By Steve Donovan, Head of Technical (UK & Ireland) and Giulio Stangarone MIET, Technical Engineer, at Segen Ltd.
When designing an effective solar photovoltaic (PV) system, it is well worth considering oversizing the array. This approach allows the system to generate power more consistently throughout the day, and maintain better performance during winter months. The result is higher overall energy output – and potentially more revenue too!
Inverters are rated by their AC output, which you’ll find on the data plate. For example, a 3.6kW inverter can deliver a maximum 3.6kW of AC power to your customer and to the grid. However, most inverters will accept a higher kW DC input from the solar array relative to the AC rating of the unit. This means you can connect more solar panels to the same inverter – a strategy known as oversizing.
When designing a PV system, always consult the inverter data sheet. There you will find the recommended maximum DC input compared to the rated AC output, which can be as high as 150%, or even 200%. This headroom exists for a reason. Panels rarely operate at nameplate output. Temperature losses, seasonal irradiance, orientation and shading all eat into real-world performance. Designing at a 1:1 ratio assumes conditions that almost never occur. However, it’s important to note that any oversizing must comply with the limitations set out by the manufacturer of your chosen product. It is also essential to review your plans with a relevant technical specialist before installation.
Design Considerations: Hybrid Inverters and East–West Arrays
Oversizing is particularly beneficial when using a hybrid inverter with battery storage. As a rule of thumb, the default operation mode of many hybrid inverters is to charge the battery first and then export any excess energy. Once the battery is full, generation will drop to the maximum limit that you can export on the AC side.
It’s worth noting that you won’t get the full oversizing benefit from pairing a PV inverter to an AC-coupled battery inverter. In this setup, the bottleneck will be the rating of the PV inverter. While the AC-coupled battery inverter will be able to draw excess from the PV inverter’s AC output, a system with separate AC-coupled inverters will not allow DC oversizing advantages in the same way a hybrid inverter does.
Oversizing is also particularly beneficial for east-west configurations. Typical solar systems are angled south to maximise irradiance at midday, but an east–west setup has some panels facing east and others facing west. The east-facing panels produce more energy in the morning, while the west-facing panels produce more in the afternoon, meaning the system never reaches the inverter’s theoretical maximum DC rating.
East-west configurations are growing in popularity because they provide more energy outside peak hours, which can increase revenue from exporting to the grid and boost self-consumption. When designing such a system, remember to observe the inverter’s minimum and maximum voltage limits constraints and keep the east and west arrays on separate MPPT trackers to ensure the correct performance of the system.
Grid Benefits: Smoother Energy Supply
By undersizing the inverter you are also doing a good deed by helping to reduce grid congestion. The wider, fatter generation curve from the solar system will result in more consistent energy to the grid and less peak inconsistent power. On a large scale, this helps grid operators by reducing the duck curve effect.
The duck curve describes the shape of daily electricity demand (excluding solar), specifically the difference from high demand to low demand. In the morning, when there is little solar, demand is stable, then at midday when all the solar is generating demand drops, only to rise back up to the daily peak in the late afternoon. These daily differences make energy trading complex and expensive as operators need to turn on peaking powerplants burning gas and biomass to cover the demand peak.
By oversizing solar arrays, you contribute, albeit modestly, to a smoother, more consistent power supply in the afternoon, helping delay the use of peaking plants. This effect becomes even more significant when paired with battery storage systems that trade energy.
Managing Excess Energy: Clipping
As mentioned previously, solar panels rarely operate at their nameplate output. However, in an oversized array, there are times—typically on very sunny days—when the panels can produce more DC power than the inverter can handle. The excess energy is managed through a process called “clipping.”
During clipping, the inverter adjusts the maximum power point along the IV curve, with the MPPT (maximum power point tracker) deliberately operating at a slightly suboptimal voltage. This reduces the power drawn from the array, keeping the inverter within its AC rating. Any energy above this threshold is “clipped” and cannot be converted, although in systems with a hybrid inverter, the surplus can instead be directed to charge the battery.
Despite these brief losses, oversizing still increases total daily energy production and extends the periods during which the inverter operates at its maximum capacity.
Plot of solar generation through a sunny day: The lower line is a 4kWp array on a 4kWAC PV inverter, the higher line is a 4.8kWp array on the same 4kWAC PV inverter. The green area is the additional solar energy generated. The red area is the energy being clipped by the inverter.
The bottom line: more power, more cost effective
To conclude, there are numerous benefits to installing an oversized PV array. It allows for better utilisation of the inverter’s AC rated output and can reduce the levelized cost of electricity produced. Additionally, the system owner can enjoy a more consistent output throughout the day and across the year. Pairing the system with a hybrid inverter and battery storage further enhances self-consumption, making the most of the energy your panels generate.
As always, ensure your design stays within the limits of the equipment. If you are unsure, refer to the manufacturer’s specifications or design tools. Alternatively, reach out to Segen’s technical team for guidance.
Segen is online at www.segen.co.uk/info/.
