When does solar pay for itself?

How we calculate it

• Year 0 is the system cost, as a minus — the line starts below zero and climbs as the savings come in.
• Each year saves the kWh you no longer buy x the price that year; the price rises by the increase you set, and the panels produce a little less each year.
• What you still buy from Eskom on cloudy days comes off the saving — a hybrid system still draws some grid power; that kWh is priced at the same rising tariff.
• Off-grid shows the days a year without power — a day the panels plus what is left in the battery cannot cover that day’s use. It is built day by day from your region’s solar output and rainy days; a bigger battery carries you through more cloudy spells.
• A battery replacement comes off in its year, so the line can dip back below zero before it recovers. Break-even is the year it stays above zero.
• Paying cash is not free money — the cash could have earned interest, so while you are still behind the shortfall grows at the rate you could have earned. The calculator shows it as Interest forgone.
• On a loan, the shortfall grows at the loan rate — interest runs until the savings have paid it off, shown as Loan interest paid.
• Once you are ahead, the savings earn interest too — at the same ‘could have earned’ rate, whether you paid cash or took a loan.
• Rand as spent, not today’s money; maintenance, insurance and the value of backup power during load-shedding are left out.

Where the typical figures come from

• Price increases — Low 6%, Moderate 9%, High 13%: NERSA’s Eskom decisions: 12.74% for 2025/26, then 8.76% (Eskom) and 9.01% (municipal) for 2026/27 after the R54.7bn correction; Low is the original 2026–28 path (5.36% / 6.19%). Engineering News, 2026, Eskom, 2026.
• System and battery prices: published South African installer price guides — OurPower, 2026, EnergyBee, 2026, EnergyBee batteries, 2026.
• Off-grid days without power — solar output by region: EU JRC PVGIS 5.2 (satellite data 2005–2020), panels at 30° facing north, 14% losses — Johannesburg 4.8, Cape Town 4.6, Durban 4.1, Bloemfontein 5.0, Gqeberha 4.4 kWh a day per kWp over the year. PVGIS, European Commission.
• Off-grid — rainy days a month: days with 1 mm of rain or more, 1991–2020 normals (South African Weather Service / NOAA), from each city’s climate table — Johannesburg 67 a year, Cape Town 65 (counts 0.1 mm, so a little pessimistic), Durban 86, Bloemfontein 54, Gqeberha 71. Johannesburg, Cape Town, Durban, Bloemfontein, Gqeberha.
• Off-grid — a rainy day makes 25% of a clear day: the top of the 10–25% usually quoted for heavy overcast; a general figure, not measured in South Africa. Solar Calculator.
• Interest you could have earned — 7.25%: the SARB repo rate after the 25-basis-point hike of 23 September 2026. Money-market funds pay close to it (Old Mutual Money Market Fund 6.61% a year in April 2026, before the hike); a 12-month fixed deposit pays a little more (GoTyme 8.75%, July 2026). Trading Economics, Sep 2026, Old Mutual, GoTyme.
• Loan interest — 10.75%: prime (repo 7.25% + 3.5) from 24 September 2026; your bank’s offer may be higher. Briefly News, Sep 2026.
• Our estimates, not sourced: household usage per size, the cloudy-day Eskom top-up (about 15% of use), the extra-large system’s price and the labour in a battery swap. Off-grid: rain comes in spells of up to 3 days, half of each day’s use is after dark, and the battery figure is usable storage. Use your own bill and quote.