Stop counting panels and start counting what you use
In the first part of this series, we looked at why reducing baseload demand before installing solar is the sequence that protects the investment. Once a business is ready to have a solar install discussion, the important metric to focus on is the self-consumption rate i.e. the proportion of generated electricity used on-site.
The export gap
There are two ways a solar PV system can earn its keep. It can displace electricity a business would otherwise buy from the grid or it can export surplus generation to the grid. These two outcomes are not equal, and understanding the difference between them is the most important thing a business can do before signing off on any solar investment.
A unit of solar electricity used on-site offsets a unit imported at the commercial rate and avoids the additional use of systems costs. A unit exported earns a wholesale-linked rate, a fixed tariff, or in some commercial arrangements, nothing at all. According to the Sustainable Energy Authority of Ireland (SEAI), on-site consumption of solar-generated electricity can be around three times more valuable than exporting it.
Once a system is generating more electricity than a business is drawing down at that moment, additional generation earns at the export rate, not the import rate. Every panel beyond the point at which generation exceeds daytime demand returns progressively less value. The system size that maximises kilowatt-peak is rarely the system size that maximises return on investment. The self-consumption rate is the figure that drives payback and lifetime value far more reliably than installed capacity.
What ‘daytime demand’ means
Peak solar generation in Ireland falls broadly between 10am and 2pm. That four-hour window is the one that matters most for sizing decisions, and a business’s consumption during those hours is a very different figure from its total daily or annual usage.
A business with a steady operational load running through the working day typically has a demand profile that sits comfortably under the generation curve during those hours, making it a strong solar candidate. A business that fires up heavily before dawn, or peaks late in the afternoon when solar output is already tailing off, will self-consume far less of what it generates, regardless of how large the system is.
This is why consumption data matters more than roof area. SEAI recommends that businesses with large electricity demand obtain half-hourly consumption data for a full twelve months and overlay it against a modelled generation profile for the specific premises. Large energy users can request this data from the Meter Registration System Operator through ESB Networks. Smaller businesses can use a power logger over a few weeks to build an equivalent picture.
What self-consumption rate should a business target?
As a general principle, a well-designed commercial system should aim to keep self-consumption high enough that exported electricity represents a small proportion of total generation rather than a structural part of the financial case. Designing to a self-consumption rate of 70–80% or above is a reasonable starting point for most business profiles, though the right figure depends on the specific demand pattern, operating hours, and whether battery storage or load-shifting is part of the design. Customers who expect to reduce their overall demand for imported energy should speak to their supplier. Their existing tariff may no longer be the optimal choice to meet their needs. In some cases, a drop in import consumption can lead to extra charges when a supply contract includes a volume tolerance.
Battery storage can extend the self-consumption window beyond generation hours, capturing surplus that would otherwise be exported and making it available later in the day. Batteries sized purely to increase self-consumption may not yet make economic sense in isolation, though the business case strengthens where they can offer additional value through demand charge reduction.
Load-shifting is often a more immediately practical lever. Scheduling high-draw equipment such as compressors, pumps, water heaters, or EV charging to operate during peak generation hours can meaningfully raise the self-consumption rate without any additional capital expenditure.
Solar PV is one of the few capital investments where the most impressive-looking system on paper can deliver a materially weaker return in practice. The metric that matters is kilowatt-hours used, not kilowatts installed.