How to compare solar installer quotes

A solar installation is designed to run for 25 years. On a decision with that horizon, choosing on lowest price usually turns out expensive: behind an attractive figure there may be lower-quality equipment, an incomplete installation or a legalization process that is not included and will show up later as a cost overrun.

The point of comparing is not to find the smallest number, but to identify the proposal that offers the best system for your money, with real support over time. For that, every quote has to speak the same language.

There is an obvious information asymmetry: the installer knows the technical detail and you, almost always, are buying this for the first and only time in your life. The method that follows aims to close that gap, not to turn you into an engineer. Simply reviewing the right points in the right order already makes you a far better-informed buyer than average.

Before looking at prices, make sure each proposal describes exactly the same thing. If one includes legalization and another does not, or if one uses panels of a named brand and another does not specify one, they are not comparable even if the totals look similar.

A serious proposal fits in two or three pages and answers the questions below without ambiguity. If finding out which panels you are going to get means calling and asking, that in itself tells you something about what the relationship will be like after signing.

The usual situation is that the three quotes you receive are not directly comparable: one proposes 4.2 kWp, another 5.0 kWp and the third 4.6 kWp with a different inverter brand. That is not a problem, it is normal. The trick is to normalise them before deciding.

Start by separating what is inside and outside each scope. Make a list of every item that appears in any of the three — legalization, meter, special structure, civil works, transport, first-year maintenance — and mark for each proposal whether it is included, excluded or not mentioned. The items that are 'not mentioned' are the ones that cause arguments later.

When an item is missing from a proposal, ask for it to be quoted separately rather than assuming it is included. With that column of extras, the totals become comparable again. This step, which takes half an hour, is the one that saves the most money in the whole purchase.

The total price depends on the size of the system, so two quotes of different sizes cannot be compared on their final figure. The useful metric is the price per installed watt: divide the total value by the capacity in peak watts (the kWp multiplied by a thousand).

That number lets you compare on a level footing and spot proposals that are out of line with the market, both above and below. A price per watt far lower than the rest is not automatically a bargain: it usually indicates something has been trimmed, and it is worth asking what.

Bear in mind that larger systems usually have a lower price per watt, because some fixed costs — technical visit, paperwork, transport, commissioning labour — are spread across more capacity. Compare systems of similar size so the metric is fair, and if the sizes differ a lot, expect and accept that difference.

An important nuance: the price per watt measures how much you pay for installed capacity, not how much energy you will receive. Two systems with the same price per watt can produce different amounts if one has better orientation, less shade or better equipment. That is why the next criterion is estimated output.

Almost every proposal includes an annual generation figure in kWh. It is the number that later translates into savings, so it deserves a critical look rather than being taken at face value.

The quick way to audit it is to divide the estimated annual output by the system's kWp. The result — kWh per kWp per year — is a specific yield measure you can compare across proposals and check against the radiation in your area. If two installers propose the same size on the same roof but one estimates considerably more generation, someone is using different assumptions and it is worth asking which.

Ask for the proposal to state its assumptions: peak sun hours used, overall system performance ratio (which bundles losses from temperature, wiring, soiling and the inverter), the orientation and tilt assumed, and whether shading was deducted. A realistic performance ratio on a well-executed installation usually sits between 0.75 and 0.85; values well above that indicate an optimistic estimate.

Be especially wary of estimates that do not deduct shading when your roof clearly has some. It is the most common way of inflating promised output without telling any explicit lie.

Not all warranties are equal or cover the same things. A solar installation involves at least three: the panels', the inverter's and the installation's own (workmanship). A quote mentioning '25-year warranty' does not say much if it does not clarify on what.

For panels there is a distinction between the product warranty — which covers manufacturing defects and is usually 10 to 15 years, although high-end manufacturers now offer 25 — and the performance warranty, which guarantees a minimum percentage of output over time. The latter is expressed as a curve: a percentage of loss in the first year and a maximum annual degradation after that, guaranteeing around 80-90% of the original capacity at the end of the period. Two panels with a '25-year warranty' can have different degradation curves, and that difference is money.

The inverter usually has a shorter warranty, in line with its 10 to 15 year service life. Many manufacturers offer paid extensions; it is worth knowing what they cost before deciding, because replacing the inverter is the most likely recurring expense in the whole system.

And the installation should have its own workmanship warranty, covering mounting defects: roof leaks, badly executed fixings, poor connections. It is the warranty most often left out of proposals and the one that solves the most problems in practice.

Just as important is knowing who answers for each warranty and how it is claimed. A manufacturer warranty is not much use if the installer disappears and nobody handles the claim, or if the manufacturer has no representation in Colombia and the equipment has to be exported to enforce it. Ask explicitly who files the claim and who covers the labour for the replacement, which is not always included.

You do not need to be a specialist to spot relevant differences between equipment. Reviewing a handful of datasheet parameters already tells a carefully prepared proposal from an improvised one.

For panels, the nominal capacity in peak watts is measured under standard laboratory conditions (1,000 W/m² of irradiance and a 25 °C cell temperature), which are not the conditions on your roof at midday. That is why the temperature coefficient matters: it indicates how much capacity is lost for every degree the cell exceeds those 25 °C. In Colombia, with cell temperatures usually well above that, the parameter genuinely counts.

For the inverter, check the maximum and the European or weighted efficiency, the number of maximum power point trackers (the MPPTs, which determine how the system behaves if different groups of panels receive different light) and whether monitoring is included. Also check that the equipment carries the certifications required by Colombian electrical regulations.

A technical detail almost nobody checks and which explains price differences: the ratio between the capacity of the panels (direct current) and that of the inverter (alternating current).

It is common and correct to install somewhat more panel capacity than the inverter's nominal rating, because panels rarely reach their peak output. That oversizing makes better use of the inverter and usually improves the economics of the whole. But taken to an extreme it cuts output during the hours of highest generation, because the inverter clips the output.

If one proposal is noticeably cheaper than another with the same panels, check whether the inverter is smaller. It is not necessarily a mistake — it can be a legitimate design decision — but it should be explained and reflected in the estimated output.

The part you cannot see is what determines whether the system is safe. An electrical installation in Colombia must comply with the Technical Regulation for Electrical Installations (RETIE), and that includes the photovoltaic part: protections, correctly sized conductors, earthing and labelling.

Ask whether the proposal covers direct and alternating current protections, system earthing and surge protection, which is especially relevant in areas with frequent atmospheric electrical activity. These items have a cost; a proposal that leaves them out looks cheaper and is not.

It is also worth clarifying who is liable if the installation causes a roof leak. It is the most common damage and the one that generates the most disputes, and a serious proposal covers it explicitly in its workmanship warranty.

Some practices should raise an alarm and lead you to ask for clarification before going further. None of them proves bad faith on its own, but all of them justify a direct question.

A handful of questions reveal how serious a proposal is and give you grounds for comparison beyond price. The answers also tell you a lot about how you will be treated if something goes wrong later.

With the proposals normalised, the price per watt calculated and the warranties compared, the decision is rarely a tie. One proposal usually emerges that does the same thing at a reasonable price and with better backing.

If two are very close, break the tie on the factors that do not appear in the price: verifiable experience on projects like yours, the quality of communication during the quoting process — which predicts after-sales rather well — and the solidity of whoever will answer for the warranty ten years from now.

And do not rule out negotiating. With three comparable proposals on the table and a clear sense of what you are comparing, you are in a far better position than someone asking for a discount with no argument.