What is the most energy efficient boiler UK?

The most energy efficient boilers in the UK are electric boilers, no ifs or buts. Just a fact. They are 100% energy efficient boilers, with all the energy that is supplied to the immersion heaters in the boilers directly converted into heat output for the heating system.

There is a misconception that modern condensing boilers – either a gas boiler or oil boiler version – are the most energy efficient boilers; this is factually incorrect.

Our electric boilers have high modulation ratios, meaning they adjust their heat output to the heating system demands, which results in highly efficient heating systems. This ensures your energy bills/energy costs for your heating system are reduced.

Central heating with electric boilers

Electric resistance boilers convert virtually all electrical energy passing through their heating elements directly into heat within your property. Electric resistance heating is 100% energy efficient in the sense that all the incoming electric energy is converted to heat for the central heating system.

Such conversion happens through Joule heating: when current flows through a resistive element, electrons collide with atoms, generating thermal energy with almost zero waste. Unlike gas boilers, there are no flue gases carrying heat outside your home.

Electric system boilers

An “electric system boiler” is defined by its design and integration with other components. It’s a central heating boiler that works with a separate hot water storage cylinder.

• Key Feature: It has built-in components. Unlike a basic “regular boiler”, a system boiler has key parts like the pump and expansion vessel built inside the unit. These features makes installation in a modern home with a sealed heating system simpler and neater.
• How it works for heating: It heats water that circulates through your radiators and heats a large volume of water stored in a cylinder.
• Best for: Homes with multiple bathrooms where several people might need hot water at the same time. The stored hot water in the cylinder allows for strong, simultaneous showers and baths.

Our full range of these system electric boilers is detailed here – Electric system boiler

Heat only boilers electric boilers

The term “heat-only” (also historically called a “regular” or “conventional” boiler) refers to the simplest type of central heating boiler.

They are used on open-vented heating systems (usually with a feed tank in the loft).

They require external components: Unlike a system boiler, a heat-only boiler is a “bare bones” unit. It needs an external pump, expansion vessel, and feed and expansion (F&E) tank (usually in the loft for an open-vented system) to function. This makes it a more traditional or legacy system setup that was common in the 1980s in the UK.

The can-do heating hot water with an open vented indirect hot water cylinder.

Our full range of these electric boilers is detailed here –Heat only electric boilers

Electric boiler efficiency ratings

Why are electric boilers “D” rated then?

Energy efficient boilers are a crucial (and often confusing) piece of EU/UK energy labelling. The “D” rating for electric boilers is not about their thermodynamic efficiency—it’s about their cost efficiency under a specific, now-outdated, calculation framework.

Here’s the breakdown:

1. The ErP boiler efficiency ratings label has a flawed methodology

The Energy-related Products (ErP) Directive label (the one with the A+++ to G scale) you see on boilers is designed to help consumers compare the overall energy efficiency of similar products.

However, its calculation for heating devices has a critical quirk:

• It uses a “Total Energy Consumption” formula that is heavily weighted towards running costs based on standardised energy prices.
• For the ErP label, the EU standardised “primary energy factors” (PEF) to account for generation and transmission losses:
  • Electricity PEF: 2.5 (meaning for every 1 unit of electricity at your home, 2.5 units of primary energy—like gas at a power station—were theoretically used).
  • Gas PEF: 1.1 (gas delivery to your home is very efficient).

2. The “Primary Energy” Penalty for Electricity

This is where the electric boiler gets hammered. Let’s compare the label calculation for a 100% efficient electric boiler vs. a 92% efficient gas boiler:

• Electric Boiler (100% efficient at point of use):
  • Label Efficiency = (Useful Output / Energy Input) x Primary Energy Factor.
  • The calculation is (100% / 100%) x 2.5, which equals 2.5 or 250%. This sounds high, but wait—the label inverts this number to reflect the “primary energy” waste. Effectively, it’s penalised for losses in power generation before it reaches your home.
• Gas Condensing Boiler (92% efficient):
  • The calculation for the gas condensing boiler’s efficiency is (92% / 100%) x 1.1 = 1.012, which equates to 101.2%.

Under this specific “primary energy” metric, the gas boiler appears more efficient than the 100% efficient electric one, because the calculation unfairly burdens the electric technology with the inefficiencies of the entire national grid.

The Resulting Scale Distortion

Because of this PEF factor:

• Gas and oil boilers, due to their low PEF (1.1), cluster in the A and B bands (e.g., 90%+ = A).
• Direct electric boilers and heaters, despite being 100% efficient in your home, are pushed down to D, E, or even F on the scale.

• It’s not technology neutral. It heavily penalises all direct electric heating, regardless of how green the grid becomes.

The UK government and regulatory bodies have been under sustained pressure to reform heating system labelling, including for electric boilers, due to the widespread confusion and misalignment with national climate goals.

Renewable electricity production in the UK

As of 2024, over 50% of the UK’s electricity was generated by renewables. This means less than 50% was produced by non-renewable sources (fossil fuels and nuclear).

In early 2025, about 24.8% of Britain’s electricity came from fossil fuels, while 45.2% was from renewables.

The UK aims for a 95% low-carbon (renewables + nuclear) grid by 2030.

With updated figures for 2025 from this BBC report – Record year for wind and solar electricity in Great Britain in 2025

The “D” rating was a product of an era of fossil-fuel-dominated electricity for electric boilers. With the growth of renewables, the label and its underlying assumptions are becoming obsolete.

Efficient gas condensing boilers

The most efficient modern gas condensing boilers typically operate with a gross efficiency of roughly 90–95% (often A-rated ErP) and net efficiency, which excludes latent heat recovery, exceeding 100% (often 92–98% advertised). While marketing sometimes uses net values, gross efficiency is a more accurate measure of energy usage.

The most efficient gas boilers in the UK are modern condensing models, which can achieve efficiency ratings of 90% or so with condensing technology.

Key efficiency rating data:

• Gross Efficiency (GCV): Typically, 90–95%, as they recover latent heat from water vapour in the flue gas.
• Net Efficiency (NCV): Can appear over 100% (e.g., up to 103%) because it does not account for the latent heat recovered during condensation.
• Real-World Performance: While often rated near 94% efficient, seasonal efficiencies (SEDBUK) in practice may be closer to 80–83% for combination boilers due to operating conditions.
• In comparison to older inefficient boilers, i.e., non-condensing gas boilers in the UK.

The Core Principle: Return Temperature is Everything

A condensing gas boiler only achieves its headline 90–95% efficiency when the return water temperature stays below 55°C for the heating system, ideally below 50°C. The lower the return temperature, the higher the energy efficiency.

This is the dew point temperature, where water vapour in the flue gases condenses, releasing latent heat that the boiler recovers through its secondary heat exchanger. Above this temperature, the boiler operates in “non-condensing mode” and loses 8–15% energy efficiency.

• Gas condensing boilers are the most efficient gas boiler technology available, with theoretical efficiencies over 90%. To achieve high efficiency, the return water pipework must be cool enough (typically below 55°C) to cause the water vapour in the flue gases to condense, releasing extra heat. 
• In practice, heating system design and controls heavily influence the real-world energy efficiency of gas condensing boilers in typical UK homes, which typically ranges from 80 to 88%.
• They offer substantial savings over any older gas boilers and are currently the most cost-effective and energy efficient boilers.

Heating controls

For maximum efficiency and lower bills, pair condensing gas boilers with improved controls (especially weather compensation) and aim to run it at the lowest possible flow temperature to these energy efficient gas boilers.

A condensing boiler installed correctly in a suitable system will save 15–30% versus non-condensing. But one installed poorly may only achieve 5–10% savings—barely justifying the extra cost. The technology works brilliantly—but only when the entire system is designed to enable condensation.

Conclusion

Electric boilers are the most energy efficient boilers in the UK, better than gas or oil modern condensing boilers in the UK. When powered by renewable energy sources, electric boilers have zero carbon emissions and help reduce the carbon footprint for the UK boiler market.