"How big a battery do I need?" is one of the first questions we hear from homeowners in Cape Town and Gauteng. Get it too small and the battery runs flat before morning. Get it too big and you've paid for storage you never use. The good news is that sizing a home battery isn't guesswork: it comes down to how much electricity your home uses, when you use it, and what you want the battery to do.
Start With Your Electricity Bill
Battery storage is measured in kilowatt-hours (kWh), the same unit your municipality or Eskom bills you in. Your bill shows how many kWh your home used in a month. Divide that by 30 and you have a rough daily figure. As an example, a bill showing 900 kWh for the month works out to about 30 kWh a day.
That daily number is the starting point, but not the whole answer. What matters more is how much of it you use after the sun goes down, because that's the part a battery has to cover. During the day, well-sized solar panels can run your home directly and charge the battery at the same time.
What Do You Want the Battery to Do?
Most homeowners want one or more of these three things, and each one points to a different size:
- Essential backup. Keep the lights, Wi-Fi, fridge, TV and security running during an outage. This needs the least storage, because only selected circuits are backed up.
- Whole-home backup. Keep everything running, including the oven, kettle and pool pump, when the grid goes down. This needs more storage and an inverter powerful enough to carry the bigger loads.
- Lower bills every day. Store your own solar power in the afternoon and use it in the evening instead of buying from the grid. Here the battery is sized to your evening and overnight use.
Remember that a battery can only deliver power as fast as its inverter allows, so storage (kWh) and inverter power (kW) always have to be designed together.
Usable Capacity: The Number That Counts
When you compare batteries, look at usable capacity, not just the headline figure. The Tesla Powerwall 3, for example, is a 15 kWh battery with 13.5 kWh usable, and it has an 11 kW inverter built in. The Powerwall 3 Expansion adds another 13.5 kWh usable, with up to three Expansions per Powerwall 3, so storage can grow with your home.
Sigenergy takes a modular approach. Its 6, 8 and 9 kWh battery modules stack on top of each other, with the hybrid inverter sitting on top, so you can start small and add modules later.
Typical Battery Sizes for Homes
These are the battery sizes in our installed home packages. Prices are indicative and include VAT, installation, certification and council registration:
- Sigenergy Small: 6–8 kWh battery with a 6 kW inverter, from R100–125k.
- Sigenergy Regular: 12–16 kWh battery with a 10 kW inverter, from R160–200k.
- Tesla Regular: one 15 kWh Powerwall 3 battery with its 11 kW inverter, from R180–220k.
- Sigenergy Large: 16–24 kWh battery with a 15 kW three-phase inverter, from R210–270k.
- Tesla Large: 30 kWh of battery (two 15 kWh units) with an 11 kW inverter, from R290–310k, or with a 22 kW inverter (Large+), from R310–340k.
- Tesla and Sigenergy XL: 32–48 kWh with a 30 kW inverter (Sigenergy, from R340–460k) or 45 kWh with a 33 kW three-phase inverter (Tesla, from R430–500k) for large homes.
Every package also includes solar panels sized to match, and you can see the full breakdown on our residential solar page. If you'd rather spread the cost, we also offer solar finance.
Plan for Tomorrow, Not Just Today
Your electricity use won't stay the same forever. An electric car, a new pool pump or a home office can all add to it. That's why we like battery systems that can grow. With the Powerwall 3 Expansion and stackable Sigenergy modules, you can start with the storage you need now and add more later without replacing the whole system.
Let Us Do the Maths
The quickest way to the right battery size is a design based on your real usage. MLT Power is a family business founded in 1986, with more than 1,000 installations and a 4.9-star Google rating from 143 reviews. We're a Tesla Premium Installer, South Africa's first installer of the full Tesla range, and a Sigenergy Certified Installer, so we'll show you honestly which battery size and system suit your home.
Send us your latest electricity bill or call 087 943 2299, and we'll design your system with a free, fixed-price quote. Still deciding between brands? Read our Tesla Powerwall 3 vs Sigenergy comparison.
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