Why We Insist on Only Getting a Heat Pump If You Have Solar Power (A Deep Dive)

There is a specific feeling that comes with opening a utility bill in Ireland. It’s not quite fear, and it’s not quite anger. It’s a unique cocktail of emotions that sits somewhere between “resignation” and “I wonder how much a kidney sells for on the black market these days.”

For years, the solution to this problem was simple: complain about it. But recently, a new character has entered the chat. The Heat Pump.

If you have been researching home energy upgrades, or just existing as a human adult in the 21st century, you have heard of the Heat Pump. It is the Beyoncé of the retrofit world. It is sleek, it is modern, and the government really, really wants you to get one. And for good reason—they are incredible pieces of engineering.

But here is the thing. If you install a heat pump in a standard Irish home without thinking about where the electricity comes from, you might be inviting a new kind of anxiety into your life. At Retrofit Dublin, we have a bit of a golden rule: Heat Pumps and Solar Panels are like tea and biscuits. You can have one without the other, but it just feels wrong.

Today, we are going to dive deep—Wait But Why style—into the physics, the economics, and the sheer logic of why we insist on pairing these two technologies. Grab a cup of tea (biscuits optional). We are going to talk about thermodynamics.

The Magic Box (How a Heat Pump Actually Works)

First, we need to understand what a heat pump actually is. Because to the uninitiated, it sounds like magic. You plug a box into the wall, it takes cold air from outside (where it is currently raining and miserable), and somehow turns it into warm air inside. This violates every instinct we have about how heat works.

In our heads, heat is something we create. We burn a log. We light some gas. We pass electricity through a wire until it glows red hot. We are Cavemen making fire.

A heat pump doesn’t make fire. It moves heat. It is essentially a fridge running in reverse. Your fridge takes the heat out of your milk and dumps it into your kitchen (feel the back of your fridge; it’s warm). A heat pump takes the heat out of the air outside—yes, even when it is 2°C, there is still thermal energy in the air—and dumps it into your living room.

The efficiency of this process is measured by something called the Coefficient of Performance, or COP.

If you burn gas, the efficiency is maybe 90%. For every 1 unit of energy you pay for, you get 0.9 units of heat. The rest goes up the flue.

With a heat pump, the COP is usually around 3 or 4. This means for every 1 unit of electricity you buy, you get 3 or 4 units of heat. This is why the physics of heat pumps is so revolutionary. You are not creating energy; you are stealing it from the atmosphere. It is the only time stealing is legally and socially encouraged.

So, if a heat pump is 300% efficient, why am I writing a blog post warning you about them?

The Electricity Trap

The problem is not the machine. The problem is the fuel.

In Ireland, electricity is expensive. Really expensive. We have some of the highest utility rates in Europe. While gas and oil are also pricey, electricity has historically been the premium product. It is the champagne of energy sources. You use it for lights and Netflix, not for boiling 200 litres of water for a bath.

When you rip out your oil boiler and put in a heat pump, you are moving your heating bill from a relatively cheaper fuel source (oil/gas) to a premium fuel source (electricity).

Even with that magical COP of 300%, if the price of electricity is four times the price of gas, you aren’t saving money. You are just breaking even, but with a much more expensive piece of kit in your garden.

This is the “Gap of Doom.” It is the gap between the efficiency of the machine and the cost of the fuel. And this is where the Irish grid comes in to ruin the party.

The Commission for Regulation of Utilities (CRU) oversees these prices, but they can’t change the global market. If you are buying 100% of your electricity from the grid to run a heat pump, you are at the mercy of international markets, wind availability, and whatever mood the grid is in that day.

Enter the Hero: The Solar Panel

This is where Solar PV (Photovoltaic) enters the story wearing a cape.

Solar panels do one thing: they generate electricity. But they don’t just generate any electricity; they generate free electricity. Once you have paid for the panels, the electrons they push down the wire are yours. You are the utility company now.

When you combine a heat pump with solar, something magical happens to the math.

Let’s say your heat pump needs 1 unit of electricity to run.

Scenario A (Grid only): You buy that unit for 35c.

Scenario B (Solar + Grid): You generate that unit on your roof for 0c.

Suddenly, your heating cost isn’t just “efficient”—it is effectively zero for that hour.

Now, I can hear the cynic in the back of the room (it’s the Irish weather, isn’t it?). “But wait,” you say. “It’s Ireland. We don’t have sun. We have 50 shades of grey drizzle.”

This is a common misconception. While we aren’t the Sahara, Met Éireann’s solar data confirms that Ireland receives a surprising amount of solar irradiation. Solar panels don’t need blazing heat; they just need daylight. And crucially, they work well in spring and autumn—the exact times you need your heating on but don’t want to burn a fortune.

The “Battery” You Didn’t Know You Had

There is another layer to this. A heat pump doesn’t just heat the air; it heats the water in your cylinder and the concrete in your floors (if you have underfloor heating).

This allows you to act like an energy ninja.

Imagine it is 11:00 AM on a Tuesday in April. The sun is out. Your solar panels are generating 4kW of power. Your house is empty because you are at work (or working from home, pretending to be busy).

You can tell your heat pump to turn on now. It uses that free solar electricity to heat up your hot water tank and warm up your floors. By the time the sun goes down and you finish work, your house is a thermal battery, storing heat that you generated for free. You aren’t buying electricity at 6:00 PM when the peak rates kick in; you are coasting on the sunshine from lunch time.

This requires a bit of technology. You need a smart setup. This is why ESB Networks are rolling out Smart Meters across the country. It allows you to track exactly when you are using power. When you combine Solar, a Heat Pump, and a Smart Meter, you are essentially gamifying your utility bill.

The Elephant in the Room: The Leaky Bucket

Before you run out and buy a heat pump and solar panels, we need to pause. There is a third character in this story, and he is the villain. He is the Drafty Wall.

Imagine you are trying to fill a bucket with water. The Heat Pump is a very efficient hose. The Solar Panels are a free source of water. But if your bucket has massive holes in the bottom, it doesn’t matter how efficient the hose is or how free the water is. You are going to get wet feet, and the bucket will never be full.

In the world of retrofitting, this is your insulation.

If you put a heat pump into a house with single-glazed windows and no wall insulation, the heat pump will work overtime. It will run constantly, trying to replace the heat that is leaking out through your walls. It will eat electricity—even your free solar electricity—faster than you can generate it.

This is why we often talk about external wall insulation Dublin residents need so badly. Many Dublin homes are solid block or hollow block built in the mid-20th century. They bleed heat. Wrapping the outside of your house in insulation is like putting a tea cosy on a teapot. It keeps the heat in, meaning your heat pump doesn’t have to work as hard.

We wrote a detailed breakdown of this in our Irish Energy Report Card, where we looked at how leaky homes are effectively burning money. You have to fix the fabric first.

The Financial Magic Trick (And The Battery Cheat Code)

Let’s talk about money. Because as much as we love saving the planet, we also love not being broke. And this is where the strategy gets really interesting, specifically when you add a nice, large battery to the mix.

The Irish government, via the SEAI, has thrown a lot of money on the table to help you do this. The SEAI heat pump grants are generous, but the real victory is in the day-to-day running costs.

Most of the year—let’s say March to October—your solar panels are the sugar daddy. They are generating enough power to run the heat pump, boil the kettle, and probably charge the laptop you are using to read this. Your bill during these months should be practically non-existent.

But what about the deep, dark winter? What happens in December when the sun clocks off at 4 PM?

Enter the Battery (Your Night-Time Ninja)

If you install a decent-sized battery system (we are talking 5kWh or 10kWh), you unlock a cheat code called “Time of Use Arbitrage.”

Here is how it works. You sign up for a smart tariff where electricity is incredibly cheap at night—sometimes as low as 8c or 9c per unit between 2:00 AM and 5:00 AM. This is often discussed in news reports on smart tariffs as a way to balance the grid.

While you are asleep, your battery wakes up. It sucks in that cheap grid electricity until it is 100% full. It costs you peanuts.

Then, the next day, when electricity prices spike to 35c or 40c, your house ignores the grid. It runs off the battery. You are running your heat pump, cooking your dinner, and watching TV using power you bought for pennies at 3 AM.

The Math looks like this:

Spring/Summer: Solar runs the show. Cost: €0.

Winter Days: You run off the battery (charged cheaply at night). Cost: Minimal.

Winter Nights: You charge the battery again.

You effectively bypass the expensive “Peak Rates” entirely. You have insulated your wallet from the volatility of the energy market.

The Global Context

It’s easy to get lost in the weeds of Dublin grants and MPRN numbers, but zoom out for a second. We are part of a massive global shift. The data on climate change from NASA is unequivocal. We have to stop burning things to stay warm.

Electrification is the future. By installing solar, a large battery, and a heat pump, you aren’t just saving money; you are future-proofing your home. You are building a little fortress of independence.

The Verdict: Don’t Half-Bake the Cake

Retrofitting a home is stressful. It’s dusty, it’s expensive, and it involves men in high-vis jackets drinking all your tea. It is tempting to cut corners. It is tempting to say, “I’ll just get the heat pump now and do the solar later.”

But at Retrofit Dublin, we see the data. We see the bills of customers who did it right, and the bills of customers who did it halfway.

A heat pump without solar is like buying a Ferrari and towing a caravan. It works, but it’s dragging a heavy load. A heat pump with solar and a battery is like that same Ferrari on the open road.

If you are going to do it, do it right. Insulate the shell. Capture the sun. Store the cheap power. Pump the heat. It is the holy trinity of home energy, and when you get it right, that feeling of dread when the utility bill arrives? It vanishes.

If you want to stop feeding the inflation monster and start generating your own power, we should talk. Check out our guide to Solar Panels in Dublin to see how to start your journey.