The conversion itself is one division. The confusion is that a solar quote contains three different numbers measured in kilowatts, they are all different, and quotes are not comparable until you know which one you are reading.
The sizing method is in sizing a home solar array.
The conversion
panels = system kW × 1,000 ÷ panel watts
A 9 kW system in 400 W panels: 9,000 ÷ 400 = 22.5, so 23 panels. You round up, and the system becomes 9.2 kW.
| System | 350 W | 400 W | 450 W | 500 W |
|---|---|---|---|---|
| 5 kW | 15 | 13 | 12 | 10 |
| 7 kW | 20 | 18 | 16 | 14 |
| 8 kW | 23 | 20 | 18 | 16 |
| 10 kW | 29 | 25 | 23 | 20 |
| 12 kW | 35 | 30 | 27 | 24 |
| 15 kW | 43 | 38 | 34 | 30 |
Panel counts are also constrained by string sizing — panels connect in series into strings that must fall within the inverter's voltage window, so the practical count is sometimes nudged up or down from the arithmetic. That is the installer's problem, but it explains why a quote comes back at 24 panels when you calculated 23.
The three kilowatt numbers
DC array rating (kW-DC). Panel wattage × number of panels. This is the number in the conversion above and the one most commonly used to describe system size. It is measured at standard test conditions — 1,000 W/m², 25 °C cell temperature — which is a laboratory condition your roof will essentially never be in.
AC inverter rating (kW-AC). What the inverter can deliver to the house. It is normally smaller than the DC rating, deliberately.
Annual energy (kWh). What actually reaches your meter over a year. This is the only number that pays for anything.
A quote saying "9 kW" without specifying DC or AC is ambiguous, and the difference between the two readings can be 20 %.
Why the inverter is smaller
Intentionally undersizing the inverter relative to the array is standard practice, and the ratio has a name: the DC-to-AC ratio, typically between 1.1 and 1.3.
The reasoning: the array only reaches its DC rating in rare, brief conditions — cold, bright, perfectly clear, sun square onto the panels. For nearly all of the year it produces well below that. Sizing the inverter for a peak that happens for a few hours a year means paying for capacity that sits idle.
The cost is clipping — during those rare peaks the inverter caps output and a small amount of energy is lost. At a sensible ratio the clipped energy is a fraction of a percent of annual production and far cheaper than the larger inverter.
So a smaller inverter number on a quote is not a defect. A DC-to-AC ratio outside about 1.1–1.3 is worth asking about in either direction.
Why neither rating predicts your bill
Standard test conditions are not roof conditions. Between the DC rating and the meter sit the derate losses — inverter efficiency, wiring, soiling, temperature, orientation, panel tolerance — which together take roughly 20 %.
Temperature is the largest of them and the least intuitive: panels are rated at 25 °C cell temperature, and a panel in full summer sun runs far hotter than that, losing output exactly when the sun is strongest. The full breakdown is on the main sizing page.
Comparing quotes properly
Given the three-number problem, compare on these instead:
- Annual kWh production estimate. The only number that matters and the one every quote should contain. If it does not, ask.
- DC kW and panel count and panel model, so you can check the arithmetic and look up the panel yourself.
- Inverter model and AC rating, so you can see the DC-to-AC ratio.
- Price per watt, computed on DC — and confirm which basis they used, because per-watt on AC looks higher for an identical system.
A quote with 26 × 400 W panels and one with 23 × 450 W panels are 10.4 kW and 10.35 kW — effectively the same system, differing in roof area and panel count. Comparing them on panel count alone would be meaningless.
What this page does not cover
- String sizing and voltage windows, which constrain the practical panel count.
- Microinverters versus string inverters, which changes how shading is handled and how the AC rating is arrived at.
- Panel degradation — output falls slowly over decades, and warranties are written around a guaranteed percentage at year 25.
- Bifacial panels, whose rating depends on what is behind them.
One division for the conversion. The rest is knowing which kilowatt you are looking at.