· Built around the Solar payback calculator
Photo: Robert So on Pexels (free licence)
Load-shedding pushed hundreds of thousands of South African homes into solar and batteries, and electricity prices keep climbing: municipal tariffs went up 9.01% on 1 July 2026. The sales pitch is easy to find; the sums are not. This post works through each one with real numbers: when solar pays for itself, how many panels you need, the inverter and battery for an outage, watts to amps for your wiring, gas or electricity for cooking, and what an electric car costs to charge at home.
Run your own numbers
The calculator starts on the example home: a R120 000 system, a R2 500 monthly bill, 70% of it covered by solar and R1 000 a year of upkeep. Type your own quote and bill.
Checked against published examples
Before publishing, CalcSA ran the same inputs through its calculators:
- Battery: 150 W for 2.5 hours = 375 Wh, about 470 Wh of lithium at 80% depth of discharge - the streaming-setup example in BizNews's load-shedding backup guide. Our inverter and battery calculator gives 521 Wh: the same 375 / 0.8, then a further 90% for inverter losses, which the BizNews sum leaves out. Without that step ours is 469 Wh - the same answer.
- Battery: 0.5 kW for four hours = 2 kWh of energy, and a 3.5 kWh battery will do - electrical engineers answering readers in Daily Maverick / The Conversation (1 March 2023). Ours says 2.78 kWh at the minimum (2 kWh / (0.8 x 0.9)), so their 3.5 kWh is the same sum with a margin on top. Inverter: 625 W needed, so a 1 kW unit.
- Tariff increase 9.01% for municipal customers from 1 July 2026, 8.76% for Eskom direct customers from 1 April 2026 - NERSA's decision of 5 March 2026 as published by Eskom. Our solar payback calculator uses 9.01% as its default.
- City Power prepaid, 351-500 kWh block: 333.18c a kWh before VAT = R3.83 with VAT - City of Johannesburg Electricity Tariffs 2026-2027. Our EV charging and gas or electricity calculators start on this price.
Not verified from a primary source: there is no published South African standard for solar payback, panel count or gas-hob efficiency. The 5.5 peak sun hours, 20% system losses, 80% / 50% battery depth of discharge, 90% inverter efficiency, 40% gas and 75% electric-plate efficiency and 13.8 kWh of energy a kg of LPG are installer and textbook rules of thumb - each calculator says so. The BizNews figures were read through a search summary, not the full article.
Worked example 1: when R120 000 of solar pays for itself
- Year one saves R21 000: R2 500 a month x 12 x the 70% solar covers, or R1 750 a month (solar payback calculator).
- Less R1 000 a year of upkeep, the savings reach R120 000 after 4.98 years. Tariffs going up 9.01% a year do a lot of the work - by year ten the same panels save R3 804 a month.
- Ten years of savings add up to R309 204 - R189 204 more than the system cost.
- What the sum leaves out: a battery replacement after 8 to 12 years, panels losing about 0.5% of output a year, and the interest on a solar loan. A R40 000 battery swap in year nine still leaves you well ahead in this example.
- Check your municipality's fixed charges. Several now charge solar homes a monthly grid fee, which cuts the saving.
Photo: Spaxido on Pexels (free licence)
Worked example 2: how many panels for a 600 kWh home
- 600 kWh a month is 19.73 kWh a day (600 x 12 / 365) (solar panels calculator).
- Each kWp of panels makes about 4.4 kWh a day: 5.5 peak sun hours less 20% for heat, dust, wiring and the inverter.
- 19.73 / 4.4 = 4.48 kWp, so 9 panels of 550 W = 4.95 kWp on about 23.4 m² of north-facing roof.
- That makes about 662 kWh a month averaged over the year - more in summer, less in a Cape Town winter.
- Panels only cover what you use in daylight. Without a battery, cooking dinner and the evening geyser still come from the grid - which is why the payback example above assumes 70%, not 100%.
Worked example 3: the battery and inverter for a four-hour outage
- Fridge, TV, router, lights and a laptop: about 1 000 W at once, for four hours = 4 kWh (inverter and battery calculator).
- Lithium (LiFePO4) lets you use about 80%, and the inverter loses about 10%: 4 / (0.8 x 0.9) = 5.56 kWh - 116 Ah at 48 V. A common 5 kWh battery runs this load for about 3.6 hours.
- Lead-acid can only use about 50%: the same load needs 8.89 kWh - 60% more battery for the same night.
- The inverter must carry the load plus 25%: 1 250 W, so a 1.6 kW unit. A fridge compressor draws several times its rating for a second at start-up - check the surge figure.
- Leave the kettle, geyser and stove off the backup circuit. One 2 000 W kettle would double the load.
Worked example 4: watts, amps and the breaker
- A 2 000 W kettle on 230 V mains draws 8.70 A - 2 000 / 230 (watts to amps calculator).
- The same 2 000 W from a 48 V battery bank pulls 41.7 A, and from a 12 V battery 167 A - why battery cables are so thick and why 12 V systems stay small.
- A pump or compressor with a power factor of 0.8 draws 25% more amps for the same watts.
- One hour of that kettle is 2 kWh - about R7.66 at R3.83 a unit (appliance running cost).
Photo: Dinara Guseinova on Pexels (free licence)
Worked example 5: cooking on gas or electricity
- A 9 kg refill at R360 is R40 a kg (gas or electricity calculator).
- 100 kWh a month on electric plates costs R383 at R3.83 a unit, and puts 75 kWh of heat into the pots.
- The same 75 kWh of heat on gas takes 13.59 kg = R543.48, because a gas flame loses about 60% of its heat around the pot.
- Electricity is R160 a month cheaper here; gas only wins below R28.19 a kg (R254 a 9 kg refill).
- Gas still cooks through a power cut - which no sum prices. A two-plate gas hob as backup, used only on bad nights, is cheap insurance.
Worked example 6: what an electric car costs to charge at home
- 1 500 km a month at 16 kWh per 100 km = 240 kWh into the battery, 264 kWh from the wall with 10% charging losses (EV charging calculator).
- At R3.83 a unit that is R1 011 a month - R0.67 a km.
- A petrol car using 7 litres per 100 km burns 105 litres = R3 176 at R30.25 a litre (fuel cost).
- The EV saves R2 165 a month on energy - before its higher price, insurance and finance, which usually decide the sum.
- Charge from solar during the day and the energy is close to free - 264 kWh is about 40% of what the 4.95 kWp system above makes in a month.
What to do this week
- Pull 12 months of kWh from your bill or prepaid history - every sum above starts there.
- Get two written quotes that state the kWp, battery kWh and inverter kW, and run each through the payback calculator.
- Ask for the certificate of compliance and register the system with your municipality or Eskom - grid-tied systems must be registered.
Sources
- NERSA decision of 5 March 2026 - municipal increase 9.01% from 1 July 2026, Eskom direct 8.76% from 1 April 2026: eskom.co.za, read by CalcSA 16 October 2026.
- City of Johannesburg, Electricity Tariffs 2026-2027 (final), Residential Prepaid blocks: joburg.org.za, read by CalcSA 16 October 2026.
- Daily Maverick / The Conversation, Home power backup systems: electrical engineers answer your questions (1 March 2023): dailymaverick.co.za.
- BizNews, Power your way through load-shedding - battery sizing at 80% depth of discharge: biznews.com, read through a search summary, 16 October 2026.
- Petrol 95 inland R30.25 a litre from 7 October 2026: Department of Mineral and Petroleum Resources fuel price adjustment, as used across CalcSA.
- Peak sun hours: World Bank / Solargis Global Solar Atlas, globalsolaratlas.info - look up your own address.
Calculators used in this post
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