The words the industry uses, in plain English

Availability

The share of time a system was switched on and able to generate. Monitoring apps love this number because it is usually close to 100%. It says nothing about how much the system actually produced while it was on.

Battery (BESS)Battery energy storage system

A battery, its inverter and controls. Stores electricity from your solar or from cheap overnight tariffs to use later. Whether one pays for itself depends on your tariff and your evening usage, not on the size of your roof.

Degradation

The slow loss of output as solar panels age, typically a fraction of a per cent a year. Panel warranties promise a maximum rate. Measuring whether yours is within it is part of a health check.

DNODistribution network operator

The company that owns the local electricity wires. Anything that generates or stores electricity needs their permission to connect, and the permission sets limits on how much you can export. Small systems are notified after the fact; larger ones need approval first.

EPCEngineering, procurement and construction

On larger projects, the contractor who designs, buys and builds the system, and the contract that governs it. For a house or a small business, your installer is doing the same job under a simpler contract.

Export

Electricity your system produces that you do not use, which flows out to the grid. You are paid for it, but usually a fraction of what you pay to buy electricity in, which is why using your own generation matters more than making lots of it.

Export limitation

A cap, set by the network operator, on how much power a system may send to the grid. It can make a larger system pointless unless a battery or your own usage absorbs the surplus.

Feed-in Tariff and SEG

The Feed-in Tariff paid for generation and export on systems installed up to 2019; those payments continue. The Smart Export Guarantee replaced it for export only, at rates set by each supplier. Which one you have changes what a battery is worth.

G98 and G99

The rules for connecting generation to the UK grid. G98 covers small systems, which are notified to the network operator after installation; G99 covers larger ones, which need approval before.

Inverter

The box that converts the direct current from solar panels into the alternating current your house and the grid use. Inverters fail long before panels do, usually after ten to fifteen years, and are the first big replacement cost an owner faces.

Irradiance

How much sunlight is landing on the panels. Every performance calculation depends on it, and the figures a quote uses for your location are worth checking.

kW, kWp, kWh

A kilowatt is a rate of power; a kilowatt-peak is a solar system’s rated size under standard test conditions; a kilowatt-hour is an amount of energy, and is what your bill counts. A "4 kW system" is 4 kWp and might produce about 3,500 kWh a year in central Scotland, more in the south of England.

Load profile

How much electricity a building uses, hour by hour, through the year. The single most important input to sizing a system correctly, and the one most quotes ignore in favour of roof area.

MCSMicrogeneration Certification Scheme

The UK quality mark for small renewable installations and installers. Needed for export payments and for most warranties. It certifies that a system was installed to a standard, not that it was the right system for you.

O&MOperations and maintenance

The contract and the contractor that look after a system once it is built. What it includes, what it excludes, and how quickly they must respond to faults are where most owners lose money without noticing.

Owner's engineer

An engineer who works for the owner rather than the installer, specifying the job, checking the quotes and confirming what was built matches what was promised. Standard on large projects; almost unheard of for a house. That is what project support is.

Payback

How many years until the savings and export income equal what you paid. Quotes often assume electricity prices rise every year and that you use all your generation; neither is guaranteed. A second opinion recalculates it on your usage and tariff.

Performance ratio (PR)

The headline health number for a solar system: what it produced divided by what a perfect system of the same size would have produced under the same sunlight. Useful, but it hides sensor errors, downtime and temperature effects.

PPA and roof leasePower purchase agreement

Arrangements where someone else owns the panels on your roof and you buy the electricity, or lease them the roof. "Free solar" is usually one of these. The contract decides who is responsible for what for twenty years or more, so it is worth an engineer’s read before signing.

Self-consumption

The share of your solar generation you use yourself rather than export. The higher it is, the better the economics. It is raised by timing usage, by a battery, or by sizing the system to the building rather than the roof.

String

A group of solar panels wired together in series. Faults often affect one string at a time, which is why monitoring that only shows whole-system output can hide them for years.

Yield

The energy a system produces over a period, usually a year, in kilowatt-hours. "Specific yield" divides it by system size so systems of different sizes can be compared. A quote should state the yield it assumes and where the figure comes from.

Missing a term? Tell me and I will add it.