Solar PV on a new build: the Part L renewable requirement

Ask your designer for the DEAP result before you decide how many solar panels to buy. New houses must reach a Renewable Energy Ratio (RER) of at least 20%, and photovoltaic (PV) panels are the most common way for a one-off house to close the gap.

What the renewable requirement says

The nZEB guide sets out the three whole-house targets in the Part L guidance for dwellings. The RER is the share of the house’s primary energy that comes from renewable sources, and it must be at least 20%. The regulations count energy produced on site or nearby: solar PV, solar thermal and the renewable share of heat from a heat pump all count.

That means the RER is a result, not a product. Your designer enters the fabric, the heating system and any panels into the Dwelling Energy Assessment Procedure (DEAP) and reads the result. If the RER is below 20%, they add renewables or change the heating until it passes.

When do you need panels?

It depends on your heating system:

• Heat pump. The renewable heat counts towards the RER. In a well-insulated house, a heat pump can reach or come close to 20% alone. Small additions, such as a few panels, then provide margin. See the heat pump guide.
• Oil or gas boiler. These supply no renewable energy, so the house needs a large renewable contribution from elsewhere. In practice that means a bigger solar array, or switching to a heat pump.
• Other renewable sources. Biomass boilers and solar thermal collectors count. They add cost and maintenance, so ask for the DEAP result with and without them.

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. If it only passes with them, treat them as part of the build, with the cost in the budget and the array on the drawings.

How many kWp?

Panels are sized in kilowatts peak (kWp), the output of the array in standard test conditions. One Irish comparison site, which is run to generate installer leads, reports that most Irish homes use 8 to 12 panels, which is 3.5 to 5.3 kWp, and that 10 panels (4.4 kWp) is the usual choice. It reports yields of about 817 to 965 kWh per kWp a year depending on the county, so a 4 kWp system generates roughly 3,270 to 3,860 kWh a year.

Those are retrofit and typical-home figures. For a new build, your designer fixes the size from three things:

1. the RER the DEAP calculation needs;
2. the roof area and orientation: south, east or west faces without shading produce the most;
3. your own electricity use, especially if you run a heat pump and charge an electric car.

A bigger array costs less per kWp but stops being useful if you cannot use or export the power.

What it costs

The same comparison site publishes these prices for 2026, based on quotes from SEAI-registered installers, before any grant:

System	Panels	Typical price
3.5 kWp	8	€7,500 to €8,500
4.4 kWp	10	€8,300 to €9,000
5.3 kWp	12	€9,200 to €9,900
6.2 kWp	14	€10,500 to €11,500

Source checked 21 October 2026. Prices vary with the installer, the equipment and the complexity of the roof. A battery or a complicated roof adds to the price. Get itemised quotes that name the panels, the inverter, the mounting system and the commissioning.

Ask each quote to state the VAT rate it assumes. Do not assume a grant is deducted: see below.

Grants

The SEAI solar electricity grant requires a house built and occupied before 2025, and approval before work starts. A new house fails the age test, so budget the full price. Our SEAI grants guide explains the rules and the order of steps for an existing house.

Do you need planning permission?

Most roof-mounted panels on a house do not. The exemption in the Planning and Development Regulations, as amended in October 2022, applies where:

• the panels sit no more than 15 cm above a pitched roof or 50 cm above a flat roof;
• they are at least 50 cm from the roof edge;
• there is no area limit on the roof, and they may be on the front slope.

Ground-mounted panels at a house are exempt up to 25 m² and 2.5 m high, if they are not in front of the house and you keep at least 25 m² of open space at the back or side. Panels fixed to a wall, and panels on protected structures, always need planning permission. In a conservation area, check the frontage rules.

If your house is under planning permission, putting the panels on the drawings now avoids questions at completion. Ask your architect to show them, and confirm with the council if the layout is unusual.

Connecting and being paid for exports

Panels need an inverter and a connection agreement with the electricity network, which your installer normally arranges. Ask your electrician to run the cable routes at first fix, so you do not open finished walls later. Ask your supplier about the export tariff when you move in. Tariffs change often, so do not rely on a figure quoted years before you complete.

Batteries and electric cars

A battery stores daytime power for the evening. It adds a four-figure sum to the price and is not needed to meet Part L, so treat it as an optional extra and price it separately. If you plan to charge an electric car at home, the new-build guidance also expects the electrical installation to allow for a charge point. Tell your electrician now, so the cable and the consumer unit are sized for it. A car charging in daylight is also one of the best uses of the panels’ output.

Insurance and the lender

Tell your insurer about the panels before they go on. They are part of the roof and add to the rebuild cost. Add them to the sum insured at completion and to your lender’s schedule if it asks for one.

What this guide adds to a comparison site’s pages

A solar comparison site explains panels, prices and payback well, but it does not cover how a new house meets Part L, what the RER is, or why a design may need panels at all. This guide adds the compliance route, the DEAP link to the heating choice, and the order in which decisions have to be made on a new build. Use the comparison site for installer prices and county yields, and your designer for the number of panels your house needs.

Questions to ask your designer and installer

1. What is the RER in the DEAP result with and without panels?
2. How many kWp does the design assume, and where do the panels go on the roof?
3. Does the roof structure and the cable route allow for them now?
4. What are the quotes for 3.5, 4.4 and 5.3 kWp, so I can see the price of each extra step?
5. Who registers the system and arranges the grid connection?

Before the first day on site, check the cover you need: your site and contract works policy should be in place on the day work starts. See what cover you need.