When an array can out-produce its inverter, the excess is simply lost. This works out how much, in which months, and what it is worth — from every hour of a simulated year.
What do kWp and kWh mean?
kWh — kilowatt-hour
A unit of energy, and the thing your electricity bill charges you for. A
1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between
2,000 and 10,000 kWh a year depending on where it is and how it heats.
kWp — kilowatt-peak
A unit of capacity — how big the solar array is, not how much it makes.
It is the output the panels would produce under standard test conditions: bright, cold and
perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not
generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten
panels.
Putting them together
kWp is the size of the system; kWh is what it produces over time. The ratio between them
— kWh generated per kWp installed, per year — is the honest way to compare
locations, because it strips out how big the system happens to be. It runs from roughly 700 in
cloudy high latitudes to over 1,800 in sunny deserts.
485 kWhclipped per year
4.71%of what the array could make
£97the clipped energy is worth
1.47DC:AC ratio (5.40 kWp ÷ 3.68 kW)
This array loses
4.71% of its potential output to the inverter ceiling —
about £97 a year.
Recovering it means an inverter of 5.40 kW, which costs about £327 more and would take 4 years to repay that difference out of the energy it rescues.
When the clipping actually happens
Clipping is a summer phenomenon: 38% of the annual loss falls between April and September, and the worst month is Mar. For much of the year the inverter is never troubled at all.
Energy clipped, by month
Month
kWh
Jan
33
Feb
58
Mar
88
Apr
78
May
51
Jun
23
Jul
0
Aug
6
Sep
27
Oct
66
Nov
40
Dec
14
Total
485
Why clipping is smaller than it sounds
An array reaches its nameplate rating only when several things coincide: full sun, a low angle of
incidence, and cold modules. Panels are rated at 25°C and spend most of a sunny afternoon well
above that, losing roughly 0.38% of their output per degree. So a
5.40 kWp array does not produce 5.40 kW
on a hot clear day — it produces meaningfully less, and the gap between nameplate and
reality is headroom the inverter never has to pass.
That is why the loss is bounded. Even an inverter well below the array's rating can only lose the
energy in the hours that would have exceeded it, and a year contains few of those. The cost of
clipping is real, but it is a slice off the top of the best hours of the best months, not a
percentage taken from every kilowatt-hour.
It also depends almost entirely on where you are. This page opens in Phoenix,
because that is where the effect is worth seeing. Move the location control to London and the same
array, on the same inverter, clips essentially nothing — a maritime climate delivers its
annual total through a great many mediocre hours, and mediocre hours never reach a ceiling. If you
have been told your array is oversized for its inverter, the first question is not how big the gap
is but what your sky does with it.
What this does not model
Inverter efficiency is constant at 96%. A real inverter's efficiency varies
with load, so the energy recovered by a larger unit is slightly overstated here.
No inverter thermal derating. A hot inverter reduces its own output, which
looks like clipping and is not modelled.
No export limit separate from the inverter. Where a connection agreement caps
export rather than generation, a system can self-consume through the cap and lose less than
this page shows.
Shading is declared as none. A shaded array clips less, because it rarely
reaches the ceiling in the first place.