Panel installation and wiring

Solar PV Mistakes to Avoid: What UK Installers See Homeowners Get Wrong

Most solar PV problems are preventable, and most of them happen before the scaffolding goes up. This guide covers the mistakes UK homeowners most commonly make when choosing a system, vetting an installer, handling DNO notification, and managing the paperwork after installation.

Useful reference. Artisan Electrics is a Cambridge-based solar and electrical installation company that publishes practical, installation-focused advice for UK homeowners. "Don't get Solar PV until you've watched this video!" covers key considerations homeowners overlook before committing to a solar purchase. Channel: Artisan Electrics.

1. Do not size the system around the panels alone

Sizing a solar system based on available roof space is one of the most common and costly errors. Before talking to any installer, pull 12 months of electricity bills and work out your average daily consumption in kWh. Your installer should be designing the system around that figure, not around how many panels will fit in a neat arrangement on the south-facing section of your roof.

Oversizing relative to your consumption means you generate electricity you cannot use or export at a useful rate. Undersizing, when a larger system was feasible, leaves cheap generation capacity unused. If you plan to add an EV charger or a battery in the next two or three years, tell the installer now so the system design can accommodate the additional load.

2. Do not skip the shading assessment

A chimney, a neighbouring roof line, or a tree that casts shade across your panels for two hours around solar noon can reduce annual yield by 20 to 30 percent. A proper site survey includes a shading analysis using a tool such as the Solar Pathfinder or equivalent software, not just a visual inspection on the day of the survey.

If shading is identified, micro-inverters or DC optimisers can reduce the impact by allowing each panel (or a small group of panels) to operate independently rather than pulling the whole string down to the output of the most-shaded panel. These add cost, so weigh the additional outlay against the projected yield improvement. Ask the installer to show you the yield calculations with and without optimisers so you can make the comparison.

3. Do not assume the cheapest inverter is acceptable

Panels are typically warrantied for 25 years. Inverters are the component most likely to need replacement within that period. A budget inverter with a five-year warranty from a brand with no UK distribution network is a false economy: when it fails out of warranty, getting a replacement can take weeks and cost as much as the original unit.

Ask the installer to name the inverter manufacturer, state the warranty length, and confirm that spare parts and service are available in the UK. Established brands with UK distributors and 10 to 12 year warranties cost more upfront but are cheaper over the life of the installation. The inverter should be specified by model name in the written quote.

4. Do not energise the system before DNO notification is complete

Every grid-connected solar system must have a notification submitted to the local distribution network operator (DNO) before it is switched on. G98 applies to most domestic installations (up to 3.68 kW inverter output per phase, no battery export to grid); G99 applies to larger systems and to any installation where a battery will export to the network. G99 requires formal DNO approval, which can take up to 45 working days.

A professional installer handles DNO notification as a routine part of the job and provides you with the submission reference before energising the system. If an installer tells you DNO notification is not required for your installation, or that you can just plug it in and it will be fine, that is a serious red flag. Energising without notification can invalidate your home insurance and creates liability if a fault on your system affects the wider network.

5. Do not pay for the full system before work is complete

A deposit of between 10 and 25 percent of the total contract value is standard practice in the solar industry. Paying the full balance before installation begins is not. Structure the payment schedule so that the final payment is made only once the system is commissioned, the MCS certificate is in your hand, and you have received all supporting documentation.

Documentation to receive before making final payment: the MCS certificate (in the installer's registered legal name), the DNO notification reference or G99 approval letter, the inverter warranty confirmation, the commissioning report, and any roof warranty for penetrations. Never pay the full amount in cash. If an installer insists on full payment upfront or in cash, walk away.

6. Do not ignore the MCS certificate once you have it

The MCS certificate is your key to Smart Export Guarantee payments. To receive SEG payments for electricity you export to the grid, you must register with an energy supplier that offers a SEG tariff, and registration requires the MCS certificate. Most suppliers accept applications online; do it as soon as you receive the certificate, not months later when you notice the bill is smaller than expected.

File the MCS certificate, the DNO notification reference, the inverter warranty, and the commissioning report somewhere you can retrieve them. They are required if you sell the property, if you make an insurance claim involving the solar system, or if you apply for future grant schemes. Losing them does not make them unrecoverable, but replacing them takes time and paperwork you would rather avoid.

Frequently Asked Questions

What is the most common mistake when sizing a solar PV system in the UK?

The most common sizing mistake is choosing the number of panels based on available roof space rather than on actual electricity consumption. Before getting quotes, pull 12 months of electricity bills and calculate your daily average usage in kWh. This gives the installer a target to design against. A system that generates electricity you cannot use or export profitably is oversized; one that meets only a fraction of your consumption when a larger system was feasible is undersized. Both mistakes cost money in different ways.

Can I turn on my solar panels before DNO approval?

No. Any grid-connected solar system must have a G98 or G99 notification submitted to and acknowledged by your distribution network operator before the system is energised. G98 is a simpler notification for systems up to 3.68 kW per phase with no battery export; G99 requires formal approval and can take up to 45 working days. Energising without notification can invalidate your home insurance and puts you in breach of your grid connection agreement. A competent installer handles this as a matter of course and provides you with the submission reference.

What documents should I receive after a UK solar installation?

After a complete and compliant solar installation you should receive: the MCS certificate (issued in the installing company's registered name), the DNO notification reference or approval letter (G98 or G99), the inverter warranty card or digital registration confirmation, a commissioning report showing measured output at switch-on, and any roof penetration warranty from the installer. Keep all of these. The MCS certificate is required to register for Smart Export Guarantee payments and to evidence the installation when you sell the property.

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