If you're weighing up whether solar panels are right for your home in Ireland, you've probably come across two technical but essential terms: kWp and kWh. They sound similar, but they measure entirely different things. kWp refers to the system's peak power capacity, while kWh measures the actual energy it produces or uses over time. Understanding both is crucial if you want to get the best return on your solar investment.
At WattCharger, we often speak with homeowners in Dublin, Cork, and across Ireland who are comparing system sizes or evaluating quotes but feel unsure what these terms actually mean. This guide is here to help clarify the difference so you can make an informed, confident decision.
The core distinction lies in potential vs performance.
If you think of your solar panels like an electric car:
The higher the kWp, the more energy your system is capable of producing, but only real-world conditions will determine how many kWh you get out of it.
Ireland’s weather isn’t exactly Mediterranean. With frequent cloud cover and shorter winter days, understanding how kWp and kWh relate in real-life Irish conditions is important.
Let’s say you install a 4 kWp system. In perfect lab settings, you’d get 4 kilowatts of electricity per hour of sunshine. But in Ireland, you might average closer to 950–1,100 usable sun hours per year. Combine that with a performance ratio of around 80% (factoring in shade, inverter losses, dust, and wiring), and that 4 kWp system might realistically generate around 3,000–3,400 kWh annually.
That’s enough to cover a significant chunk of a typical Irish home’s electricity needs — and could save you hundreds on energy bills every year.
Good question! If kWh is what you’re ultimately interested in (because that’s what affects your bill), why do solar companies still quote kWp?
Because kWp helps define the system’s size. It tells installers and homeowners how much solar capacity is being installed, how much roof space is required, and how much energy might be generated in the best-case scenario.
It’s a baseline metric. But to understand how much power your household will actually see in savings or feed back into the grid, you need to translate kWp into estimated kWh, based on your specific location, roof orientation, and energy habits.
Let’s take a three-bedroom home in Cork with south-facing panels and standard usage.
3.5 kWp × 1,000 hours × 0.80 = 2,800 kWh per year
This could save between €700–€1,000 annually, depending on usage patterns and electricity rates. It also reduces reliance on the grid and supports long-term energy independence.
If you've already read our guide, What Is kWp? Understanding the Key to Solar Panel Performance in Ireland. You'll understand the basics of capacity and how installers define system size. Her we are adding the output dimension kWh which gives you a better idea of what to expect from your system month to month.
Again, while kWp gives you a label, kWh gives you the lived reality of your solar setup in Ireland.
"A higher kWp always means more savings."
Not always. If your roof is shaded, angled awkwardly, or you’re not consuming much electricity during the day, a massive system might be overkill.
"kWh and kWp are interchangeable."
They’re linked, but not interchangeable. kWp is the potential, kWh is the performance.
"I should just install the biggest kWp system I can afford."
That depends. Bigger systems might not qualify for full SEAI grants, and may not pay back as quickly if your electricity usage is low. The smartest approach is a tailored solution.
At WattCharger, we don’t believe in “one-size-fits-all” installations. When helping homeowners in Ireland choose a solar system, we consider:
This all feeds into a balanced view of what kWp suits you, and what kind of kWh output you can expect from it.
Whether you're in the early research phase or already comparing quotes, understanding the difference between kWp and kWh can empower you to ask smarter questions and avoid mismatched systems.
At WattCharger, we specialise in designing and installing solar systems across Ireland, tailored to real-world Irish conditions not just lab figures. Our goal is to give you clear performance expectations, transparent pricing, and maximum return on your solar investment.
📍 Serving Dublin, Cork, Galway, and nationwide.
Get in touch for a personalised quote based on your location, roof size, and energy use, no commitment required.