A tracker follows the sun, which sounds like it must always win. Whether it does depends almost entirely on your latitude — and at some of them, a fixed frame beats it outright.
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.
-0.5%single-axis, against fixed
+8.2%dual-axis, against fixed
Fixed framebest value over 25 years
1,014kWh per kWp, fixed frame
At London, United Kingdom, a single-axis tracker changes
output by -0.5%
— and on cost, the best choice here is fixed frame.
A tracker is compared against a well-oriented fixed frame here, which is the only
honest comparison and the one that makes trackers look worst. Set the fixed frame flat, or point
it the wrong way, and the tracker's advantage grows — but the money saved by fixing the
frame's orientation costs nothing at all.
All three, on your array
All three on the same array, the same site and the same value per unit. "Best value" is decided
on money over the horizon, not on output — a tracker that generates more than it earns is not
the better buy.
Mounting
Annual output
Against fixed
Extra cost
Upkeep a year
Net a year
Premium repaid in
Fixed frame — best value
9,125 kWh
—
—
—
—
—
Single-axis tracker
9,078 kWh
-0.5%
£2,340
£108
−£117
never
Dual-axis tracker
9,875 kWh
+8.2%
£6,300
£108
£42
beyond 60 years
When a tracker wins, and when it loses
Month by month against the fixed frame. A horizontal axis cannot tilt towards the equator, so it
does best in the months when the sun is high and passes overhead, and worst in midwinter when a
tilted fixed frame is pointing straight at what little sun there is.
Single-axis gain, by month
Month
%
Jan
-23
Feb
-16
Mar
-9
Apr
2
May
7
Jun
10
Jul
10
Aug
5
Sep
-5
Oct
-16
Nov
-25
Dec
-25
Total
-85
Tracking gain is mostly a question of latitude
The same 9.0 kWp array on the same 35° fixed
frame, moved around the world and given the same trackers. Sorted by distance from the equator.
A fixed comparison. These rows do not move with the controls above — every system is
identical, and only the site changes.
Site
Latitude
Fixed kWh/kWp
Single-axis
Dual-axis
Nairobi, Kenya
1.3°
1,402
+32.8%
+35.1%
Chennai, India
13.1°
1,478
+16.0%
+19.3%
Phoenix, United States
33.5°
1,914
+12.0%
+26.1%
Santiago, Chile
33.5°
1,860
+14.8%
+25.9%
Denver, United States
39.7°
1,738
+6.0%
+22.1%
London, United Kingdom
51.5°
1,014
-0.5%
+8.2%
Oslo, Norway
59.9°
911
-0.7%
+10.5%
Tromso, Norway
69.7°
719
-3.4%
+11.0%
Nairobi, Kenya sits closest to the equator and gains
32.8% from a single axis alone — nearly everything
a dual-axis tracker would add, for a fraction of the hardware. Tromso, Norway is
the furthest from it and loses 3.4%.
The reason is geometry. A horizontal north–south axis rotates east to west and nothing else,
so it can follow the sun's daily arc but never lean towards the equator for the winter. On the
equator there is no winter lean to give up and the daily arc is everything, so the match is
perfect. At high latitude the sun stays low, most of the light arrives as diffuse from the whole
sky rather than as a beam worth chasing, and a fixed frame tilted towards the equator beats a
tracker lying flatter than it should be. Trackers built for those latitudes use a tilted axis,
which this page does not model.
What this does not model
Horizontal north–south axis only. Tilted-axis, east–west-axis and azimuth-only
trackers all exist and all behave differently. A tilted-axis tracker would recover much of what
the high-latitude rows above give up.
One row, with nothing behind it. Real tracker fields shade each other at low
sun and run backtracking algorithms to manage it, which costs output the single-row figures
here never lose. Gains for a multi-row field are therefore overstated.
No downtime, no wind stow, no drive failures. A fixed frame has nothing to
break. The upkeep field is the only place that difference appears, and it is a number you set
rather than one this page knows.
No clipping. These runs are the 288-interval representative year, which
cannot see clipping — and a tracker's flattened, broader output curve interacts with an
inverter ceiling in ways the clipping calculator
is the right tool for.
Prices are per kWp and linear. Tracker economics are dominated by scale; a
twenty-panel garden array and a twenty-megawatt field are not on the same cost curve.