What is the Part L renewable energy ratio (RER)?

How it works in Ireland

The RER is one of the three targets that make up NZEB. It measures how much of the house’s energy is renewable, counting energy produced on site or nearby, such as solar PV and solar thermal, and the renewable share of a heat pump’s output.

It is a calculated result. If the house falls below 20%, the designer adds renewables or changes the heating until it passes. In a well-insulated house a heat pump can reach or come close to 20% alone, and a few panels then provide margin. If the house only passes with panels, treat them as part of the build: put the cost in the budget and the array on the drawings.

Ask your designer for the RER with and without panels. If the house passes comfortably without them, you can choose panels for their running-cost benefit instead of compliance. This is also the point to ask how many kilowatts peak (kWp) are needed and where the array can sit on the roof.

Panels are part of the roof and add to the rebuild cost, so tell your insurer before they go on and update the sum insured at completion.

If you are choosing between systems, compare the whole package rather than the panels alone. A heat pump can meet or approach the ratio with little extra equipment, while a design with a weaker fabric result needs more renewables to compensate. The ratio also interacts with the other two targets, so a change that improves one can shift another. Ask your designer to re-run the calculation after any change to windows, insulation or heating.

A common mistake

Buying panels before knowing the result. Without the DEAP calculation, you can overspend on panels the house does not need or underspend and fail the check at the end. Get the RER from your designer first, and ask what changes if you swap the heating system.