Two solar quotes for the same ‘4kWp system’ can describe genuinely different installations while showing a single headline price. The terminology used — kWp, degradation warranty, inverter clipping, DNO notification — isn’t explained on most quotes, which makes comparing value rather than just price difficult for anyone outside the industry.
This guide explains what the standard terms on a UK solar quote actually mean, so you can evaluate any quote you receive on its merits rather than its headline number alone.
kWp is the maximum power a system can produce under standardised test conditions — full sunlight, optimal panel angle, 25°C. It’s a capacity rating, not a real-world output prediction. A ‘4kWp system’ will never actually generate 4kW continuously in the UK; it’s a specification used to size the system, not a promise.
This is the number that actually determines your savings — the total electricity, in kilowatt-hours, the system is projected to generate over a full year, accounting for UK daylight hours, weather patterns, and your specific roof orientation. A quote should state this figure explicitly. If it doesn’t, ask for it, and cross-check it against an independent estimate using the EU’s free PVGIS tool (ec.europa.eu/pvgis).
Self-consumption is the portion of generated electricity you use directly in your home. Export is the surplus sent to the grid, which earns Smart Export Guarantee payments. A quote may include an assumed self-consumption percentage (commonly 30–50 percent without a battery) — this assumption affects the savings projection significantly, so check what it’s based on.
Panels are informally categorised in Tiers based on manufacturing scale and financial stability — Tier 1 manufacturers (LONGi, JA Solar, Canadian Solar, Jinko Solar, REC) have large-scale automated production and established track records. A quote should name the specific brand and model, not just ‘Tier 1 panels’ as a category.
All solar panels lose a small amount of efficiency each year — typically around 0.4 to 0.6 percent annually for quality panels. The degradation warranty guarantees a minimum output level at a set point, commonly 25 years — usually 80 to 87 percent of original rated output. This is separate from the product warranty, which covers manufacturing defects.
The inverter converts DC electricity from your panels into the AC electricity your home uses. A string inverter is a single central unit; microinverters are individual units per panel. Inverter clipping occurs when the panels’ potential DC output exceeds what the inverter can convert — a small, deliberate amount is standard design practice, but excessive clipping wastes generation capacity you’ve paid for.
The Microgeneration Certification Scheme is the UK’s accreditation standard for small-scale renewable installations. A quote from a non-MCS-certified installer, or using non-MCS-certified equipment, means the installation will not qualify for the Smart Export Guarantee — verify this explicitly, don’t assume it.
The Smart Export Guarantee pays you for surplus electricity exported to the grid. A properly written quote confirms in writing that the completed installation will be SEG-eligible. If this isn’t mentioned, ask for it directly before signing.
Before a grid-connected solar system can be switched on, your installer must notify (or in some cases apply to) your Distribution Network Operator — the company managing the local electricity grid. This should be included in the quoted price, not billed separately afterward.
Part P of the Building Regulations covers electrical safety work in dwellings. A compliant solar installation requires a Part P completion certificate, confirming the electrical work meets UK safety standards. This should be included in your quote as standard, not an optional extra
System size in kWp and projected annual output in kWh
Named panel brand, model, and degradation (performance) warranty length
Named inverter brand, type (string/microinverter/optimiser), and warranty length
Written confirmation of MCS certification and SEG eligibility
Scaffold hire, DNO notification, and Part P certificate — all included, not extras
Workmanship warranty, separate from the panel and inverter warranties
kWp stands for kilowatt-peak — the maximum output your system can generate under ideal laboratory conditions (strong direct sunlight, optimal panel angle, 25°C). It is a rating of system capacity, not a promise of actual output. The more useful figure for judging real-world performance is annual kWh (kilowatt-hours) — the total electricity the system is projected to generate over a year, accounting for UK weather and daylight patterns.
A degradation warranty (also called a performance warranty) guarantees that a panel will still produce a minimum percentage of its original rated output after a set number of years — typically 80 to 87 percent after 25 years for quality Tier 1 panels. It matters because all solar panels lose a small amount of output annually; the degradation warranty is your protection if a panel underperforms that guaranteed curve.
Inverter clipping happens when a solar array’s peak DC output exceeds the inverter’s maximum AC output capacity, so the excess generation is ‘clipped’ and lost rather than converted to usable electricity. A small amount of clipping (a slightly undersized inverter relative to panel capacity) is normal industry practice and often improves value for money. Significant clipping — noticeable underperformance on sunny days — indicates a poorly matched system design.
Yes — ask directly. Any MCS-certified installation using MCS-certified equipment should qualify for the Smart Export Guarantee, but this should be explicitly confirmed in writing by the installer, not assumed. If a quote is silent on SEG eligibility, request written confirmation before signing.
A string inverter is a single unit that converts DC electricity from all panels connected in a series (a ‘string’). Microinverters are individual small inverters attached to each panel, converting DC to AC at the panel level. String inverters are typically cheaper and suit unshaded, uniformly-oriented roofs. Microinverters cost more but perform better on roofs with partial shading or multiple orientations, since each panel operates independently.
Abhishek Prajapati
Abhishek Prajapati
Abhishek Prajapati
