How to read a solar PV quote: 9 points that set a good proposal apart
Hybrid Solar team · 26 July 2026 · 5 min read
Two proposals for the same house can differ by €3,000 and look identical on paper. The difference almost always lies in what isn't written down.
Why comparing proposals is so hard
Ask for three quotes for the same house and you will get three documents with similar capacities and very different prices. The natural reflex is to choose the cheapest. It is also the decision that causes the most problems three years later.
The trouble is that most proposals describe what is easy to compare — the number of panels and the total capacity — and say nothing about what determines the real cost: the quality of the inverter, the type of mounting structure, who does the installation and what happens when something breaks down.
These are the nine points we check when someone asks us for a second opinion on a proposal from another company.
1. Make and model of everything, not just the capacity
A serious proposal identifies the make and model of the panels, the inverter, the mounting structure and the battery. If it just says “12 × 500 W panels” and “5 kW hybrid inverter”, you are buying blind — and it leaves the door open to being supplied with different equipment from what you had in mind.
Always ask for the datasheets. A company that does good work has them to hand and sends them without hesitation.
2. Inverter capacity versus panel capacity
It is normal, and desirable, to install more panel capacity than the inverter's rated power — this is called oversizing, and it makes better use of the hours of lower irradiance. Ratios of up to about 1.3 are common practice.
Above that, output starts to be clipped at peak hours. If the proposal has 8 kWp of panels on a 5 kW inverter, ask why. It may be a valid technical choice, or it may be a way of lowering the price at the expense of output.
3. Warranties: there are three, and they are different
Product warranty: covers manufacturing defects. On panels it now runs from 12 to 25 years, on inverters between 5 and 12.
Performance warranty: promises that after 25 years the panel will still produce a certain percentage of its output. It is not the same thing as the previous one.
Installation warranty: covers the company's work — fixings, cabling, keeping the roof watertight. It is the one most often missing from proposals and the one that solves the most problems, because most faults come from the installation, not the equipment.
4. Does the company install, or is it subcontracted?
Many companies sell and then subcontract the installation to crews who move from job to job. That isn't necessarily bad, but it changes everything when it comes to after-sales service: when there is a problem two years from now, who comes back?
Ask directly. The answer tells you more about the company than any brochure.
5. Is registration with the DGEG included?
Systems up to 30 kW require prior registration with the DGEG, the Portuguese energy authority. It is mandatory, it is what allows you to sell surplus, and it is what keeps your home insurance valid in the event of a claim related to the installation.
Check that the registration is included in the price and that the company handles it. If it appears as an extra or isn't mentioned at all, that is a warning sign.
6. Electrical protection itemised
A photovoltaic installation needs protection on the direct-current side — a DC circuit breaker, fuses and, in most cases, a surge protector. AC equipment won't do: in DC, an electrical arc does not extinguish itself.
If the proposal doesn't itemise this protection, either it has been left out of the price or it will be skimped on during the job. Lightning strikes cause the most expensive faults in a solar system and are rarely covered by insurance.
7. The mounting structure and what happens to the roof
Anodised aluminium and stainless steel are the standard. Painted steel structures are cheaper and, near the sea, rust in plain sight within a few years.
Ask how the fixing points are made watertight and whether the company takes responsibility for leaks. A badly sealed roof tile costs far more than the saving on the proposal.
8. Monitoring and who owns the data
Make sure you get access to the monitoring app in your own name, not through an account belonging to the installer. We have seen customers lose access to their own production data after switching maintenance company.
If the system has a battery, also check whether you can configure the operating modes yourself or whether that depends on a paid technician visit.
9. Timescales, payments and what is put in writing
Advance payments of more than 50% before any equipment is on site are inadvisable. The usual arrangement is a deposit, a payment on delivery of the equipment and the balance on completion.
Everything promised over the phone — cleaning, grid connection, handling the registration, estimated output — must be in the document. A verbal promise is worth nothing two years from now.
The question that sums it all up
If you remember only one thing: ask each company how much after-sales service costs two years on, and who provides it. The answer separates those who sell systems from those who look after them.
If you like, we will go through any proposal you have with you — even one from another company. We would rather you decided with information than decided in a hurry.
