Eight Months of Data Through a Full Winter — 74% Solar, R800/Month Saved

The Story

Johan and Elsa Botha have lived in the same Pretoria East house for twenty-two years. Their two adult children — both studying at UP — still live at home, which means four people, two showers running back to back every morning, and a geyser that Johan describes as “the most expensive appliance we own.”

They installed a 1.8 kWp solar-PV array in late October 2024, just before the summer heat kicked in. What makes this case study particularly valuable is the length of the monitoring window: eight full months, from November 2024 through July 2025. That span covers peak summer, the autumn transition, and the depths of a Highveld winter.

Across the entire period, the system averaged 158 kWh of solar energy per month against 55 kWh from the grid — a 74% overall solar fraction. The cumulative numbers tell the story at scale: 1,227 kWh from the sun, 424 kWh from Eskom. Monthly grid cost for water heating sits at R207, with total estimated savings of R800 per month.

What’s instructive about the Botha data is the seasonal curve. Summer months pushed the solar fraction well above 80%, while June and July pulled it down — but even at the winter low point, solar still carried the majority of the load. The 74% annual average accounts for every cold front and overcast week Pretoria threw at it.

Elsa is pragmatic about it: “We’re not off-grid, and we don’t pretend to be. But cutting our water heating bill by three-quarters without changing anything about how we live? That’s not nothing. That’s a family holiday every year.”

For prospective buyers, the Botha case is the most useful stress-test in this series. It’s real performance through real weather, with no cherry-picked months. The 74% number is what you can plan around.