Inverter and Battery Calculator

How the Inverter and Battery Calculator Works

Sizing a backup power system correctly requires two separate calculations: the inverter must handle the peak power draw (in watts or VA) of all appliances running simultaneously, and the battery must store enough energy (in watt-hours) to run those appliances for the required backup duration.

Inverter sizing: Total the running wattage of all selected appliances. The inverter must handle this load with a safety margin. The calculator adds 25% headroom for surge currents (motors draw 2–3× their running wattage at startup) and divides by an 80% inverter efficiency factor to convert watts to VA:

Battery sizing: Total watt-hours needed equals total watts × backup hours. The battery capacity in amp-hours (Ah) is:

Depth of discharge (DoD) is how much of the battery's rated capacity can be safely used. Lithium batteries (LiFePO4) support 80–90% DoD without significant lifespan reduction. Lead-acid batteries should not be discharged below 50% - doing so permanently reduces their capacity. The calculator adjusts for the selected battery chemistry.

How to Use This Calculator

1. 1

   Select the appliances you need during load-shedding

   Be realistic - running a kettle (2 000 W) or microwave (1 000 W) requires a very large inverter and drains batteries rapidly. Focus on essentials: lights, internet router, laptop, and perhaps a TV. High-draw appliances like geysers, stoves, and air conditioners are usually not practical for battery backup.

2. 2

   Set the backup duration

   South Africa currently experiences load-shedding in 2-hour or 4-hour blocks. Choose a backup duration that covers the longest typical outage in your stage: 2 hours for Stage 2, 4 hours for Stage 6. A longer backup duration requires proportionally more battery capacity.

3. 3

   Choose battery chemistry

   Lithium Iron Phosphate (LiFePO4) batteries are safer, lighter, longer-lasting (3 000–6 000 cycles), and support deeper discharge than lead-acid. They cost 2–3× more upfront but typically have a lower lifetime cost. Lead-acid (AGM) batteries are cheaper upfront but last only 300–500 cycles at 50% discharge and are significantly heavier.

4. 4

   Review the recommended system specifications

   The calculator outputs the minimum inverter VA and battery Ah, rounded up to standard commercial sizes. Use these as a floor - buying the exact minimum leaves no headroom for future appliances. A 20% upsize on both inverter and battery is a practical rule of thumb.

5. 5

   Get quotes from registered installers

   Use the specifications from this calculator when requesting quotes. Common inverter brands in South Africa include Victron Energy, Sunsynk, Goodwe, and Deye. Common battery brands include BSL, Pylontech, BYD, and Freedom Won. Ensure any installation is done by a registered electrician with a valid CoC.

Frequently Asked Questions

How much does a home inverter and battery system cost in South Africa?

A basic load-shedding backup system (1–2 kVA inverter + 2–5 kWh LiFePO4 battery) costs approximately R25 000–R60 000 installed. A mid-range system capable of running most lights, internet, a TV, and a fridge for 4 hours (3–5 kVA inverter + 5–10 kWh battery) costs R60 000–R120 000. A whole-home backup system with solar integration can cost R150 000–R400 000+. Prices have fallen significantly since 2022 due to increased competition and Chinese battery imports.

What is the difference between a UPS, inverter, and hybrid inverter?

A UPS (uninterruptible power supply) provides immediate switchover (milliseconds) and is designed for computers and sensitive electronics - typically small capacity (500 VA to 2 kVA) and short backup time. A standard inverter switches within 20–50ms - fine for most appliances but may reset some electronics. A hybrid inverter (also called a solar inverter or multi-mode inverter) can simultaneously manage solar panels, grid power, and battery storage, optimising between all three sources. For load-shedding, a hybrid inverter with battery gives the best flexibility.

Can I run a geyser or stove from an inverter during load-shedding?

Technically yes, but it is generally impractical for a residential backup system. A standard electric geyser uses 3 000–4 000 W and an electric stove plate uses 1 500–2 500 W - these loads drain batteries extremely quickly. To run a geyser for 1 hour, you need approximately 4 kWh of usable battery capacity (a R30 000+ battery bank). For most households, the practical solution is to time geyser heating before load-shedding schedules and use a gas hob for cooking during outages.

What is a Certificate of Compliance (CoC) for an inverter installation?

A Certificate of Compliance is issued by a registered electrician (registered with the Electrical Contractors Board under the Occupational Health and Safety Act) after verifying that an electrical installation meets South African wiring standards (SANS 10142). An inverter or solar installation must have a CoC to be legal, to maintain your homeowner's insurance, and to comply with most HOA requirements. Installing an inverter without a CoC is a safety risk and can void your insurance if there is a fire or electrical fault.

How long do LiFePO4 batteries last in South Africa?

Quality LiFePO4 (Lithium Iron Phosphate) batteries are rated for 3 000–6 000 charge cycles at 80% depth of discharge. At one full cycle per day (one load-shedding outage), this translates to 8–16 years of useful life. In practice, with 2-4 stage load-shedding (partial cycles), batteries may last longer. The main risk to battery life in South Africa is high ambient temperatures - batteries degrade faster above 35°C, so adequate ventilation in the battery cabinet is important.

Do I need permission to install a backup system in a sectional title complex?

Yes. Sectional title owners must comply with the body corporate's rules on electrical installations. Most complexes require body corporate approval before any inverter or battery installation, and the installation must meet the complex's metering and safety requirements. Some complexes have blanket restrictions on rooftop solar panels (shared roof) but allow internal battery systems. Check your conduct rules and consult the body corporate trustees before purchasing - non-compliance can result in forced removal of the system.