How Do Solar Panels Work?
Built by Jeremy Panasuk. Published October 3, 2026.
The cell physics is a Department of Energy description. The wattage below is this site's calculator, with every input written out.
The U.S. Department of Energy separates solar into two technologies. Photovoltaics are the panels. Concentrating solar-thermal power uses mirrors to focus sunlight onto receivers that collect heat, and DOE says that form is used primarily in very large power plants. A cabin array is the photovoltaic kind.
Where the calculator starts
assets/app.js does not model the electric field inside a cell. sizeSystem starts from watt-hours you already added up. Its array line is daily watt-hours ÷ sun hours ÷ 0.75, rounded up to the next 100 W. The comment above that function describes 0.75 as wiring, controller, battery round-trip, and panel heat and soiling. The result panel on the calculator calls the same factor 75% system efficiency. The bank line is daily watt-hours × autonomy days ÷ max depth of discharge, and amp-hours round up to the next 10 Ah. sizeController then divides array watts by battery volts by 0.85 and takes the first of 10, 20, 30, 40, 60, 80, and 100 A that is at least that current. The 0.85 note in the file calls it a charge inefficiency margin.
Those divisors are the calculator's. PVWatts publishes its own loss model, and the monthly pages on this site already use that 14% figure. This walkthrough leaves 14% on those pages.
Worked example: 2,400 Wh
Battle Born's battery article uses 2,400 watt-hours as a 12-volt example: 2,400 ÷ 12 = 200 amp-hours for one day. This run keeps their 2,400 Wh and their 12 V, and it uses the calculator's own defaults for everything else. The form opens with sun hours 4, autonomy 2 days, and max depth of discharge 80% (the LiFePO4 option). Voltage on the form opens at 24 V; this example changes it to 12 V.
| Step | Arithmetic | Result |
|---|---|---|
| Array, before rounding | 2,400 ÷ 4 ÷ 0.75 | 800 W |
| Array the function prints | 800 is already a multiple of 100 | 800 W |
| Bank energy | 2,400 × 2 ÷ 0.8 | 6,000 Wh |
| Bank amp-hours at 12 V | 6,000 ÷ 12, already a multiple of 10 | 500 Ah |
| Winter array in the same function | 2,400 ÷ 2.5 ÷ 0.75, then up to the next 100 W | 1,300 W |
| Controller current | 800 ÷ 12 ÷ 0.85 = 78.4 A | 80 A |
Leave voltage at the form's 24 V default and the array is still 800 W, because voltage is not in the array line. The bank becomes 6,000 ÷ 24 = 250 Ah. Controller current becomes 800 ÷ 24 ÷ 0.85 = 39.2 A, and the listed size is 40 A. Switch depth of discharge to the 50% option and the array is still 800 W. The bank becomes 2,400 × 2 ÷ 0.5 = 9,600 Wh, or 800 Ah at 12 V.
Battle Born's article stops at 200 Ah for one day, and it doubles that to 400 Ah for two days off-grid. It does not divide by 0.80. The 500 Ah figure appears only after this calculator's 80% input. How that changes a battery count is on how many solar batteries do I need.
One common mistake
Reading a module's watt rating as if it were watt-hours per day. A nameplate watt is a power under the test condition printed on the label. Daily energy is that power times the hours the calculator asks you to type, after the 0.75 factor. DOE's description of charges moving in a cell does not supply those hours. Sun hours are an input, and the winter line inside sizeSystem swaps in 2.5 when you want that check. Four sun hours and 2.5 sun hours are why the same 2,400 Wh prints 800 W in one line and 1,300 W in the other.
Limits
DOE also says an inverter is expected to be replaced at least once in the 25-year lifetime of a PV array. Module physics and inverter replacement are different clocks. Grid connection, roof work, and the DC wiring between modules are work for a licensed electrician and the permits your jurisdiction requires. The mounting page and the fusing page stay with the manufacturer's installation manual on those points. Panel count from a finished wattage is on how many solar panels, and the rest of the system stack is on what size solar system.
FAQ
What does the Department of Energy say a photovoltaic cell does?
When sunlight hits the panel, the PV cells absorb that energy. The energy creates electrical charges that move in response to an internal electrical field in the cell, and electricity flows. Concentrating solar-thermal power is the other technology DOE describes, and DOE says it is used primarily in very large power plants.
What does this calculator print for 2,400 Wh at 4 sun hours, 2 days, 80% depth of discharge, and 12 V?
2,400 ÷ 4 ÷ 0.75 = 800 W, already on a 100 W step. The bank is 2,400 × 2 ÷ 0.8 = 6,000 Wh, which is 500 Ah at 12 V. Controller current is 800 ÷ 12 ÷ 0.85 = 78.4 A, so the listed size is 80 A. The winter line in the same function, at 2.5 sun hours, is 1,300 W.
Why is 500 Ah larger than Battle Born's 400 Ah for two days?
Their article divides 2,400 Wh by 12 V to get 200 Ah for one day, then doubles it for two days off-grid. This calculator also divides by the 80% depth-of-discharge input, so two days become 6,000 Wh and 500 Ah at 12 V.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- U.S. Department of Energy, How Does Solar Work? — accessed Oct 3, 2026.
- U.S. Department of Energy, Solar Photovoltaic System Design Basics — 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.