By Tim Stocker, Principal Product Manager, CC&I
When we tell engineers and EPCs that our inverters use “fixed string voltage,” we often get puzzled looks. But once we walk them through the concept, the benefits become immediately clear—not only in terms of improved system efficiency but also in how photovoltaic (PV) systems can be designed more flexibly and cost-effectively.
At the heart of SolarEdge’s fixed string voltage architecture are Power Optimizers—DC-to-DC converters that regulate voltage independently of the modules themselves. They ensure a consistent string voltage while allowing current to vary, maintaining constant power levels throughout the system.
String Stretching
One of the most impactful benefits of fixed string voltage is being able to do string stretching. Determining the maximum number of modules per string in the traditional way involves calculating the maximum open circuit voltage of the module based on the coldest temperature at the project site and then dividing the maximum DC voltage allowed by the inverter, 1500 Vdc in most cases, by that open circuit voltage.
This usually results in around 26–28 modules per string, depending on climate and module specs. For example, on a site using 650 W modules and targeting a DC:AC ratio of 1.2 on a 330 kW inverter, you might end up with 23 strings of 27 modules each—roughly 396 kWdc total. That means 23 pairs of #10 PV wire going to combiner boxes, each requiring fuses and terminations.
Higher DC:AC ratios or larger inverter sizes will require more strings and thus potentially more combiner boxes.
With SolarEdge’s fixed string voltage design, this constraint changes dramatically. Two modules are connected in series to each H1500 Power Optimizer, and the optimizers regulate string voltage. You no longer need to size strings based on open-circuit voltage limits.
Instead, string sizing is governed by the maximum allowed power per string (40 kWdc), the maximum continuous power (30 kWdc), or the maximum number of optimizers per string (up to 66 modules depending on module wattage). This architecture enables significantly fewer strings per inverter and therefore fewer combiners, fuses, and terminations.
Back to our earlier example: With 48 modules per string (connected via optimizers), we can achieve around 31.2 kWdc per string—reducing the total number of strings to about 13 per inverter. Not only does this shrink balance-of-system (BoS) costs, it also simplifies installation and reduces O&M touchpoints.
Design Flexibility
Another major benefit of fixed string voltage is design flexibility. Traditional string inverters require uniform string lengths and often force designers to accommodate electrical constraints by making compromises in layout. This frequently results in cross-row stringing, which complicates wiring and tracker designs.
With the TerraMax inverter, string lengths can differ by up to five optimizers (equivalent to 10 modules). This allows designers to tailor string configurations to specific racking or tracker layouts and to better accommodate terrain, obstructions, or irregular site geometries.
In short, fixed string voltage enables designers to create standardized building blocks that can be deployed across projects regardless of climate or layout—or to fully customize system design based on site-specific constraints without incurring extra cost or complexity.
Mismatch Mitigation – Long-term Gains
Lastly, fixed string voltage architecture paired with SolarEdge Power Optimizers is able to mitigate energy losses caused by module mismatch—a common and often underestimated issue in PV systems.
Mismatch occurs when modules in a string produce power at slightly different levels, which can be due to manufacturing variances, uneven degradation, shading, soiling, or environmental factors. In traditional string inverter systems, where modules are wired in series and the inverter performs MPPT (maximum power point tracking) at the string level, the current through all modules must remain the same. This means the lowest-performing module in the string limits the output of all others, reducing the overall energy production from that string.
Another contributing factor is bifacial gain variability, which depends on the albedo, or reflectivity, of the ground surface beneath the modules. Since this reflectivity can vary across a site and over time—due to changing surfaces, vegetation, or ground cover—bifacial modules may generate inconsistent backside gains from one row to the next. In conventional systems, this introduces further string-level mismatch and negatively impacts MPPT accuracy, leading to a reduction in total energy harvest.
SolarEdge Power Optimizers overcome these issues by enabling MPPT at the optimizer level (typically one optimizer per two modules). This allows each optimizer to adjust its voltage independently while contributing to a fixed string voltage. The result is maximum power extraction from each module pair, regardless of mismatch, shading, or bifacial variability.
In year one, the gain from mismatch mitigation may seem modest—but by year 20, the cumulative difference in energy production is substantial. This enhanced long-term performance improves ROI, reduces degradation risk, and gives asset owners greater confidence in the reliability and predictability of their PV investment.
Conclusion: The Swiss Army Knife of PV Design
Fixed string voltage delivers broad, real-world advantages. With SolarEdge TerraMax inverters and H-Series Power Optimizers, you get:
- Longer strings with less wiring and fewer components
- Flexible layouts that adapt to any site
- Higher lifetime energy yield by minimizing mismatch losses
Think of it as the Swiss Army knife of PV system design—a versatile tool that helps you build smarter, more efficient solar projects from the ground up. Watch the explainer video here.
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