Glossary: every technical term on VersusIndex explained

VersusIndex compares products on their specifications. Many of those figures come from European standards and official registers and have a precise meaning. This glossary covers every term you will find on our product and comparison pages, grouped by product category. Most fields on the site also have an ⓘ with a short explanation; here you get a little more background and what to look out for.

General terms

kW and kWh

A kilowatt (kW) is power: how fast a device uses or delivers energy. A kilowatt-hour (kWh) is energy: 1 kW for one hour. Your bill counts kWh; your main fuse and supply limit kW.

Phases

Most UK homes have a single-phase supply (230 V); some larger or newer homes have three-phase. The number of phases limits how much power an EV charger, heat pump or inverter can draw or export.

Sound power

Sound power, in dB(A), is the total noise a unit emits, measured to a standard. It is the fair figure for comparing products. Lower is quieter; 3 dB less halves the sound power.

Sound pressure

Sound pressure is what you hear at a given spot and depends on distance and surroundings. Some manufacturers quote sound pressure at 1, 3 or 5 metres; that figure is always lower than the sound power, so the two are not directly comparable.

IP rating

The IP code shows protection against dust (first digit) and water (second digit). IP54 is splash-proof and IP65 protects against water jets; IP54 or better is usual for equipment outdoors without cover.

IK rating

The IK code shows impact resistance, from IK00 to IK10. IK10 withstands a 20-joule impact – useful for a charger on a driveway where it might get knocked.

Refrigerant

The fluid circulating in a heat pump or air conditioner that picks up and releases heat. Common ones are R290 (propane), R32 and the older R410A. Most heat pumps in our database use R290.

GWP

Global Warming Potential: how strongly a refrigerant warms the climate if it leaks, compared with CO₂. R290 is close to zero, R32 is in the hundreds and R410A is over 2,000.

Refrigerant charge

The amount of refrigerant in kilograms. Together with GWP it determines the climate impact of a leak; for flammable refrigerants such as R290 it also affects where the unit may be installed.

Minimum operating temperature

The lowest ambient temperature at which the manufacturer says the product still works – relevant for chargers and batteries outdoors or in an unheated garage.

Warranty

The manufacturer’s product warranty in years. Check the conditions: the full term sometimes applies only after registration or installation by an approved installer. Solar panels and batteries also have a performance warranty (see below).

Heat pumps

Heat pump type

An air-to-water heat pump takes heat from the outside air and heats the water in your radiators or underfloor heating. A ground source heat pump takes heat from the ground through boreholes or loops. Air-to-air heat pumps blow warm air into rooms and are covered under air conditioners below.

Flow temperature

The temperature of the water going to your radiators or underfloor heating. Standards test at 35 °C (underfloor, low temperature) and 55 °C (radiators). The lower the flow temperature, the more efficient the heat pump.

SCOP

The Seasonal Coefficient of Performance is the average efficiency over a heating season in an average European climate. SCOP 5 means 5 kWh of heat per kWh of electricity on average. We show SCOP at 35 °C and 55 °C; the median in our database is about 4.9 and 3.7 respectively. The Boiler Upgrade Scheme uses its own SCOP figure from the MCS design for your home.

Seasonal efficiency ηs

The seasonal space heating energy efficiency ηs (in %) is the official figure behind the energy label. For heat pumps, roughly ηs = SCOP ÷ 2.5 × 100 − 3 ¹. Where a source only gives ηs, we convert it with this formula; for ground and water source units it slightly understates SCOP.

Energy label

The energy label for space heaters runs from A+++ to D and is based on ηs ¹. There is one class at 35 °C and one at 55 °C. Most modern heat pumps reach A+++ at 35 °C.

COP

The Coefficient of Performance is the efficiency at a single test point at full load. A7/W35 means 7 °C outdoor air and 35 °C water; A2/W35 and A-7/W35 show how efficiency drops in colder weather. COP is a snapshot and not the same as SCOP.

Heating capacity (A7/W35)

The heat output at 7 °C outdoors and 35 °C water. In frost an air source heat pump delivers less. Sizing should follow a room-by-room heat loss calculation, which MCS installers carry out.

Rated output (Prated)

The design output from the energy label at 35 °C or 55 °C, based on a design temperature of −10 °C in the average climate ¹. It says more about capacity on cold days than the A7/W35 figure.

Maximum power input

The highest electrical power the heat pump draws from the grid. Useful for checking your supply and whether your network operator needs to be notified.

Maximum flow temperature

The highest water temperature the heat pump can reach without a backup heater. A high maximum (60–75 °C) helps if you keep existing radiators or need a lot of hot water, but efficiency falls at those temperatures.

Minimum outdoor temperature

The lowest outdoor temperature at which the heat pump still produces heat. Below that, an immersion or backup heater takes over.

Capacity control

How the heat pump adjusts its output. A modulating (inverter) compressor slows down when little heat is needed, which is more efficient and quieter than switching on and off.

SG Ready

A standard interface that lets an energy management system, solar inverter or battery control the heat pump, for example to heat more when there is surplus solar or cheap power. Almost every heat pump in our database has it.

Boiler Upgrade Scheme

The grant in England and Wales for replacing fossil-fuel heating: £7,500 for an air-to-water or ground source heat pump ². Both the product and the installer must be MCS certified. See our heat pump grants guide.

MCS 020(a)

The noise standard for permitted development in England. An air source heat pump usually needs no planning permission if its level, calculated from sound power, stays at or below 37 dB LAeq,5min one metre from a neighbour’s door or window ³. That is why we show sound power for every outdoor unit.

EV chargers

AC and DC charging

At home you charge with alternating current (AC); the car’s on-board charger converts it to direct current for the battery. Rapid chargers supply direct current (DC) and are far more powerful – and far more expensive.

Maximum charging power

The charger’s highest output: 7.4 kW on a single-phase supply, 11 or 22 kW on three-phase. Most UK home chargers are 7 kW; what you actually get also depends on your supply and your car’s on-board charger.

Current per phase

The maximum current in amps per phase, usually 32 A for a 7 kW single-phase charger. Power ≈ voltage × current × phases: 230 V × 32 A ≈ 7.4 kW.

Minimum charging current

The lowest current at which charging can run; under the charging standard it is 6 A. On single-phase that is about 1.4 kW, on three-phase about 4.1 kW – which determines how small a solar surplus can be.

Type 2 connector

The European standard plug for AC charging, used on practically all new cars sold in the UK. A charger is either tethered (fixed cable) or untethered (a socket for your own cable); we list cable length separately.

1↔3 phase switching

The charger switches between single-phase and three-phase while charging, so it can keep charging on a small solar surplus. Only relevant if you have a three-phase supply.

Load balancing

Dynamic load balancing adjusts charging power to the rest of the home’s consumption so the main fuse is not overloaded. It usually needs a CT clamp or meter on the supply, and is especially useful in homes with a smaller main fuse or an electric shower, heat pump and charger on the same supply.

P1 port

The data port on Dutch and Belgian smart meters. It is not used in the UK; here chargers measure household demand with a CT clamp instead.

Solar surplus charging

The charger uses only, or mainly, the power your solar panels are exporting at that moment. It usually needs a CT clamp on the supply or a link to the inverter or energy management system.

Smart charging (UK regulations)

Since 2022, home and workplace chargers sold in Great Britain must be smart: able to connect to the internet, respond to signals and come with default settings that avoid charging at peak times, unless you change them ⁴.

OCPP

The Open Charge Point Protocol is the open standard a charger uses to talk to a management platform ⁵. It keeps you free to choose the app or provider. Version 1.6J is common, 2.0.1 is newer.

OCA certification

The Open Charge Alliance tests and certifies a charger’s OCPP implementation ⁵, which reduces compatibility problems when you switch platform.

ISO 15118 and Plug & Charge

A standard for communication between car and charger. It enables Plug & Charge (charging and paying without a card or app) and is required for bidirectional charging.

Bidirectional charging (V2H/V2G)

A bidirectional charger can feed power from the car battery to your home (vehicle-to-home) or the grid (vehicle-to-grid). Both the charger and the car must support it.

MID meter

An energy meter certified under the Measuring Instruments Directive (MID) ⁶. It gives trustworthy readings, for example when an employer reimburses home charging.

Eichrecht

German calibration law for chargers that bill third parties. It has no legal meaning in the UK but shows the metering has been independently verified.

RCD protection and 6 mA DC detection

A charger must be protected against leakage currents, including DC faults from the car. If the charger detects DC leakage from 6 mA itself, a Type A RCD is usually sufficient instead of a more expensive Type B. On a PME supply, UK wiring rules also require protection against a broken PEN conductor, which many UK chargers have built in. Your electrician decides based on the wiring regulations.

RFID

A card reader so charging only starts with an authorised card – useful for shared driveways or tracking sessions.

4G, Wi-Fi and Ethernet

How the charger connects to the internet for the app, updates, smart tariffs and OCPP. A cable is the most reliable; 4G helps when Wi-Fi does not reach the driveway.

Standby power

The charger’s consumption while not charging, in watts. A charger using 5 W on standby uses just over 40 kWh a year.

OZEV eligible

The Office for Zero Emission Vehicles keeps a list of authorised chargepoint models ⁷. A charger on the list meets the technical requirements for OZEV grant schemes.

Home batteries

Usable capacity

The energy in kWh you can actually take out of the battery. This is the figure to compare batteries on.

Nominal capacity

The total cell capacity according to the manufacturer. Part of it is held back to protect the cells, so usable capacity is usually a little lower. If no usable capacity is stated, we show the nominal figure.

Smallest configuration and expandability

Modular batteries are built from separate modules. The smallest configuration is the minimum set-up; “expandable up to” is the largest capacity you can reach later by adding modules.

Charge and discharge power

The power in kW the battery can take in or deliver continuously. A 10 kWh battery with 2.5 kW discharge power cannot fully cover an evening peak with the oven and kettle on.

Peak power

The power the battery can deliver for a short time (seconds to minutes), for example to start an appliance with a high inrush current.

Round-trip efficiency

How much of the stored energy you get back after charging and discharging. At 90%, you get 9 kWh out for every 10 kWh in; the rest is lost as heat.

Depth of discharge (DoD)

The share of nominal capacity that can be used. With 90% DoD and 10 kWh nominal, 9 kWh is usable.

Cell chemistry (LFP)

The type of lithium cell. Almost every home battery in our database uses lithium iron phosphate (LFP): slightly less energy per kilogram than other lithium types, but safer, cobalt-free and good for many cycles.

AC and DC coupling

An AC-coupled battery has its own inverter and works alongside any existing solar system – the usual choice for a retrofit. A DC-coupled battery connects to a hybrid inverter; slightly more efficient, but tied to that inverter brand.

Backup power

During a power cut the battery can keep (part of) the home running. This usually needs extra switchgear, and often only selected circuits are backed up.

Off-grid

The battery can run an installation with solar panels entirely without a grid connection. Rarely needed for an ordinary home on the grid.

Plug-in battery

A battery that plugs into an ordinary socket, often up to about 800 W, instead of being hard-wired by an electrician. Check with the manufacturer and your electrician whether a model may be used this way in the UK.

Dynamic tariff

The battery charges automatically when electricity is cheap and discharges when it is expensive, based on a time-of-use or half-hourly tariff. Many UK suppliers offer such tariffs.

Cycles and throughput warranty

A cycle is one full charge and discharge. Manufacturers guarantee a number of cycles or a total throughput in MWh; whichever comes first often ends the performance warranty, even if the years have not run out.

Air conditioners

Split, monoblock and portable

A split unit has an indoor and an outdoor unit connected by refrigerant pipes. A monoblock sits entirely indoors on an outside wall. A portable unit vents warm air through a hose and is the least efficient.

Cooling and heating capacity

The rated output in kW when cooling or heating. Too small and it won’t keep up; too large and it cycles on and off, which is less efficient.

SEER

The Seasonal Energy Efficiency Ratio is the seasonal efficiency when cooling: kWh of cooling per kWh of electricity over an average cooling season ⁸. Higher is better.

SCOP for air conditioners

The seasonal efficiency when heating, in the average European climate ⁸. It works like a heat pump’s SCOP but applies to warm-air heating, so it is not directly comparable with an air-to-water heat pump. Since April 2026, air-to-air heat pumps in homes can qualify for a £2,500 Boiler Upgrade Scheme grant in England and Wales ².

Energy label for cooling and heating

Air conditioners carry separate energy classes for cooling and heating, from A+++ to D, based on SEER and SCOP ⁸.

Indoor sound pressure

The sound pressure of the indoor unit on its quietest setting, measured close by. Important in a bedroom; around 20 dB(A) is very quiet.

Maximum pipe length

The longest distance allowed between indoor and outdoor unit, which determines how freely you can place the outdoor unit.

Heats down to

The lowest outdoor temperature at which the unit still heats. If you want to heat with it all winter, choose one that works to −15 °C or lower.

Solar panels

Watt-peak (Wp)

A panel’s output under standard test conditions (STC): 1,000 W/m² of sunlight and 25 °C cell temperature. Yearly generation also depends on orientation, pitch, shading and temperature.

Efficiency

The share of sunlight hitting the panel that is turned into electricity. Higher efficiency means more power per square metre – handy on a small roof. If the source doesn’t state it, we calculate it from peak power and panel area.

Cell type

The cell technology: monocrystalline (with variants such as PERC, TOPCon and heterojunction), polycrystalline or thin film such as CdTe and CIGS. Newer monocrystalline cells usually have higher efficiency and a better temperature coefficient.

Bifacial

Bifacial panels also capture light on the back. The gain is mostly on light-coloured surfaces or raised mounting on a flat roof or field; on a pitched roof it is small.

All black

Panels with black cells, backsheet and frame. Purely cosmetic; efficiency is often slightly lower because the panel runs hotter.

Temperature coefficient Pmax

How much output the panel loses per degree above 25 °C cell temperature. At −0.30 %/°C, a panel at 55 °C produces about 9% less than at STC. Closer to zero is better.

NOCT

Nominal Operating Cell Temperature: the cell temperature at 800 W/m², 20 °C ambient and a light breeze. A lower NOCT means the panel runs cooler and produces a little more.

PTC rating

Output measured under more realistic conditions than STC (1,000 W/m², 20 °C ambient, light wind), as used in the Californian CEC list. It is lower than the watt-peak rating.

Degradation

Loss of output as the panel ages. It is usually largest in the first year (often 1–2%) and slower afterwards, typically a few tenths of a percent a year. Both are set out in the performance warranty.

Performance warranty

The manufacturer guarantees the panel still delivers a minimum share of its original output after a number of years, for example 87% after 30 years. This is separate from the product warranty on materials and workmanship.

Voc, Isc, Vmp and Imp

The panel’s electrical characteristics: open-circuit voltage (Voc), short-circuit current (Isc), and voltage and current at maximum power (Vmp, Imp). Installers use them to design strings that suit the inverter.

Maximum system voltage

The highest voltage the panel is rated for in a string, usually 1,000 or 1,500 V. Together with Voc it limits how many panels can be wired in series.

Snow load

The mechanical load in pascals the panel can take on its front. 5,400 Pa is a common rating.

Fire class

The classification of the panel’s fire behaviour under a European or US standard. Relevant for roofs with specific fire requirements.

Solar inverters

AC power

The maximum alternating-current output the inverter delivers to your home and the grid.

Maximum PV array power

How much panel capacity (kWp) may be connected. It is often more than the AC power because panels rarely reach their full rating; in our database the median is 1.5 times the AC power.

MPPT

A Maximum Power Point Tracker constantly finds the operating point where the connected panels produce most. With several MPPTs you can connect roof faces with different orientations or shading separately.

Strings per MPPT

How many strings (panels in series) may be connected in parallel to one MPPT. Parallel strings must face the same way and have the same number of panels.

MPPT voltage range

The voltage window in which the tracker works best. The string voltage must stay inside it on cold and hot days alike.

Maximum input current per MPPT

The highest current one tracker can handle. Modern panels with large cells produce high currents; if they don’t fit, the inverter loses some yield.

European efficiency

A weighted average efficiency across load levels for a Central European climate. It is more realistic than peak efficiency; in our database it ranges from about 96.4% to 98.2%.

Maximum efficiency

The best efficiency at the ideal operating point. It sounds impressive, but inverters mostly run at part load.

Hybrid inverter

An inverter that a home battery can connect to directly (DC-coupled). If you plan a battery later, a hybrid inverter can save a second inverter; check which battery brands it supports.

Microinverter

A small inverter per panel (or per two to four panels). Each panel works independently, which helps with shading and complex roofs.

Inverter backup function

Some (hybrid) inverters can supply a backup output during a power cut, usually only with a battery and sometimes through a separate socket or changeover switch.

G98 and G99

The engineering recommendations for connecting generators, including solar inverters and batteries, to the GB distribution network. G98 covers small systems up to 16 A per phase, notified after installation; larger systems fall under G99 and need approval from the network operator first. The ENA Type Test Register lists inverters that have been type-tested against them ⁹.

Inverter noise

Inverters with a fan hum or whir. Avoid mounting a noisy unit in a living room or bedroom.