SOURCE: Chief Appraiser Knowledge Base — What a Kilowatt Means on a Solar
Permit, and How Solar System Size Is Actually Measured
Compiled from: U.S. Energy Information Administration (EIA) Glossary
(kilowatt, kilowatthour, capacity factor); National Renewable Energy
Laboratory (NREL) PVWatts Calculator methodology; 2023 National Electrical
Code (NFPA 70) Section 690.12, Rapid Shutdown of PV Systems on Buildings
(model electrical code; adopted into law by individual states and
jurisdictions)
Last reviewed: August 2026
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What a Kilowatt Means on a Solar Permit, and How System Size Is Measured
Realtors and sellers run into the same question constantly once a solar
permit or listing sheet is in front of them: the system is described as
"10kW" or "6.2kW" — what does that number actually mean, and does it tell
you anything about how much electricity the house is producing? The short
answer is that a kilowatt is a measurement of rate, not amount, and
understanding that distinction is the key to reading any solar
documentation correctly.
Kilowatt vs. kilowatt-hour: rate vs. amount.
Per the U.S. Energy Information Administration's own glossary, a kilowatt
(kW) is a unit of power — 1,000 watts — describing the rate at which
electricity is generated or consumed at a given instant. A kilowatt-hour
(kWh) is a unit of energy: one kilowatt of power sustained for one hour.
The relationship is the same as speed versus distance traveled — a car's
speedometer reading (kW) tells you how fast it's going right now; the
odometer (kWh) tells you how far it has actually gone over some period of
time. A solar system's size, as stated on a permit, is always given in kW
— a rate — not in kWh.
Why the permit shows a specific kW figure.
Building departments size a solar permit using the system's nameplate (DC)
rating: the sum of every panel's rated wattage under Standard Test
Conditions (STC) — a fixed, industry-standard test environment (full sun,
a specific cell temperature, and a specific light angle) that every panel
manufacturer uses to rate its product. If a system uses 25 panels rated at
400 watts each, the nameplate rating is 25 × 400W = 10,000W, or 10kW. This
number is used on the permit because it is fixed and independently
verifiable from the equipment specification sheets — it does not depend on
the weather, the roof's orientation, shading from nearby trees, or the time
of year, all of which affect how much the system actually produces on any
given day.
Why nameplate rating isn't the same as real-world output.
A 10kW system does not produce 10kWh every hour it's daylight, and it
certainly doesn't produce 240kWh in a day (10kW × 24 hours) — that would
only be true if the sun delivered full test-condition intensity around the
clock, which it never does. Actual output depends on the sun's angle
through the day and across seasons, cloud cover, panel temperature (panels
actually lose some efficiency as they get hotter, not gain it), shading,
dust and soiling on the panel surface, and the inverter's own AC output
limit, which is sometimes set slightly below the array's DC input — a
deliberate design tradeoff installers call "clipping."
Capacity factor: the gap between nameplate and reality, in one number.
The standard way to describe that gap is capacity factor — the ratio of a
system's actual energy output over a period of time to what it would have
produced if it ran at full nameplate rating for every hour in that period.
The EIA and DOE define it the same way industry-wide:
Capacity Factor = Actual Energy Output (kWh) ÷ (Nameplate Rating (kW) ×
Hours in the Period). Fixed-tilt residential rooftop solar (the overwhelming
majority of home installations, as opposed to ground-mount systems with
sun-tracking hardware) typically runs somewhere in the 15%–25% capacity
factor range once shading, temperature losses, soiling, and seasonal sun
angle are accounted for — which is the underlying, physics-based reason a
"10kW" system's real annual output lands well under its nameplate-times-
8,760-hours theoretical ceiling.
Translating kW into panels on a roof.
Residential solar panels sold today are typically rated somewhere between
roughly 350 and 450 watts each, depending on the manufacturer, cell
technology, and physical panel size — a figure that shows up on the same
manufacturer spec sheets building departments use to calculate the
nameplate rating described above. That means a system described as "10kW"
on a permit is usually built from somewhere around 22 to 29 individual
panels, and a smaller "6kW" system from roughly 13 to 17 panels, though the
exact count for any given installation depends on which specific panel
model the installer used. This is a useful translation for a real estate
agent walking a roof with a seller or buyer: counting the physical panels
and multiplying by a typical wattage range gives a rough, honest sanity
check against whatever kW figure appears on the permit or listing sheet,
without needing to look up the exact model number. It's also a reminder
that two systems with the same stated kW rating can look physically
different on the roof — a system built from higher-efficiency panels needs
fewer of them to reach the same nameplate number, and a system built from
older or lower-efficiency panels needs more roof area to get there.
Getting a real production estimate for a specific address.
Because actual output depends so heavily on location, orientation, and
shading, there isn't one honest single "kWh per day" figure that applies to
every home with a given system size — a fact worth being upfront about
rather than quoting a number that only holds for one particular roof.
NREL's own PVWatts Calculator, a free public tool built and maintained by
the same national lab that studies solar performance nationally, lets
anyone enter a specific address, system size, and panel orientation to get
a modeled annual and monthly production estimate using real historical
weather data for that location. That's the honest way to answer "how much
will this system actually produce" for a specific listing, rather than
relying on a rule-of-thumb number pulled from a different climate or roof.
The rapid shutdown switch or label — a real, unrelated code requirement.
Sellers and buyers sometimes notice a labeled shutdown switch or placard
near the main electrical panel or the utility meter and assume it's related
to system sizing or production — it isn't. It's a life-safety requirement.
The National Electrical Code, in the model language most states and
jurisdictions have adopted (2023 NEC Section 690.12), requires PV system
circuits installed on or in a building to include a rapid shutdown function,
so firefighters can de-energize the high-voltage DC conductors on a roof
quickly during an emergency rather than working around live wiring. The
2023 edition set clear voltage limits (conductors outside the array boundary
must drop below 30 volts, and conductors inside the array boundary must drop
to no more than 80 volts, both within 30 seconds of activation) and added an
exception for non-enclosed detached structures like carports and solar
trellises. As with the carbon monoxide alarm requirement covered elsewhere
in this library, the NEC is a model code — it only becomes binding law once
a state or local jurisdiction adopts a particular edition, so the exact
rapid-shutdown requirement in force on a given property depends on which
code edition that jurisdiction has adopted.
What this means for real estate agents.
When a listing or permit states a system size in kW, that's a fixed,
verifiable equipment specification, not a promise about the electric bill —
useful for comparing two systems' relative size, but not for predicting
savings without more information. If a seller or buyer wants a real
production estimate, point them to NREL's PVWatts Calculator rather than
quoting a generic regional average, since orientation and shading change the
answer house to house. A visible rapid-shutdown label or switch near the
meter is a normal, code-required safety feature, not a sign of a problem or
anything unusual about the installation.
Practical checklist.
Read the kW figure on the permit as a nameplate/equipment rating, not an
output promise. For an actual production estimate on a specific address,
use NREL's PVWatts Calculator (pvwatts.nrel.gov) rather than a generic
number. Don't be alarmed by a rapid-shutdown switch or placard near the
meter — it's a standard NEC 690.12 life-safety feature, required on modern
grid-tied systems, not a defect. Confirm which NEC edition and local
amendments actually govern the property with the local building department
if the rapid-shutdown detail matters to a specific transaction. If a permit
and a marketing sheet for the same system show two slightly different kW
figures, that's usually the DC nameplate rating versus the inverter's AC
output limit, not an error or a sign two different systems were installed.