By the Solar Installer Pretoria team • Last updated November 2026

How to Calculate Solar Battery Size and Backup Time (With a South African Example)

To size a solar battery, add up the energy your essential appliances use during an outage, then divide by the share of the battery you can safely use. To find backup time, divide your battery’s usable energy by the power your appliances draw. This guide walks through both with a worked example you can copy for your own home.

The Basics

kWh, kW and Ah: What the Numbers Mean

kW (kilowatts)

Measures power, which is how fast an appliance uses electricity.

kWh (kilowatt-hours)

Measures energy, which is how much electricity it uses over time. A 1kW heater running for one hour uses 1kWh.

Ah (amp-hours)

Measures charge. Convert it to kWh before you compare batteries:

volts × Ah ÷ 1,000 = kWh

For example, a 51.2V 100Ah battery stores 5.12kWh. A 12.8V 200Ah battery stores about 2.56kWh. Think of kWh as the size of your fuel tank and kW as how fast you burn fuel.

Want us to do the math for you? Send us your appliance list and inverter details and we will double-check your numbers.

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The Method

How to Calculate Solar Battery Size in 5 Steps

List the appliances you want to keep running and find their wattage on the label.

Multiply each wattage by the hours you need it. This gives watt-hours (Wh).

Add the totals and divide by 1,000 to get kWh.

Allow for inverter losses. Divide by about 0.9 as a starting assumption. Check your inverter datasheet for the real efficiency.

Divide by the usable share of the battery. Many lithium batteries allow about 90% use, but confirm the figure on the datasheet.

Real-World Example

Worked Example: A Pretoria Townhouse During a Four-Hour Outage

These wattages are typical, so check your own labels.

Appliance	Typical draw	Hours	Energy
LED lights (8 bulbs)	80W	4	0.32kWh
Wi-Fi router and fibre box	20W	4	0.08kWh
LED TV	100W	4	0.40kWh
Fridge-freezer (counted as always running)	150W	4	0.60kWh
Laptop and phone charging	80W	4	0.32kWh
Alarm and gate motor standby	30W	4	0.12kWh
Total	460W (0.46kW)		1.84kWh

Add inverter losses: 1.84 ÷ 0.9 = 2.04kWh.

Allow for 90% usable capacity: 2.04 ÷ 0.9 = about 2.3kWh of battery needed.

A 5kWh-class battery covers this household with plenty of room to spare. Here is the backup-time formula:

Backup hours = battery kWh × usable share × inverter efficiency ÷ load in kW

For a 5.12kWh battery: 5.12 × 0.9 × 0.9 ÷ 0.46 = roughly 9 hours.

Larger Loads

A Busier Household

Add a desktop PC and monitor (200W), a second TV (80W) and a chest freezer (100W). The load rises to about 0.84kW.

Option 1: The 5kWh Battery

A four-hour outage now needs roughly 4.2kWh of battery, and a 5.12kWh battery lasts about five hours.

That works, but with little margin, so a 10kWh battery gives you breathing room.

Option 2: The 10kWh Battery

If you want to run heavier loads for longer, the 10kWh battery is the better fit.

Breathing room matters: it allows for longer outages, cloudy days, or adding another appliance later.

Heavy Loads

Heavy Appliances Drain a Battery Fast

Appliance	Typical draw	Use	Energy
Microwave	1,200W	10 minutes	0.2kWh
Kettle	2,000W	10 minutes	0.33kWh
Geyser element	3,000W	1 hour	3kWh

One hour of geyser use takes most of a 5kWh battery. Most installers keep geysers and stoves off the backup circuit, or recommend a solar geyser installation instead.

Inverter Limits

How Inverter Capacity Affects Your Battery

The inverter’s kW rating caps what you can run at the same time, however big the battery is. If your essentials draw 0.46kW and you switch on a 2kW kettle, you need an inverter that handles roughly 2.5kW. A big battery paired with a small inverter wastes money. A hybrid solar system is designed so the inverter and battery match your loads.

The Battery (kWh)

Think of this as the size of your fuel tank. It determines how long your appliances can run before the battery is empty. A bigger battery gives you more time, but only if the inverter can handle the load.

The Inverter (kW)

Think of this as the size of the engine. It determines how much power you can pull at once. If your inverter is too small, it will trip or shut down, even if your battery is full.

Size Guide

How Much Battery Capacity Does a House Need?

Use these as starting points, then run your own numbers.

5kWh

Keep lights, Wi-Fi, fridge and devices running.

10kWh

Add extra loads, such as a work-from-home setup, or cover longer outages.

15kWh+

Larger homes, heavier loads, extended backup.

Your panels also need to recharge the battery. If you use a big battery but have few panels, it sits half empty, especially on short winter days. Our solar system design and consultation page explains how installers match panels to battery size.

Ready to see what these sizes actually cost? Read our guide on solar battery prices in South Africa.

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Avoiding Pitfalls

Common Battery Sizing Mistakes

Sizing from the price tag instead of from your loads.

Reading total capacity as usable capacity. Always check usable kWh.

Ignoring the inverter limit. Power (kW) and energy (kWh) are different limits.

Forgetting standby loads like gates, alarms and routers, which run all the time.

Putting a geyser or stove on the backup circuit without sizing for it.

Skipping room to grow. Choose a battery you can expand later. If you already have solar, read how to add a battery to an existing solar system.

The Price Tag

What Does the Battery Cost?

Size drives price more than anything else. See current solar battery costs in South Africa for prices by capacity, and learn how solar battery storage works if you want the full background. Once you know your size, you can also compare solar battery options.

Frequently Asked Questions

FAQs

How do I calculate solar battery backup time?

Multiply the battery’s kWh by its usable share and the inverter efficiency, then divide by your load in kW. A 5.12kWh battery running a 0.46kW load lasts roughly nine hours.

How big a battery do I need for load shedding?

Add up the kWh your essentials use during one outage, then divide by the usable share and the inverter efficiency. Many households that back up lights, Wi-Fi, TV and a fridge start at about 5kWh.

Can a solar battery run my whole house?

Only if the battery and the inverter both match your total load. Geysers, stoves and air conditioners drain batteries quickly, so most homes back up essentials only.

Does a bigger battery always last longer?

Only if your load stays the same. A bigger battery stores more energy, but a heavy appliance can still drain it quickly or exceed the inverter’s limit.

Not sure what size your home needs?

Send us your suburb, your essential appliances and a photo of your inverter. We will connect you with qualified Pretoria installers who size it properly.

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