How to read a solar energy quote before you sign
Three quotes for the same house, three different numbers. See what must be written down, how to check the promised generation with a simple calculation and what changes between microinverters and string inverters.
A client from Vargem Grande do Sul, in Brazil, laid three quotes on her kitchen table, all for the same house and the same electricity bill. One was a single page with the price and the words "complete system". Another ran to four pages, with every part listed. The third was a low-priced "kit" with one line: "installation not included". Her question was blunt: which one is lying?
None of them, necessarily. But comparing the three by the final price is like comparing an orange with a crate of oranges without knowing how many are inside.
A quote is a contract in draft
What is not written was not promised. "Complete system" does not say whether the mounting structure suits your roof tiles, whether there is surge protection, or whether the design and the utility approval are included.
It also pays to separate a kit from an installed system. The kit is the box of equipment: modules, inverters, structure, cables and connectors. The installed system is the kit plus design, an ART or TRT (the Brazilian technical responsibility record signed by a qualified professional), labor, AC-side protections, utility approval and commissioning. A cheap kit with the rest bought separately can end up more expensive, with warranties that nobody coordinates.
What must be written down
- Total power in kWp, plus the quantity and power of each module.
- Model and Inmetro registration (Brazil's certification body) of the modules and of the inverters or microinverters.
- Structure: which roof it is meant for and what it is made of.
- Protections: generation breaker, SPD (surge protective device), earthing and how the system meets rapid shutdown.
- Estimated monthly generation, with the assumptions (HSP, factor, orientation).
- Design, ART or TRT and utility approval: who signs, who follows up and whether it is included.
- Warranties in years: module (product and performance), inverter, structure and installation.
- What is not included and the documents delivered at the end: diagram, manuals and commissioning report.
How to check the promised generation
Every proposal must state how much the system generates per month. To check it, use the same calculation as the solar sizing tool on the E-Nogueira website (the site is in Portuguese): monthly generation ≈ kWp × HSP × 30 × 0.75.
HSP stands for peak sun hours: the day's energy squeezed into hours of sunlight at the module's laboratory condition. In São Paulo, the yearly average is 5.0. The 0.75 is the production factor, which lumps together the heat that lowers the module's output, dust, and losses in the cables and the inverter.
Example: a 4.1 kWp system in São Paulo generates about 4.1 × 5.0 × 30 × 0.75 ≈ 461 kWh per month. Now the client's cheapest proposal: 3.51 kWp promising 520 kWh. The calculation gives 395. To reach 520, the factor would have to be close to 0.99.
If the promised generation is well above kWp × HSP × 30 × 0.80 (the top of the market range), ask for the justification in writing. A promise without assumptions cannot be enforced later.
Microinverter or string
With a string inverter, the modules are wired in series and the cable comes down from the roof carrying hundreds of volts of direct current; shade on one module drags the whole string down, and if the inverter fails, the whole system stops. With microinverters (MLPE, module-level power electronics), the conversion happens under the module, the DC voltage on the roof stays in the 30 V to 60 V range, shade costs only the shaded modules and monitoring shows each module.
Micro is not always better. A shade-free roof facing one direction, or a large system, may call for a string inverter. What is not acceptable is a proposal that does not say how it meets the rapid shutdown required by ABNT NBR 17193 (the Brazilian fire-safety standard for PV systems on buildings).
How far to go on your own
Reading, checking the calculation and comparing are up to you. Design, work at height and direct-current electrical work belong to a qualified professional, with a signed design record. No formula replaces the site visit that sees the shade, the roof and the service entrance. The technical basis here is Brazilian; outside Brazil, follow your local electrical code, net metering rules and utility procedure.
To keep going
The "Reading a solar energy quote" checklist puts these items on one sheet to take to your conversation with the installer. It is a free PDF, sent by email.
Compare by criteria, not just by price.
Marcelo Nogueira Souza — E-Nogueira, since 2014