How solar PV TCO is modelled
For a PV plant the dominant variables are specific yield, degradation, and how much of the balance of system has to travel with the modules.
Cost lines in a PV model
| Line | What it covers |
|---|---|
| Modules | Panel class and quantity, with the FOB price as the starting point. |
| Balance of system | Mounting, cabling, combiner and protection, inverters, and monitoring. |
| Delivery and install | Freight, duty, VAT, haulage, installation and grid connection. |
| Operation | Cleaning, inspection, inverter replacement and insurance over the analysis horizon. |
| Yield | Specific yield for the stated location and mounting type, with degradation applied year by year. |
What stays open until confirmed
Specific yield is a modelled assumption unless a site measurement or a bankable study exists. Soiling, shading and grid curtailment are often unquantified in a first pass, and they can change the answer more than a small change in module price.
Worked example
The 2 MWp rooftop study shows the cost stack and cash flows for a stated case, with assumptions marked as assumptions. Its numbers belong to that case and its evidence date.
What this model does not tell you
It does not tell you the yield your roof will produce, the tariff you will be paid, or whether your interconnection will be granted. It shows which of those you still have to establish.
How this differs from the other classes
Solar is the class where the cost of the equipment is least predictive of the outcome. Two identical module orders in two locations produce different results, because the yield assumption dominates the calculation.
Questions to put to a supplier
- What power tolerance and degradation curve does the module warranty state?
- What specific yield is the model using, from which source, and is it P50 or P90?
- Which balance-of-system items are inside the quoted price and which are excluded?
- At what year is inverter replacement assumed, and at what cost?
- What soiling, shading and curtailment assumptions are applied?
The input most often left unverified
Specific yield. It is usually carried over from a regional average rather than measured at the site, and a modest error in yield moves the whole business case. A site-specific study is the only way to replace the assumption with evidence.