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What size inverter and battery do I need for my house?

By Kagiso Mahlangu, The Bill Detective · 27 October 2026 · 10 min read

Short answer, as of 27 October 2026: add up the watts of everything you want running at the same time, multiply by 1.25, and round up to the next standard inverter size. That's usually 3 kW for essentials (lights, Wi-Fi, TV, fridge), 5 kW if you want a kettle or microwave on backup too, and 8 kW or more for air conditioning, a pool pump or a geyser (SolarZA sizing guide). Then size the battery separately, by energy: the watts you'll draw, times the hours you need, divided by the share of the battery you can actually use.

Those are two different questions, and mixing them up is the most common reason people end up with the wrong system. The inverter decides how much you can switch on at once. The battery decides for how long. Here's how to work out both for your house in about 15 minutes, with real appliance figures and current prices.

Inverter vs battery: what each one does

A hybrid inverter also charges the battery from solar panels and the grid, and manages the switch-over when the supply drops. If you have panels, the inverter's solar input matters too, but that's a separate decision covered in our 5 kW solar system cost guide.

Step 1: List what must keep running

Be ruthless. Backup systems get expensive fast when everything in the house is on the list. Most homes split loads into two groups, and good hybrid inverters have separate "essential" and "non-essential" outputs for exactly this reason (the Sunsynk 5 kW hybrid is one example).

Here are typical ratings. Measured figures come from the PowerSaving appliance database, which lists the measured draw of specific models; the rest are planning estimates from SolarZA. Your own appliances' labels beat both.

Two things jump out. A fridge, Wi-Fi, TV and lights together use less than a single kettle. And the geyser alone uses more than everything else on the essentials list combined. That's why almost nobody puts a geyser on battery backup: it's cheaper to heat water another way, which we'll cover in a later guide on solar geysers.

Step 2: Size the inverter

1. Add up the watts of everything on your list that could be on at the same time.
2. Multiply by 1.25. Motors in fridges, pumps and washing machines draw two to three times their running watts for a moment when they start, and the margin covers that (SolarZA; RandTools calculator).
3. Round up to the next standard size: 3 kW, 5 kW, 8 kW, 10 kW or 12 kW.

Example A: essentials only

This is an illustration with stated assumptions, not a real household. Using the measured figures above and the hungriest fridge in the list:

• Fridge/freezer 220 W + lights 100 W + TV and decoder 103 W + laptop 25 W + alarm 12 W + router 6 W = 466 W
• 466 W x 1.25 = 583 W

On paper, even a small inverter could cope. In practice, installers rarely fit anything below 3 kW for a house, because it leaves room for the occasional extra load and a later solar upgrade.

Example B: essentials plus the kitchen

Same house, but they want to boil the kettle and use the microwave during an outage, possibly at the same moment:

• 466 W + kettle 1,750 W + microwave 1,445 W = 3,661 W
• 3,661 W x 1.25 = 4,576 W, so a 5 kW inverter

You could squeeze this onto a smaller inverter by never running the kettle and microwave together. That's a household rule, not a technical limit, and it tends to break the first busy morning.

Check the surge rating too. Inverter spec sheets list a short-term figure as well as the continuous one. The Sunsynk 5 kW single-phase hybrid, for example, is rated 5,000 W continuous, 5,500 W for five minutes and 11,000 W for one second (EnergyTech Store spec listing). That one-second headroom is what lets a fridge compressor start while the kettle is on.

Step 3: Size the battery

The formula, used by most South African calculators (RandTools; Calcy):

Battery size (kWh) = load (kW) x hours of backup ÷ usable fraction

The usable fraction is the depth of discharge (DoD), the share of the battery you can drain without shortening its life. For the lithium iron phosphate (LiFePO4) batteries sold in South Africa, makers and retailers quote 80% to 90%; old-style lead-acid batteries shouldn't go below 50% (Calcy). Energy Bee lists a 5.12 kWh Dyness BX51100 at about 4.6 kWh usable and a 5.3 kWh Hubble AM-5 at 4.77 kWh usable, both at 90% DoD (Energy Bee, 13 Oct 2026).

Example A again: how long would one battery last?

Illustration with stated assumptions. Take the essentials load of 466 W and assume, to be safe, that everything runs flat out the whole time:

• One 5.12 kWh battery, about 4.6 kWh usable
• 4.6 kWh ÷ 0.466 kW = about 9.9 hours

That's a cautious figure, because a fridge doesn't run constantly. Eskom's own residential calculator assumes a fridge's compressor runs about three hours a day to hold its temperature (Eskom residential calculator assumptions). On that assumption the same battery lasts noticeably longer. Inverter losses and the inverter's own standby draw pull the other way, so treat about 10 hours as a fair planning number for this load and let your installer refine it.

Example B: when the kettle changes the sum

Big loads matter less to the battery than you'd think, because they run briefly. Boiling a kettle for three minutes at 1,750 W uses about 0.09 kWh (1.75 kW x 0.05 hours). Ten boils a day is under 1 kWh. The fridge, router and alarm, quietly running all night, are what drain a battery.

The detail most quotes skip: can the battery feed the inverter?

A battery has a maximum power it can deliver, not just an energy capacity, and it's often quoted as a current or a "C-rating".

• Energy Bee lists the Dyness BX51100 at about 50 A continuous (0.5C class) (Energy Bee). At its 51.2 V nominal voltage, 50 A x 51.2 V is about 2,560 W.
• The Hubble AM-5 Plus is a 1C battery rated for 5.12 kW continuous (GeeWiz product listing).

So in Example B, with the grid down and no sun, a single 0.5C battery couldn't supply the full 3,661 W the kettle, microwave and essentials need together, even though a 5 kW inverter could handle it. You'd need a higher-rated battery or two batteries in parallel. SolarZA puts it simply: match the battery to the inverter so it can deliver the inverter's full power (SolarZA). Ask every installer to confirm this in writing.

What it costs right now

These are listed retail prices for the hardware only, including VAT. They exclude installation, cabling, protection gear, the Certificate of Compliance (CoC) and any solar panels.

As a worked sum, the cheapest 5 kW inverter and the Hubble battery above come to R34,982 in hardware (R17,140 + R17,842). Add Energy Bee's installation range and a basic backup-only system lands at roughly R40,000 to R50,000, before any panels. That's arithmetic on one-off listed prices, so use it as a sense check against real quotes, not a quote itself. Prices move with the rand and with retailer specials; several of the inverter listings above were marked as discounted.

Energy Bee also points out that bigger battery banks cost less per usable kWh, so it can be cheaper to size right first time than to add a second battery later (Energy Bee). The flip side: don't buy a big battery purely for outages that may not come.

Is a big backup system worth it now?

Load shedding has largely stopped for now. As we covered in is solar still worth it now that load shedding has eased?, Eskom reported more than 500 consecutive days without it by October 2026, although local load reduction still affects some areas. That changes the sizing advice:

• If you only want outage cover, size for essentials and a few hours. Example A on one 5 kWh battery covers most short outages and municipal faults.
• If you want to cut your bill, the battery matters less than the panels. A battery mainly lets you use daytime solar in the evening, so match it to your evening usage, not to a worst-case outage.
• If you work from home or run a medical device, reliability is worth more than the sum suggests, and a second battery may be sensible.
• When it isn't worth it: if your bill is small, you rent, or your area rarely loses power, a portable power station or a UPS for the router may do the job for a fraction of the price.

Registration and paperwork in Tshwane

A battery system wired into your house is electrical work. You need a registered electrician and a Certificate of Compliance.

On registration, NERSA clarified in February 2026 that an embedded generation facility of 100 kW or less with a point of connection to the grid must register with its distributor (Eskom or the municipality), and that this depends on the grid connection, not on whether you export power. Systems with no point of connection to the grid are exempt (SAnews, 3 February 2026). The City of Tshwane handles applications through sseg@tshwane.gov.za and says nobody may feed power into its grid without approval (City of Tshwane FAQ).

If you add solar panels and register as a grid-connected generator in Tshwane, the City's 2026/27 tariff adds a fixed monthly charge; the details and what it does to payback are in our solar payback guide. Ask your installer how they'll handle registration and whether your setup counts, before you sign.

Your 15-minute checklist

1. Walk the house and list what must stay on in an outage. Note the watts from each label.
2. Mark which of those could run at the same time, add them up, and multiply by 1.25. Round up for your inverter size.
3. Estimate your outage load and hours, and divide by 0.9 (lithium) for the battery size.
4. Check the battery's continuous power rating covers the load you'll draw from it.
5. Decide whether you'll add panels now or later, and tell the installer, so the inverter has the right solar input.
6. Get three itemised quotes naming the exact inverter and battery models, the installation cost, the CoC and how registration will be handled.

If you're in Pretoria, Centurion or elsewhere in Tshwane, you can request free quotes from vetted local installers. Take your list from steps 1 to 3 with you; it makes quotes far easier to compare.

Sources

All checked on 27 October 2026.

• SolarZA, What size inverter do I need? South Africa 2026 guide.
• PowerSaving, Database of appliance power ratings.
• RandTools, Inverter and battery calculator South Africa 2026.
• Calcy, Inverter and battery calculator.
• Eskom, Residential calculator and its assumptions.
• Energy Bee, Solar battery prices South Africa 2026 and Dyness vs Hubble comparison, 13 October 2026.
• SolarNevs, Sunsynk 5 kWh battery price survey, 16 September 2026.
• Retail listings: Get Green Energy, ARB Electrical Wholesalers, EnergyTech Store, Solar Shop, GeeWiz.
• SAnews, NERSA clarifies Small-Scale Embedded Generation regulations, 3 February 2026.
• City of Tshwane, Electricity FAQs.

Kagiso Mahlangu is the pen name of The Bill Detective's editorial team. Figures come from the published sources linked above and were checked on the date shown; tariffs and prices change, so confirm with your municipality and get written quotes. Example scenarios are illustrations, not real customers. If you ask for quotes through this site we may be paid a referral fee by the installer, at no cost to you.