Renewable heating and renewable energy are very jargon and acronym rich subjects. To help navigate through them, here is a glossary of some of the most frequently used ones on the forum. If you find any others that aren't explained here or have any suggestions, please let us know by posting a response to this thread. We'll incorporate the suggestions  as appropriate into this post and delete the responses to keep the thread clean and useable.

AC - Alternating current

An electrical current that keeps changing direction. The UK operates domestic mains electricity at 230V ac at a frequency of 50Hz, but both voltage and frequency can vary by country.

API - Application programming interface

A standardised means of sending and receiving data, often over the Internet.

ASHP - Air source heat pump

An appliance that cools the air outside and dumps that heat into a home, thereby keeping the occupants warm. Strictly, an air source heat pump could be configured to work the opposite way round as well, at which point it would be working as we commonly think of as air conditioning, although this is not always practical for every setup.

BESS - Battery energy storage system

Exactly as it says, a means of storing electricity in one or more batteries for use later. Typically used to store home-generated electricity from domestic solar panels or electricity bought from the grid at cheap times so it can be consumed when the grid unit price is high.

BUS - Boiler upgrade scheme

A Government funded incentive scheme for homeowners wanting to move from fossil fuel heating to renewable energy heating. The BUS took over when the RHI was retired.

CAN (bus) - A communications protocol commonly for talking wit pieces of hardware

See also Modbus. CAN bus or controller area network bus in the context of this forum's areas of discussion is most commonly used for communication with electric vehicles, home storage batteries and domestic inverters. It is a protocol (a language, if you like) normally used over a two-wire cable. See also RS485.

CoP - Coefficient of Performance

See also SCoP. The coefficient of performance is a measure of how efficiently a heat pump or boiler converts fuel energy (electricity, gas, oil) into heat energy. By dividing the output energy in kWh by the input energy in kWh you get a ratio and that particular ratio is called the CoP, and a CoP of 3 is the same as saying the heat pump is 300% efficient, or that 1 kWh in is converted into 3 kWh out. However, a CoP figure is calculated as a point in time measure or, more commonly, as a measure of efficiency over a short period of time. Therefore, a heat pump could vary its CoP quite significantly over a heating season.

DC - Direct current

An electrical current that flows in a single direction

DCC - Data Communications Company

The company who operate the encrypted messages system between smart meters and energy suppliers

DESNZ - Department of Energy Security and Net Zero

The government department, headed by the Secretary of State, which defines energy policy for the UK

DHW - Domestic hot water

The water in a hot water tank, as opposed to the water running through radiators and underfloor heating

DNO - Distribution Network Operator

A company which holds a licence for maintaining the distribution grid for electricity up to 132kV within a given geographical region. There are multiple geographical regions and so multiple DNOs. DNOs are distinct and separate companies to the energy suppliers like EDF, Octopus, OVO and the like.

FT - Flow temperature

The temperature of the water just after it has left the heat pump on its way to the radiators or water tank. See also LWT.

HV - High voltage

The distinction between high and low voltage electrical circuits depends hugely on context. However, typically in a domestic setting, mains electricity (230 volts AC) is seen as high voltage whereas LED lights and so forth may well be run on 12 volts DC which is thought of as low voltage.

IAT - Internal (or indoor, or inside) air temperature

The temperature of the air inside a home

kW - kilowatt

A unit used to express power, most typically electrical power; the more kilowatts, the more powerful something is. Not to be confused with kWh

kWh - kilowatt hour

A unit used to express energy. It combines the power rating (usually in kW) with duration (typically hours) where running something at 1 kW for one hour will use 1 kWh of energy and running it at 2 kW for 3 hours will use 6 kWh energy. Not to be confused with kW (which expresses power, not energy)

LV - Low voltage

The distinction between high and low voltage electrical circuits depends hugely on context. However, typically in a domestic setting, mains electricity (230 volts AC) is seen as high voltage whereas LED lights and so forth may well be run on 12 volts DC which is thought of as low voltage.

LWT - Leaving water temperature

Another acronym meaning the same as flow temperature; the temperature of the water just after it has left the heat pump on its way to the radiators or water tank. See also FT.

MCS - Microgeneration Certification Scheme

What this scheme is or isn't is a matter of active debate. The important point here is that any renewable heating solution must be installed by an MCS-registered installer and in accordance with MCS regulations in order to qualify for the BUS grant.

Modbus - A communications protocol commonly for talking with pieces of hardware

Many pieces of renewables hardware (solar inverters, heat pumps and so on) are designed to be hooked up to a two-wire cable so systems can communicate with the kit. That communication could be carried out over that wire using several different protocols (languages, if you like) and modbus is one of those protocols. Not all kit supports modbus, however, and lack of standards in this area is a common bugbear. See also RS485

NESO - National Energy Systems Operator

This is an organisation which runs the electricity, gas and heat networks for Great Britain

NGET - National Grid Electricity Transmission

This is an organisation which is licensed to maintain the electricity transmission grid for England and Wales. This part of the national grid operates at high voltages - 275kV and 400kV

OAT - Outside air temperature

The temperature of the air outside any buildings

OFGEM - Office for Gas and Electricity Markets

OFGEM is the regulator for the energy industry. It issues licences for energy suppliers and operators.

PV - Photovoltaic

Typically used in conjunction with the term "solar". Solar PV - generating electricity from sunlight - is generally mentioned in order to distinguish it from solar thermal which uses solar energy to heat water.

RHI - Renewable heating incentive

A Government funded incentive scheme for homeowners wanting to move from fossil fuel heating to renewable energy heating. Superceded by the BUS

RS485

RS485 is an electrical communications standard which, without overcomplicating, lets a two-wire cable be used to send and receive signals. Modbus and CAN bus are two communications protocols that are commonly used transmitted over RS485 cables. In the well known telephone analogy of two cans and a piece of string, RS485 represents the string and cans (the hardware bit) and modbus or CAN bus represent the chosen language of the two people holding the cans; communication won't work if one speaks German and the other speaks Japanese.

RWT (or sometimes RT) - Return water temperature

The temperature of the water just as it returns to the heat pump on its way back from the radiators or water tank. N.B. RT can occasionally also be used to denote room temperature which is synonymous with IAT (internal air temperature)

SCoP - Seasonal Coefficient of Performance

See also CoP. Where the CoP is a measure of heating efficiency at a point in time, the SCoP is a measure of efficiency over a whole heating season. It is calculated by dividing the heat energy in kWh delivered over a whole year by the fuel energy (electricity, gas, oil) consumed in order to deliver that heat. This averages out the performance across good and bad periods and gives a far more accurate understanding of a heat pump or boiler's true efficiency.

SMETS2 - A standard for smart meters

A smart meter is a means not only of measuring consumption of water, gas or electricity but also of communicating that consumption regularly back to the utility provider. There's a lot of infrastructure involved in making that happen and SMETS2 is a list of standards the meter has to abide by to be allowed access to that infrastructure.

TOU - Time of use

Some electricity tariffs vary the price of a unit of electricity according to when that electricity is consumed. These are termed Time Of Use tariffs. The term can cover relatively predictable tariffs designed for electric car charging (one normal price for most time periods during a day and one lower price for times set aside for car charging) as well as highly volatile tariffs - such as Octopus Agile - with different prices per half-hour period with those prices being decided one day ahead in line with wholesale energy prices.

UFH - Underfloor heating

Snakes of water pipes laid under a room's flooring to make the floor one big radiator. The large surface area means it doesn't have to get very warm to raise the room temperature a significant amount and so it is an excellent pairing to a heat pump.

ΔT (or DeltaT) - The difference between two temperatures

The difference in temperature between two points in a heating system - most commonly the water leaving a heat pump and returning - combined with the flow speed to determine the volume of water, is used to work out how much energy has been released as heat. This information is important in any tweaking of a heat pump.