How to Buy Solar Panels
Built by Jeremy Panasuk. Published October 3, 2026.
Shopping starts from the watts the calculator already printed. The module numbers below are one model's published ratings, used as the label you should be able to find on any panel you buy.
sizeSystem. The CS6A-220M in Canadian Solar's Annex C is 220 W at Standard Test Conditions. 800 ÷ 220 = 3.64, so the count rounds up to 4 modules. Four times 220 W is 880 W of nameplate. Three modules are 660 W, which is short of 800 W.What the label is measuring
The Installation Manual of Standard Solar Modules (EN-Rev IM/GN-AM-EN/2.2, July 2020) measures electrical ratings at STC: 1,000 W/m², an AM1.5 spectrum, and a cell temperature of 25°C. Annex C lists the CS6A-220M at Pmax 220 W, Vmp 24.8 V, Imp 8.87 A, Voc 30.6 V, Isc 9.31 A, and a maximum series fuse rating of 15.00 A. The manual says those electrical characteristics are within ±10% or [0, +5 W] of the indicated values of Isc, Voc, and Pmax. A 220 W nameplate is that test point, inside that tolerance band. It is not a promise of 220 W at noon on a hot roof.
The manual tells the installer to multiply STC short-circuit current by 1.25 when determining component ratings, and it says local rules may apply a further multiplier it states as 1.56 when sizing conductors and fuses. Open-circuit voltage gets the cold-temperature factor in the manual's Table 1. Series strings have to stay within the maximum system voltage printed on the nameplate. Only modules with similar electrical outputs go in the same string. The manual is marked for professional use only. The fuse arithmetic for this same model is on how to fuse a solar panel array.
Do not stack a second loss factor on 0.75
The 800 W already includes the calculator's ÷ 0.75. The comment in assets/app.js describes that factor as wiring, controller, battery round-trip, and panel heat and soiling. Adding another 25% after the calculator has printed 800 W sizes a different system than the one on the screen. The monthly pages on this site use PVWatts' 14% default losses for a different method. Use one method for a given run. The 0.75 path is the off-grid calculator. The 14% path is the monthly-bill pages.
The controller follows the nameplate you actually buy, not the 800 W before the round-up to whole modules. Four CS6A-220M modules are 880 W. At the form's default 24 V, sizeController does 880 ÷ 24 ÷ 0.85 = 43.1 A and lists a 60 A controller. At 12 V the same 880 W is 880 ÷ 12 ÷ 0.85 = 86.3 A, which is past the 80 A step, so the listed size is 100 A. Buying the controller against the three-module 660 W figure sizes the controller for an array that does not cover the 800 W result.
| Purchase | Nameplate | Against the 800 W result |
|---|---|---|
| 3 × CS6A-220M | 660 W | Short by 140 W |
| 4 × CS6A-220M | 880 W | Covers 800 W |
| Controller for 880 W at 24 V | 880 ÷ 24 ÷ 0.85 = 43.1 A | 60 A |
| Controller for 880 W at 12 V | 880 ÷ 12 ÷ 0.85 = 86.3 A | 100 A |
Panel count for other wattages is on how many solar panels. If the bank already exists and you are only refilling it, use panels to charge a battery, which divides stored watt-hours by sun hours and by 0.85. That is panelsForBattery, a different function from sizeSystem.
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One common mistake
Comparing a module's STC watts with a daily watt-hour total as if they were the same unit. 220 W is the Annex C power at 1,000 W/m² and 25°C cell temperature. 2,400 Wh is a day of loads. The bridge is the calculator: divide the watt-hours by sun hours and by 0.75, round to the next 100 W, then divide by the nameplate and round the module count up. Stopping at "2,400 divided by 220 is about 11 panels" skips sun hours and the 0.75 factor. 2,400 ÷ 220 = 10.91, and that 11-panel pile is not the 4-module result above.
Limits
Canadian Solar's ratings describe Canadian Solar modules installed as that manual specifies. Another manufacturer's label replaces every number in the table. The manual says the mounting design is certified by a registered professional engineer, and a licensed electrician handles the conductors, the overcurrent devices, and the code adopted where you build. This page does not rank brands and does not report a test of a panel.
FAQ
How many CS6A-220M modules cover an 800 W calculator result?
800 ÷ 220 = 3.64, which rounds up to 4 modules, 880 W of nameplate. Three modules are 660 W, 140 W under 800 W. The 800 W is 2,400 ÷ 4 ÷ 0.75 from this site's sizeSystem function.
What does the ±10% or 0 to +5 W band apply to?
The Canadian Solar manual says the electrical characteristics are within ±10% or [0, +5 W] of the indicated values of Isc, Voc, and Pmax. STC for those indicated values is 1,000 W/m², AM1.5, and a cell temperature of 25°C.
What controller does 880 W ask for?
At 24 V, 880 ÷ 24 ÷ 0.85 = 43.1 A, so the calculator lists 60 A. At 12 V, 880 ÷ 12 ÷ 0.85 = 86.3 A, so the listed size is 100 A. The list of standard sizes in sizeController is 10, 20, 30, 40, 60, 80, and 100 A.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- Canadian Solar, Installation Manual of Standard Solar Modules, EN-Rev IM/GN-AM-EN/2.2, July 2020 — accessed Oct 3, 2026.
- Battle Born Batteries, How to Size a Deep Cycle Battery Bank, May 22, 2026 — accessed Oct 3, 2026.
- SolarSizer calculator, assets/app.js (sizeSystem and sizeController) — accessed Oct 3, 2026.