You've got three quotes from different solar installers, and no two look the same. One quotes a price for a "4kW system" and little else. Another names every component. A third includes a battery the other two didn't mention. The system sizes, brands and savings projections all differ.
Here's how to make sense of them.
Start with the system size
Every quote should state the total system size in kilowatts (kW): the combined output of all the panels. A 4kW system means four kilowatts of peak generation capacity under standard test conditions.
But system size alone doesn't tell you much. A 4kW system using eight 500W panels is not the same as one using twelve 330W panels. The first uses fewer, more efficient panels and takes up less roof space. The second uses more panels of an older, less efficient design. Both are "4kW" on paper.
What to check: the number of panels, the wattage per panel, and the panel brand. If a quote doesn't name the panel brand and wattage, ask for it.
Check the inverter
The inverter converts DC electricity from the panels into AC electricity your home can use. It's the most technically complex component in the system and the one most likely to need replacing within the system's lifetime.
What to check: inverter brand and model, warranty length, and whether it's a standard string inverter or a hybrid inverter (which can also manage a battery). If you might add a battery later, a hybrid inverter now saves replacing the inverter later.
Battery: included or not?
Some quotes include a battery. Some don't. Make sure you're comparing like with like. A quote with a battery will always cost more than one without, but the two are solving different problems.
If a battery is included, check the brand, capacity in kWh, chemistry (LFP is generally preferred over NMC for safety and longevity), and warranty length. If the battery is optional, ask for the quote with and without, so you can compare panel and inverter costs directly.
What the price includes
Check what each total price covers: panels, inverter, battery, scaffolding, labour, the DNO application and MCS registration. Two quotes only compare fairly when they cover the same things.
If an item isn't mentioned, ask the installer to confirm in writing whether it is included.
Warranty comparison
There are three warranties to compare.
- Panel product warranty. Covers manufacturing defects. Tier-1 manufacturers typically offer 25 years. Budget brands may offer 10 to 15 years.
- Panel performance warranty. Guarantees the panels will still produce a minimum percentage of their rated output after 25 years. Look for guaranteed output of at least 80% at year 25.
- Workmanship warranty. Covers the quality of the installation itself. This is the installer's warranty, not the manufacturer's. Check how long it lasts, whether it's insured (through a scheme like IAA's IBG), and what happens if the installer goes out of business.
Savings projections
Every installer will give you a savings estimate. These are projections, not guarantees. The figure depends on your electricity usage, how much you use in daylight hours, your tariff rate, the Smart Export Guarantee rate for surplus, and future energy prices that nobody can predict.
What to check: are the savings based on your electricity bill, or generic national averages? Does the projection assume current energy prices, or rising ones? If it assumes rising prices, what rate is the installer using and where does it come from?
Who does the installation?
Ask whether the company uses its own employed installers or subcontractors. If they subcontract, ask who the subcontractor is and whether they're separately MCS-certified. It affects quality control, accountability, and what happens if something goes wrong after install.
The comparison table to build
Put all three quotes side by side and fill in these rows.
- System size (kW)
- Number of panels and wattage per panel
- Panel brand and product warranty
- Inverter brand, model, and warranty
- Battery: yes or no, brand, capacity, warranty
- Total price (including VAT)
- Price per kW installed
- Scaffolding: included or extra?
- DNO application: included or extra?
- MCS certification: yes, and which scheme?
- Workmanship warranty: length and insured by whom?
- Own installers or subcontractors?
- Savings projection: based on your bill or generic?
An installer who can't or won't fill in every row isn't giving you a complete quote. The best quote isn't always the cheapest. It's the one that gives you the clearest picture of what you're buying, who's installing it, and what happens if something goes wrong.
