If you are considering solar for your home, one of the first questions you are likely to ask is: how long will the battery actually last when the power goes off?
The answer is not simply “a 5kWh battery lasts X hours” or “a 10kWh battery lasts all night”. Battery runtime depends on how much energy is usable, how much power your home is drawing, which appliances are running, how long the outage lasts and how the battery and inverter are configured.
That is why two homes with exactly the same battery can experience very different backup times.
Battery size does not equal backup time
A battery's capacity is normally shown in kilowatt-hours (kWh). Think of kWh as the amount of energy stored. Your appliances use energy over time, while the inverter supplies the power needed to run them.
A simplified starting point is:
Backup time ≈ usable battery capacity ÷ average load
For example, if a battery provides about 4.5kWh of usable energy and the backed-up load averages 500W (0.5kW), the simple calculation is about 9 hours.
Real-world runtime will be different because batteries are not normally discharged to zero, the inverter consumes some energy, appliance loads change over time and batteries perform differently depending on their condition and temperature. Current South African battery-runtime guides use the same basic relationship while stressing that appliance selection and usable capacity are what matter.
What does a 5kWh battery really give you?
A battery labelled around 5kWh does not necessarily mean that the entire 5kWh is available to your home. The battery management system and manufacturer's specifications determine the permitted depth of discharge and usable energy.
For a modern lithium iron phosphate (LiFePO4) battery, a planning figure around 80–90% usable capacity is common, although the exact figure depends on the battery model and its operating limits.
As a simple illustration, a 5kWh battery with 90% usable capacity provides about 4.5kWh before allowing for the rest of the system's losses.
That could be enough for many essential loads for several hours, but it can be consumed much faster if you run high-power appliances.
What does a 10kWh battery give you?
A 10kWh battery gives you roughly twice the stored energy of a 5kWh battery, assuming the two batteries have similar usable-capacity specifications.
With around 90% usable capacity, a 10kWh battery might provide approximately 9kWh of usable energy. At an average 500W load, the simple theoretical calculation would be around 18 hours.
At a 1kW average load, it would be closer to 9 hours.
At a 2kW average load, the same battery would be closer to 4.5 hours before accounting for system losses and other real-world factors.
These examples show why saying “I need a 10kWh battery” is only the beginning of the sizing discussion.
Which appliances use the most battery power?
The appliances you choose to run during an outage can make a dramatic difference to runtime.
- LED lighting: generally a relatively small load when efficient bulbs are used.
- Wi-Fi router and security equipment: usually modest continuous loads, but they run for many hours.
- Fridge and freezer: important essential loads that cycle on and off rather than drawing their maximum power continuously.
- Television and electronics: usually manageable on an appropriately sized backup system.
- Laptops and phone chargers: relatively small compared with heating appliances.
- Kettle: short operating time but high instantaneous power.
- Microwave: high power for short periods.
- Pool pump: a motor load that can add significant energy use over several hours.
- Air conditioner: can substantially increase both power demand and energy consumption.
- Electric geyser: one of the biggest household loads and usually a poor use of battery energy during an outage.
Current South African battery guidance similarly highlights geysers, pumps, air conditioners and other high-power appliances as loads that can change runtime dramatically.
Why the geyser matters so much
An electric geyser is a particularly important consideration when designing backup.
A typical geyser element can draw several kilowatts while heating. If that load is allowed to run from a battery, it can consume a large amount of stored energy in a relatively short period.
For many homes, a better strategy is to heat the geyser while solar generation is available during the day and keep it off the battery-backed circuits during load shedding.
This is not a rule for every property. If hot water is a critical requirement and the system is specifically designed to support it, the inverter and battery need to be sized accordingly.
The important point is that backup should be designed around the appliances you actually want to use, rather than assuming the entire house must operate normally during an outage.
Can a battery run the whole house?
It can, but “whole-house backup” can mean very different things from one property to another.
A home with LED lighting, refrigeration, internet, security, television and a few plug points may have a relatively modest backup load.
A home with electric cooking, a geyser, pool equipment, several air conditioners, a borehole and other high-demand appliances can have a much larger load.
In the second case, simply installing a larger battery may not solve the problem. The inverter must also be capable of supplying the required power, including suitable allowance for motor starting and other short-duration surges.
This is why battery capacity and inverter size need to be considered together.
Battery size versus inverter size
Battery capacity is measured in kWh. Inverter output is measured in kW.
They answer different questions:
- Battery kWh: how much energy is available over time.
- Inverter kW: how much power the system can supply to appliances at a given moment.
A large battery connected to an undersized inverter may still be unable to run several high-power appliances simultaneously.
Conversely, a very large inverter does not automatically give you long backup. If the battery is small, the available energy can still run out quickly.
For more on this relationship, see our guide to choosing the right solar inverter size.
What about solar panels during load shedding?
Solar panels can change the backup picture because they can produce electricity during daylight hours.
With a properly designed hybrid system, solar energy can supply the home's loads while also charging the battery. During an outage, the panels may continue contributing energy if the inverter and system architecture support this operation.
This means the battery does not always have to supply the entire daytime load from stored energy.
However, solar production changes with weather, shading, orientation, temperature, time of day and household demand. A system should therefore not be designed on the assumption that the panels will always produce their maximum rated output.
Our article on how much electricity solar panels can generate explains this in more detail.
How to make your battery last longer during load shedding
The easiest way to extend backup time is often not to buy a bigger battery immediately. It is to control the loads connected to it.
1. Back up the essentials
Decide what genuinely needs to remain powered when the grid goes down. Lighting, refrigeration, internet, security, selected plug points and essential work equipment may be more important than running every appliance in the home.
2. Avoid unnecessary heating loads
Electric geysers, heaters, ovens and other resistive heating appliances can consume stored energy quickly. Where practical, schedule these loads for periods when solar generation is available.
3. Run flexible loads in daylight
If your system is producing solar power, using energy during the day can reduce the amount that needs to be stored for later.
4. Watch motor loads
Pumps, compressors and air conditioners can have starting demands that need to be considered when sizing the inverter and backup circuits.
5. Know your actual load
A smart inverter, energy monitor or electricity meter can help reveal what the home is actually using. Real data is much better than guessing from the size of the house alone.
Example: a modest essential-load setup
Imagine a household that wants the following during a power outage:
- lights
- Wi-Fi and internet equipment
- fridge/freezer
- television
- security system
- laptop charging
If the average combined load works out to around 500W, a battery providing approximately 4.5kWh of usable energy would give a simple theoretical runtime of around 9 hours.
But if the household adds a 2kW kettle or another large appliance for significant periods, the average load changes sharply. The same battery could then provide substantially less backup time.
This is why runtime estimates should always be treated as planning estimates rather than guarantees. Appliance cycling, inverter losses, battery condition, starting loads and the amount of charge available when the outage begins all affect the result.
How much battery do I need for load shedding?
There is no single battery size that is right for every South African home.
A useful starting point is to determine:
- Which appliances must stay on.
- How much power those appliances use together.
- How many hours of backup you want.
- Whether solar panels will be available to recharge or support the load during the day.
- Whether the system must support motors, air conditioning, cooking or other high-demand equipment.
- Whether you want essential-load backup or something closer to whole-home backup.
Our earlier article How Much Battery Storage Do I Need for My Home? looks at the sizing question itself. This article is specifically about what happens to that stored energy once the outage begins.
Do you need enough battery to cover the entire night?
Not necessarily.
Some homeowners want enough stored energy to cover a particular load-shedding period. Others want overnight backup. Others want the ability to operate almost normally through a long outage.
These are very different design objectives.
For example, a homeowner who mainly wants protection from short outages may choose an essentials-only backup system. Someone who works from home, needs refrigeration and security continuously and wants several hours of evening autonomy may choose more storage.
The right system is the one that matches the actual priority, rather than simply choosing the biggest battery that fits the budget.
What happens when the battery reaches its minimum charge?
Modern battery systems are controlled by battery management systems and inverter settings that protect the battery from excessive discharge.
When the battery reaches its configured minimum state of charge, the inverter can stop supplying the backed-up loads from the battery. Depending on the system architecture and grid status, the property may then wait for grid power to return, use available solar generation, or transition according to the system's configured operating mode.
The exact behaviour depends on the inverter, battery, backup configuration and installation.
Why battery runtime should be designed around your home
Online calculators are useful for understanding the basic relationship between battery size and appliance load, but they cannot replace a proper system assessment.
A professional design should consider the battery's usable capacity, inverter capability, essential circuits, appliance loads, starting currents, solar array, electrical installation and the way the system will be operated.
It should also take into account what you may want to add later. A home that plans to install additional air conditioning, a pool pump, borehole or electric vehicle may need a different system architecture from a home that only wants basic load-shedding protection.
How Solar8 approaches battery backup
Solar8 does not believe that the answer is simply to sell the biggest battery available.
The better approach is to understand the property first: your electricity usage, essential appliances, backup expectations, roof and solar potential, inverter requirements and plans for the future.
From there, the system can be designed around what you actually want to keep running when the grid is unavailable.
Get the right battery backup for your home
If you are in Pretoria, Johannesburg, Gauteng or elsewhere in South Africa, Solar8 can help you work out how much backup capacity makes sense for your home.
We can look at your electricity usage, essential loads, appliance requirements and backup goals before recommending a complete solar and battery solution.
Get My FREE Solar8 Assessment and let us help you design a battery backup system around your home.