state of charge
• A 10 year or 8,000 cycle warranty, whichever comes first

When evaluated over the full warranty period, this cycle-based warranty supports higher total energy throughput than many capacity-limited alternatives. Additional advantages include:

• Integrated battery heating for improved cold climate performance
• Support for up to 16 batteries
• Compatibility with 1 to 10 inverters, enabling flexible energy to power ratios

This approach gives homeowners the freedom to expand over time without redesigning the entire system.

Maximum Modularity with eForce Batteries

For projects that demand even greater flexibility, Fortress Power offers eForce batteries, a fully modular and stackable storage solution.

Each eForce stack supports:

• One, two, or three 9.6 kWh battery modules
• Total stack capacities of 9.6 kWh, 19.2 kWh, or 28.8 kWh
• Automatic internal module interconnection with no additional wiring required

Up to 32 modules can operate in parallel, even when stacks contain different module counts.

Each stack includes an eWay wire management cabinet, which features:

• Integrated 750 amp busbars
• Direct paralleling without third party combiners, provided current limits are respected

Like eBoost batteries, eForce systems are:

• Heated for cold environments
• Wall mountable or floor mountable
• Capable of inverter docking when configured with up to two modules per stack

This modular architecture makes eForce an ideal option for projects where flexibility and phased expansion are priorities.

Design the Right Fortress Power System for Your Home

Every home has different energy needs, and the right configuration depends on how you use power today and how you plan to use it in the future. Ready to design a system that fits your home and grows with your energy needs? Explore your options with Fortress Power today.