Solar tracker or just more panels? The math said panels.

I spent a weekend last month trying to answer what sounded like a simple question: should I put the garden solar panels on a single-axis tracker, or just buy two more fixed panels for the same money? The short answer is “more fixed panels, it’s not close.” The long answer — and the reason I built an interactive clear-sky model to get there — is more interesting.

The trap of the “40% tracker gain” headline

Vendors love to quote tracker gains in the 30–40% range, sometimes 50%+ for dual-axis systems. Those numbers are mathematically correct, but they describe a theoretical clear-sky ceiling — the maximum energy a tracker could capture if every photon came in as direct beam, every day was cloudless, the motor was perfect, and the panel was always exactly perpendicular to the sun. My model reproduces this ceiling: at Jodoigne’s latitude (50.76°N), dual-axis sits around 70% above fixed, single-axis around 30–40%. That’s what the charts show.

The real world isn’t clear-sky. Belgium averages roughly 1,600 sunshine hours per year and about half the annual irradiation is diffuse (cloud-scattered), not direct beam. Diffuse radiation doesn’t care which way the panel is pointed — a fixed panel catches it from a wide swath of sky, while a tracker gains nothing for following the sun through a cloud. After diffuse, tracker accuracy, soiling, and wind losses, real-world dual-axis gains typically land around 25–35%, and single-axis around 15–25%. Suddenly a different question matters: does the mechanical complexity pay for itself at those gains?

The break-even inequality

Here’s the thing that surprised me. The break-even condition isn’t complicated:

A tracker wins when its effective cost (including motor rebuilds) is less than the cost of buying enough extra fixed panels to match its real-world energy gain at the same orientation.

That gives a break-even gain G_BE = ctEff / (n × cp), where ctEff is the tracker cost after motor rebuilds, n is panels per tracker, and cp is the cost of one panel. The interactive calculator lets you plug in your own numbers and see the verdict instantly.

For my setup — 4 panels per tracker at about €280 each (~€1,120 of panels), with a tracker quoted at €1,800 plus one motor rebuild during the 25-year lifetime — ctEff works out to €2,160, which means the break-even gain is 193%. That is, the tracker would need to give me nearly three times the energy of the equivalent fixed panels just to pay for itself. The physical ceiling for dual-axis is ~70%. There is no real-world scenario, no weather, no tariff structure, that gets me to 193%. The tracker is dominated.

Put differently: the €1,800 I’d spend on a tracker plus motor rebuilds could instead buy me 7.7 additional panels at €280 each. Those panels would generate about 12.9× more energy per euro than the tracker would. It’s not close.

Where a tracker could win

I want to be fair to tracking systems — they’re not a dumb idea in general. The break-even math flips in a few specific situations:

1. Very cheap trackers. If you can find (or build) a single-axis tracker for €300 and a couple of hours of labor, the break-even gain drops to ~30%, which is within striking distance of real-world single-axis performance. Some hobbyists build their own; a closed-form catalog tracker at €300 is rare.
2. Very expensive panels or scarce roof space. If panels cost €500 each (premium, high-efficiency), or if you physically cannot fit more panels because the roof or garden area is full, the break-even shifts toward the tracker.
3. High electricity value. In places with €0.50/kWh feed-in or higher, every extra kWh is worth more, which shrinks the tracker’s payback period faster than panels (because trackers get more kWh per panel, but each kWh is worth proportionally more to both options).
4. You just want a tracker for the hobby. Which is legitimate! My model doesn’t price engineering joy. If building and tuning a tracker is the point, the economics are irrelevant.

What I’d actually do

For my garden, at current prices in Wallonia, with real weather and real diffuse radiation, the answer is clear: add more fixed panels at 35° south, skip the tracker. The same math also says that monthly re-tilting (adjust for winter in November, flat in April) captures about 97% of the dual-axis ceiling — so if I ever wanted to squeeze out a bit more, that’s where I’d start, not a motor-driven tracker. And 35° south is genuinely close to the clear-sky annual optimum (the model puts it at 47°, but the difference in annual yield between 35° and 47° is under 3%).

If you want to run the numbers for your own site, the interactive model lets you plug in panel cost, tracker cost, expected gain, and electricity value, and tells you the verdict live. The geometry and heatmap sections above it are also useful for building intuition about why a particular tilt works best in your latitude. I built the whole thing in an afternoon using vanilla canvas and a pile of nerdy excitement; moving it into lav.ren as a native React experiment took another afternoon on top of that.