Comparing solar quotes

Put every quotation on the same basis

Write down the job before comparing equipment. Include the property, roof areas, present electricity use, daytime demand, expected EV or heat-pump load and whether a battery is planned.

Mark any differences between surveys:

• roof planes included
• obstructions and access
• shade sources
• roof repair or replacement
• panel layout
• battery now or later
• scaffolding and lifting
• consumer-unit or meter-area work
• planning or listed-building restrictions

A cheaper quotation may use fewer panels, exclude a difficult roof plane or assume that another contractor will complete electrical or roof work. A more expensive one may include scaffolding to several elevations, structural input and a new distribution enclosure. Compare scope before dividing the total by the number of panels.

For the full survey-to-handover sequence, see the home solar installation guide.

Roof condition and structural evidence

The roof survey should establish condition, covering, structure, access and remaining service life. Installing PV above a roof that soon needs replacement creates avoidable removal and reinstatement work.

Ask each installer to identify:

• roof covering and fixing approach
• rafter, purlin or deck information used
• visible defects and areas not inspected
• proposed attachment positions
• array edge zones and set-backs
• wind and snow design inputs
• additional dead and imposed loads
• responsibility for structural calculations
• weathering detail around every fixing

Under the applicable MCS installation standard, a competent person assesses a pitched roof for the proposed loading. An unusual roof or unresolved doubt needs suitable structural input. A ballasted flat-roof design has a different evidence route and needs a qualified structural engineer.

MCS 012 concerns certification of a mounting product and its use conditions. It does not prove that the property’s roof can carry the selected array. Ask for both the mounting-system certificate or project-specific route and the site assessment.

Where installers propose different attachment systems, compare exact compatible components and instructions. A rail brand by itself does not establish the tested roof hook, fixing, module clamp or roof-covering detail.

Array layout, orientation and shade

Request a scaled plan showing every module, roof edge, obstruction, orientation and pitch. Keep materially different roof planes separate in the performance information.

Shade assessment should represent the sun path, seasonal movement and the exact array area. A remote image may miss recent trees, chimneys, aerials, neighbouring buildings or a dormer. If a remote survey is used for the quotation, the installer should state which assumptions will be confirmed on site.

Compare the physical response to shade before the electronics:

• moving a module or whole array
• shortening or separating a shaded string
• assigning roof planes to separate MPPTs
• leaving a persistently poor area unused
• using module-level electronics where the design supports them

MPPT is the inverter input control that keeps an attached array section near its useful operating point. Different roof orientations or shade patterns may need separate trackers.

Optimisers and microinverters can change how mismatch is managed, but they do not make shade disappear. They also add roof-mounted electronics, connectors and product dependencies. The quote should explain the actual problem they address.

Comparing the performance estimates

The MCS annual PV estimate uses the array kWp, an orientation and location factor and the shade factor. It provides a common method for comparison. It is not a guarantee of weather or annual generation.

Check that each estimate shows:

• postcode or location basis
• array kWp
• orientation and pitch for each plane
• shade factor and assessment method
• estimated annual AC kWh
• changes expected after the site survey

kWp is the array’s peak DC nameplate rating under standard test conditions. kWh is energy over time. Neither is the inverter AC power.

An advanced interval model may add explicit treatment of temperature, inverter loss, clipping, wiring, soiling and time of generation. It should add information rather than replace the standard estimate with an unexplained higher number.

Annual generation must also remain separate from:

• self-consumption
• grid export
• self-sufficiency
• bill reduction

Self-consumption depends on the timing of household loads, battery operation and EV charging. Ask for the household profile and tariff assumptions used in any financial model.

Remove current tariff rates and export payments from the technical comparison, then apply a consistent set yourself. Otherwise a quotation can look better because it uses a more favourable energy-price assumption rather than a better system.

Panels and usable roof area

Compare the exact module manufacturer, model and datasheet. Panel wattage helps determine how much nameplate capacity fits on the roof, but it is not a complete quality measure.

Record:

• module dimensions and weight
• nameplate W
• electrical current and voltage data
• mechanical-load and fire information used by the design
• connector type
• product warranty
• performance warranty
• replacement and labour terms

A higher-wattage module may produce more kWp from a constrained roof. On a roof with spare area, a lower-wattage proposal may reach the same target with a different layout. Compare the complete array and annual estimate.

Check whether the proposed module is compatible with the mounting, inverter and any optimiser. Similar connector housings should not be assumed compatible without the declared combination.

Inverter, strings and MPPT allocation

Ask for a single-line schematic or design drawing showing:

• module count per string
• roof plane and modules on each string
• string allocation to each MPPT
• DC voltage and current checks
• inverter DC input limits
• inverter AC rating
• isolators and protection
• cable routes

Series string voltage rises in cold conditions and falls as modules heat. Parallel string current adds. The design must stay within the inverter’s maximum voltage, start-up, MPPT and current limits under the applicable conditions.

Do not attempt to validate the design only by multiplying the module count by one voltage from the datasheet. Ask the installer for the completed check and exact product data.

DC array oversizing can allow the inverter to operate nearer its useful output for more of the day. It can also create some clipping when potential DC power exceeds the active AC limit. Compare modelled annual energy, not only the size of the highest visible peak.

A flat top in monitoring data can also result from export limitation, thermal derating, grid response or another inverter limit. A proposal should distinguish intentional inverter sizing from a network export constraint.

Battery-ready claims

“Battery ready” can refer to several different arrangements:

• spare space for a later AC-coupled battery
• a hybrid inverter with a compatible battery DC input
• communication support for a named battery range
• an installed meter or CT (current transformer) for energy control
• a DNO application that already includes storage

Ask which one is meant. For a hybrid inverter, request the compatible battery voltage range, power limits, available MPPT capacity, backup options and current supported product list.

A later AC-coupled battery uses a separate inverter and may be a practical retrofit. It also changes the aggregate generation and storage information considered by the DNO.

If a battery is likely, compare a solar-only proposal, a hybrid-inverter proposal without a battery and a complete battery design. Do not pay for an undefined future feature that has not been checked against the desired battery or backup job.

DNO connection route

The DNO route should be explicit in each quote. In Great Britain, qualifying Fully Type Tested generation with aggregate Registered Capacity no greater than 16 A per phase can use G98. Other installations normally follow G99.

Ask for:

• existing generation and storage considered
• inverter and aggregate Registered Capacity
• G98 or G99 route
• whether connection can occur before notification
• any required application before installation
• proposed export limit
• responsibility for DNO forms and responses
• costs if network work or a revised design is required

A G100 export-limitation scheme controls net flow at the grid connection. It does not erase the installed Registered Capacity or automatically make a larger system G98.

Northern Ireland uses separate G98/NI and G99/NI documents through NIE Networks. A Great Britain route should not be copied into a Northern Ireland quotation.

Electrical installation and cable routes

Compare the visible and concealed electrical work:

• inverter position
• DC and AC cable routes
• roof entry and fire stopping
• DC and AC isolation
• surge protection assessment
• consumer-unit or separate-enclosure work
• earthing and bonding
• meter and CT placement
• labels and system diagram
• Building Regulations notification

A spare consumer-unit way does not prove that the board, supply or existing installation is suitable. The installer must assess the existing electrical equipment affected by the work. That assessment determines whether a full EICR is required.

For an inverter in a loft or other restricted space, compare access, temperature, fire and maintenance arrangements. A short cable route does not override product or building constraints.

Make good should be stated. Roof penetrations, internal cable routes, exterior conduit, distribution equipment and monitoring hardware can all leave work outside an electrical-only quote.

Planning and other permissions

Solar PV can be permitted development where the current nation-specific conditions are met. The rules and exceptions differ across England, Wales, Scotland and Northern Ireland.

Ask every bidder to state:

• whether the proposal relies on permitted development
• the rules checked for the UK nation
• listed-building or conservation-area implications
• landlord, freeholder or leasehold permission
• responsibility for an application or certificate where needed

Do not accept one installer’s planning assurance as a technical advantage unless it relates to the same final layout and has been checked against the actual property.

Warranties and financial protection

Separate the documents:

• module product warranty
• module performance warranty
• inverter and optimiser warranty
• mounting-system warranty
• installer workmanship warranty
• labour or access cover
• insurance-backed guarantee
• deposit or advance-payment protection

A long module performance warranty usually concerns a defined output-retention curve. Check separately whether it pays for fault diagnosis, scaffolding, removal, transport or fitting a replacement.

Read registration, installer-status, maintenance, internet-connectivity and transfer conditions. The legal entity offering workmanship cover should match the contracting installer.

Manufacturer longevity is relevant, but it cannot be established by a “Tier 1” label alone. Ask how claims are made in the UK, who supplies replacement labour and what happens if the original installer is unavailable.

Monitoring and owner access

Compare the data included:

• inverter generation
• household consumption
• grid import and export
• panel-level data where applicable
• alerts and event history
• local display or local access
• cloud account and subscription
• API or data export

Some values require an additional meter or CT. Generation measured inside the inverter is not the same boundary as smart-meter export. Confirm which sensors are included and where they will be installed.

The owner should control the main account. Ask whether changing installer, router, supplier or mobile phone affects access.

Installation and handover scope

Use one final schedule for all bidders:

• scaffolding and edge protection
• roof survey and structural evidence
• mounting and weathering
• modules, inverter and electrical work
• DNO submission
• planning support
• testing and commissioning
• waste removal and making good
• MCS certificate where applicable
• Building Regulations evidence
• electrical certificate
• array plan, schematic and test results
• product datasheets and warranties
• monitoring setup and owner demonstration

The quote should state the handling of hidden roof defects, an unsuitable consumer unit, revised DNO conditions and layout changes found after access is erected.

Compare the final contract after clarifications have been incorporated. An email answer that materially changes equipment or scope should not remain separate from the signed quotation.