Solar-plus-storage systems – also known as hybrid projects – have taken over the global market faster than almost anyone predicted. This trend is particularly pronounced in leading markets like the U.S. where over half of the solar interconnection queue capacity is now paired with storage, with CAISO reaching 98% hybrid penetration.1 The U.S. deployed 11.9 GW of battery storage in 2024—over 60% increase from 2023—with 18.2 GW projected for 2025.sup>2 Globally, deployment reached over 200 GWh in 2024, a 53% increase year-over-year.3
The business case is compelling. Battery costs dropped 40% in 2024, driven by manufacturing overcapacity, economies of scale, and increased adoption of lower-cost lithium iron phosphate (LFP) chemistries. Supportive policies like the U.S. IRA’s 30% standalone storage ITC and similar incentives in Europe and Asia further accelerate adoption. Hybrid systems can stack revenue from energy arbitrage, capacity markets, and ancillary services.4
But there’s a gap between pro forma projections and operational reality—one that often doesn’t surface until commissioning begins or, worse, after commercial operation starts. Traditional solar operations and maintenance (O&M) weren’t designed for hybrid complexity. The “set-it-and-forget-it” model that works for photovoltaic (PV) doesn’t translate to battery storage. And the consequences of treating them the same can significantly damage project economics.
In this post, we’ll walk through three critical operational challenges that hybrid projects face—and the strategic considerations that translate directly to smoother commissioning, stronger performance, and better project economics.
What Makes Hybrid Operations Different
Standalone solar is operationally mature. The industry has decades of experience managing predictable generation patterns, scheduled maintenance protocols, and gradual performance degradation. Remote monitoring systems, established best practices, and proven O&M models deliver reliable outcomes.
Battery storage introduces fundamentally different operational demands.
Solar runs on natural cycles, where batteries operate continuously, responding to real-time market signals, grid conditions, and optimization algorithms. Solar requires scheduled maintenance, where batteries demand constant monitoring, state-of-charge management, thermal control, and safety oversight. Solar performance degrades predictably over decades, where battery performance depends on how well you manage thousands of charge-discharge cycles, with significant degradation costs when poorly managed.
The operational paradigm shifts from passive generation monitoring to active energy asset management. And the consequences of getting this wrong aren’t marginal—they’re existential for project economics.
Three Critical Operational Challenges
These fundamental differences manifest in three specific operational challenges that directly impact project economics, and require solutions during development, not after commissioning begins.
1. Performance Optimization: Every Cycle is a Trade-Off
The Challenge
Every charge-discharge cycle generates revenue but also accelerates battery degradation. Deep discharge cycles maximize energy arbitrage value during price spreads but reduce battery life. Shallow cycling preserves the asset but leaves money on the table. The optimal strategy varies by battery chemistry, current state of charge, projected price signals, grid service commitments, and thermal conditions—and it changes hour by hour
The Solution
Addressing this challenge requires two layers working together. The control layer—your energy management system (EMS)—executes charging and discharging commands, enforces battery constraints, manages state-of-charge, and tracks degradation metrics. The optimization layer—whether third-party software, in-house algorithms, or managed services—analyzes market signals, forecasts prices, models degradation impacts, and determines the optimal dispatch strategy.
The key is seamless integration between these layers. Your EMS must be able to receive and execute sophisticated optimization commands while providing the granular operational data that makes intelligent optimization possible.
But here’s the catch: your optimization strategy is only as good as your control architecture allows. If your EMS and optimization layer can’t communicate seamlessly, or if your PPC adds latency to dispatch signals, you’re leaving revenue on the table before the algorithm even runs. We’ll explore this coordination challenge in depth in our next post.
Key Consideration
Select an EMS during development that provides robust control architecture and seamless integration with optimization systems. Your control architecture determines what revenue optimization strategies are possible, whether you’re using third-party software, in-house trading, or managed services providers.
2. Maintenance Coordination: Two Assets, Conflicting Schedules
The Challenge
Solar and battery equipment have completely different failure modes and maintenance requirements. Solar exhibits predictable degradation with weather-dependent cleaning and module replacement on known schedules. Battery systems require thermal management system maintenance, module replacement based on degradation curves rather than time, battery management system (BMS) diagnostics, and fire suppression system inspections.
Coordinating these different schedules while minimizing downtime becomes exponentially more complex. You can’t manage this with rotating contractors and annual site visits.
The Solution
Leading operators implement integrated maintenance programs that coordinate PV and battery schedules, use condition-based maintenance triggered from real-time monitoring data, maintain dedicated on-site personnel with cross-trained expertise in both solar and storage systems, and establish vendor coordination protocols to prevent finger-pointing during troubleshooting.
This integrated approach requires control systems that were purpose-built for hybrid assets—not separate PV and BESS controllers forced to coordinate through middleware. When troubleshooting spans multiple vendors with fragmented accountability, even the best maintenance program struggles.
Key Consideration
Determine your maintenance strategy during development, not after commercial operation date (COD). Will you build internal capability, partner with specialized O&M providers experienced in hybrid systems, or use a hybrid model? The expertise required spans electrical, chemical, and software engineering—capabilities most traditional solar O&M contractors don’t possess.
3. Grid Service Delivery: Real-Time Multi-Objective Optimization
The Challenge
Hybrid systems must simultaneously provide multiple grid services operating on completely different timescales: energy arbitrage (hourly to daily), frequency regulation (sub-second response), voltage support (sub-cycle reactive power management), capacity services, and operating reserves. Each service affects battery degradation differently and operates under different grid service and market rules that continue to evolve across regions.
The Solution
Advanced optimization and control systems enable revenue stacking by optimizing across multiple services continuously while respecting battery constraints, solar variability, and grid codes. These systems make real-time trade-offs between competing opportunities, maintain state-of-charge availability for committed grid services, and adapt to evolving market rules across different regions and grid operators.
The ability to deliver these services depends entirely on your control architecture. Can your system actually execute sub-second responses? Or have you architected latency into your design by separating your EMS and PPC? Many assets fail fast frequency response (FFR) qualification not because their hardware isn’t capable, but because their control layers are too slow.
Key Consideration
Your control architecture determines what is possible. Increasingly, sophisticated systems that can optimize across multiple services simultaneously are becoming the competitive standard. Evaluate whether your EMS can handle multi-service optimization or if you’ll be limited to simpler dual-use applications.
Building Your Operational Strategy: Key Considerations
The decisions you make during development determine your operational outcomes.
Control Architecture is Strategic, Not Just Technical
Your EMS determines what revenue streams you can access and how efficiently you deliver them. But beyond the EMS specification itself, there are deeper architectural questions that determine operational outcomes:
- Vendor coordination: Are you forcing separate PV and BESS control systems to communicate, or using unified hybrid intelligence?
- Grid service capability: Can your architecture deliver the real-time coordination that ancillary services require?
- Response speed: Have you introduced control latency that disqualifies you from premium FFR markets?
These aren’t just technical details—they’re architectural decisions that compound over the project’s life. In our upcoming posts, we’ll unpack each of these challenges and the design approaches that address them.
The Build vs. Partner Decision
Hybrid operations require expertise most solar developers don’t have in-house: battery electrochemistry and thermal management, real-time market optimization, grid service delivery and compliance, and safety management.
You can build this capability internally, partner with specialized managed services providers, or take a hybrid approach. The economics typically favor specialization. Building internal expertise for a single project rarely justifies the investment. Operators managing portfolios—like Stem’s managed services team supporting projects across multiple markets—can amortize knowledge across sites and optimize based on cross-fleet learning.
Design Decisions That Create Operational Flexibility
Certain choices during development create operational flexibility or lock in constraints:
- Unified control platforms vs. multivendor coordination
- Battery chemistry selection (LFP vs. NMC have different operational profiles)
- DC-coupled vs. AC-coupled configurations
- Duration sizing (2-hour vs. 4-hour) determines accessible revenue streams
These aren’t just specifications—they’re strategic choices that compound over the project’s life.
The Bottom Line
Operational complexity scales exponentially with hybridization. Developers treating hybrid projects like solar-plus-extras are discovering this gap during commissioning—when integration stretches from weeks to months—or after COD when underperformance erodes the revenue projections that justified the investment.
But operational expertise is only half the equation. Even with specialized teams and sophisticated management, there’s a control architecture layer that’s often overlooked during development: one that determines whether these operational challenges are manageable or unsolvable.
The question isn’t just whether you have the right people managing your hybrid project. It’s whether your control architecture was designed for hybrid complexity in the first place.
When you force separate PV and storage controllers to coordinate through communication layers, you craft performance gaps into the project from day one. Over our next posts, we’ll examine the specific control architecture challenges that turn operational problems from manageable to expensive: multivendor coordination chaos, grid service delivery gaps, and control signal latency that costs you FFR qualification.
At Stem, we’ve spent over a decade developing solutions for these challenges with an approach that combines operational expertise with sophisticated control architecture. Understanding the fundamental challenges that affect all hybrid projects, regardless of vendor, is critical for making informed development decisions.
If you’re evaluating hybrid project development or looking to optimize existing assets, connect with a Stem energy expert in the form below!
References
- Berkeley Lab. “Energy Markets & Policy”. Sept 2024. https://emp.lbl.gov/publications/hybrid-power-plants-status-2
- U.S. EIA. “Solar, Battery Storage to Lead New U.S. Generating Capacity Additions in 2025”. (Feb 2025). https://www.eia.gov/todayinenergy/detail.php?id=64586
- Energy Storage News. “Global BESS Deployments Soared 53% in 2024” (2025). https://www.energy-storage.news/global-bess-deployments-soared-53-in-2024/
- Energy Storage News. “Behind the Numbers: BNEF Finds 40% Year-On-Year Drop in BESS Costs”. (Feb 2025). https://www.energy-storage.news/behind-the-numbers-bnef-finds-40-year-on-year-drop-in-bess-costs/
This is the first post in our series on solar-plus-storage hybridization. In our next posts, we’ll explore the specific control architecture challenges that repeatedly surface during development—and how unified hybrid intelligence addresses coordination gaps, control signal latency, and other complexities that traditional approaches can’t solve.