Energy Efficiency Grade Assessment for Thermal Storage Electric Heaters

Thermal storage electric heaters charge with low-cost off-peak electricity and release the stored heat over many hours, so their real performance can only be judged through a standardized energy efficiency grade assessment. This article explains how storage heaters are tested and classified under IEC/EN 60675 and EU Ecodesign Regulation (EU) 2015/1188, covering test samples, key measurement items, laboratory methods, applications, and frequently asked questions.

Technical Overview

Thermal storage electric heaters, also known as storage heaters or heat-retention radiators, use an electric resistance element to heat a high-density thermal core, typically magnetite, ceramic, or feolite bricks, during an off-peak charging window such as a seven-hour night tariff period. The stored energy is then released into the room either by natural convection through insulated dampers or by a fan-assisted discharge system governed by an electronic room thermostat. Because a substantial share of the charged energy can be lost through the casing during the retention period, the useful efficiency of a storage heater depends on the balance between charge input, storage retention, discharge management, and control intelligence rather than on element conversion alone.

Energy efficiency grading for these appliances is driven mainly by EU Ecodesign Regulation (EU) 2015/1188 for local space heaters, commonly known as Lot 20, together with the labelling rules of Commission Delegated Regulation (EU) 2015/1186. The core performance metric is the seasonal space heating energy efficiency, calculated on a primary-energy-equivalent basis using a default electricity conversion factor of 2.5 and adjusted with percentage-point correction factors for electronic room temperature control, presence detection, and open-window detection. The underlying heat output and storage characteristics are measured according to IEC/EN 60675, the international performance standard for household directly-connected storage heaters. Safety requirements, assessed in parallel, are defined in IEC 60335-2-61 and national adoptions such as the GB 4706 series of standards.

Testing Samples

Static (Natural Convection) Storage Heaters

Classic models with a brick core, insulation package, and damper-controlled convective release. Evaluation focuses on standing losses, damper behavior, and how much of the charged energy is delivered as useful heat over a 24-hour cycle.

Automatic and Fan-Assisted Smart Storage Heaters

Models equipped with charge-prediction algorithms, fan-assisted discharge, electronic thermostats, and optional presence or open-window detection. Qualifying control functions attract correction factors that can raise the declared efficiency and the resulting label class.

Combination Storage Heaters

Units that integrate a direct-acting booster element alongside the storage core. The storage section is assessed under IEC/EN 60675, while the direct-acting section is measured according to IEC/EN 60531, and the results are combined in the seasonal efficiency calculation.

Sample submission typically involves one to three production units representing each model family, accompanied by declared data such as rated charge input, nominal heat output, charge duration, control type, and firmware version. Variants that differ in core mass, input power, or control electronics must be declared so the laboratory can select the worst-case configuration for testing.

Testing Items

The assessment program covers the electrical, thermal, and control characteristics that determine the final efficiency grade:

• Nominal charge input power and total charging energy over the declared off-peak window
• Nominal and total useful heat output delivered during a 24-hour discharge cycle
• Charge retention and standing (keeping-warm) losses through the retention period
• Discharge profile, meaning heat output versus time under thermostat control
• Charge control accuracy, or the correlation between stored energy and anticipated heating demand
• Auxiliary energy consumption of fans, damper actuators, and control boards during charging, retention, and discharge
• Input and output of any direct-acting booster element on combination models
• Standby, off-mode, and networked standby power consumption
• Functional verification of electronic room temperature control, presence detection, open-window detection, and distance control
• Calculation of seasonal space heating energy efficiency and assignment of the energy efficiency class

Testing Methods

Chamber and Instrumentation

Tests are performed in a calibrated calorimetric room or climatic chamber built in accordance with IEC/EN 60675, with characterized enclosure heat-loss coefficients and ambient temperature held within tight tolerances. Electrical input is measured with high-accuracy power analyzers, while delivered heat output is determined by the room calorimetric method or by surface heat flux instrumentation under continuous data logging.

Charge-Retention-Discharge Cycle Testing

The heater is charged at nominal input across the declared off-peak window, then passes through a retention phase and a controlled 24-hour discharge cycle that simulates a realistic heating demand. Repeated cycles verify reproducibility, and the measured energy balance yields retention losses, usable storage capacity, and the discharge characteristic used in the efficiency model.

Efficiency Calculation and Classification

The measured data feed the calculation model in Annex III and Annex IV of Regulation (EU) 2015/1188: electricity consumption is converted using the default 2.5 primary energy factor, deductions are applied for standby consumption and output delivered when no heat is demanded, and correction factors are added for qualifying control functions. The resulting efficiency value determines both compliance with the minimum threshold, which is 38 percent for electric storage room heaters since January 2018, and the energy efficiency class published on the label under Regulation (EU) 2015/1186.

Quality Assurance

All measurements should be carried out by ISO/IEC 17025 accredited third-party laboratories using traceably calibrated instruments and documented uncertainty budgets. Complete test reports support manufacturer declarations and can be re-verified by market surveillance authorities within defined tolerances.

Applications

Certified efficiency grading for thermal storage electric heaters supports a wide range of commercial and regulatory purposes:

• EU market access, where ecodesign compliance documentation and the energy label accompany CE marking for exported products
• Product development, allowing engineers to benchmark insulation materials, core mass, damper design, and control strategies against grade targets
• Utility and off-peak tariff programs, where verified efficiency data qualifies appliances for demand-side management and time-of-use heating schemes
• Green procurement and building documentation, supplying validated performance figures for public tenders and building energy assessments
• Market surveillance, providing an objective basis for verifying manufacturer declarations at the point of sale

FAQ

Which standards apply to the energy efficiency assessment of storage heaters?

Performance measurement follows IEC/EN 60675, the efficiency calculation and minimum thresholds come from EU Ecodesign Regulation (EU) 2015/1188, and the label class is assigned under Commission Delegated Regulation (EU) 2015/1186. Safety evaluation runs in parallel under IEC 60335-2-61 or the corresponding GB 4706 series standards.

What minimum efficiency must a storage heater reach in the EU?

Since 1 January 2018, electric storage room heaters must achieve a seasonal space heating energy efficiency of at least 38 percent under Regulation (EU) 2015/1188. Because the metric applies a 2.5 primary energy conversion factor, this threshold corresponds to delivering roughly 95 percent of the charged electricity as useful heat in final-energy terms.

How do smart controls influence the grade?

Electronic room temperature control, presence detection, and open-window detection each earn percentage-point correction factors in the efficiency calculation. Well-implemented control software can lift a product by one or more label classes while reducing wasted output during unoccupied periods.

How many samples are needed and how long does testing take?

Most programs require one to three production units per model family, with a typical project duration of three to six weeks covering preconditioning, repeated charge-discharge cycles, standby measurements, calculation, and reporting.

Is a national efficiency grade available for the domestic Chinese market?

Safety under the GB 4706 series is mandatory, but room electric heaters are generally not covered by the national energy label catalog. Manufacturers therefore rely on IEC/EN 60675 based assessment primarily for export compliance and voluntary performance benchmarking.