How to Size an Inverter and Battery for Your Home
Inverter Size, Battery Capacity and Runtime Explained in Plain Language
Sizing a backup-power system becomes much easier when you separate two questions: how much equipment must run at the same time, and how long must it run? The inverter answers the first question. The battery answers the second.
SIMPLE ANALOGY
Think of the inverter as the width of a doorway and the battery as the size of a water tank. A wider doorway lets more appliances operate together. A larger tank keeps them operating for longer. One cannot replace the other.
Understand kW and kWh
Kilowatts (kW) measure power at a moment in time. A 2 kW load is demanding 2,000 watts right now. Kilowatt-hours (kWh) measure energy over time. A 1 kW load running for two hours uses approximately 2 kWh before system losses.
| Measurement | What it tells you | Buying question |
|---|---|---|
| W or kW | Instantaneous power demand | Can the inverter run the appliances together? |
| Wh or kWh | Stored or consumed energy over time | How long will the battery last? |
| VA or kVA | Apparent-power rating used on some inverters | What is the actual continuous output in watts or kW? |
Step 1: build an essential-load list
Use the wattage on each appliance label or a reliable power meter. Add only the appliances that are likely to run together. A fridge, for example, cycles on and off, while a router and alarm may operate continuously. Kettles and heaters are short-duration but high-power loads.
| Example load | Planning value | Likely use |
|---|---|---|
| 8 LED lamps | 80 W total | Evening lighting |
| Fibre ONT + router | 30 W | Continuous |
| Television | 120 W | Several hours |
| Two laptops | 130 W total | Work or study |
| Efficient refrigerator | 150 W running, higher at start | Cycles through the day |
| Example running total | About 510 W | Before start-up margin |
Step 2: size the inverter for simultaneous load and surge
Add the loads that may operate at the same time, then leave a practical margin for normal variation and future additions. Motors and compressors can draw a higher start-up current than their running value, so a refrigerator, pump or power tool may need more short-duration surge capacity than the simple running-total calculation suggests.
A useful planning formula is: required continuous inverter output = expected simultaneous running load × planning margin. A 20–30% margin is a practical starting point, not a universal rule. The final design must use the inverter’s continuous output, surge rating, battery-voltage limits and appliance start characteristics.
Step 3: estimate battery runtime
Use this planning formula: runtime in hours ≈ delivered battery energy in kWh ÷ average load in kW. “Delivered energy” is lower than the battery’s nameplate capacity because the system normally keeps an operating reserve and loses some energy through conversion.
| Nominal battery | At 500 W average load | At 1,000 W average load |
|---|---|---|
| 1.28 kWh | About 2.2 h | About 1.1 h |
| 2.56 kWh | About 4.4 h | About 2.2 h |
| 5.12 kWh | About 8.7 h | About 4.4 h |
The table uses an 85% delivered-energy planning allowance. Actual runtime changes with the battery’s discharge settings, inverter efficiency, temperature, age, load variation and manufacturer requirements.
Choose a package that matches the job
- Esener 1KW 12V Pro Inverter + 1.28KW 12V Lithium-Ion Battery Combo - catalogue price R 8,999.00. Compact backup for carefully selected small loads
- Esener 3.6KW 24V Pro Inverter + 2.56KW 24V Lithium-Ion Battery Combo - catalogue price R 12,999.00. More simultaneous power and storage for a broader essential-load circuit
- Luxpower SNA5000 5KVA Inverter + Wifi - catalogue price R 7,999.00. Standalone hybrid inverter; battery, protection and installation are separate
- Revov 12V 100AH Lithium Battery With Bluetooth - catalogue price R 5,099.00. Battery option for a compatible 12 V system; compatibility must be confirmed
Five mistakes that create disappointing systems
- Sizing from appliance count instead of actual watts and simultaneous use.
- Ignoring compressor, pump or tool start-up demand.
- Treating kVA and kW as automatically identical.
- Assuming nameplate battery capacity is all delivered to the appliances.
- Adding panels, batteries or high-power loads without checking the inverter and battery manufacturer’s limits.
Build the load list before you buy
Send the team a photograph of each appliance rating label or a simple load list with the hours of use. A correctly sized smaller system is more useful than a larger system selected from guesswork.
Browse inverters and battery combinations
Contact the team for product guidance
Frequently asked questions
Is inverter power the same as battery capacity?
No. Inverter power, measured in watts or kilowatts, limits how much load can run at once. Battery capacity, measured in kilowatt-hours, determines how long the load can run.
Why do fridges and pumps need extra inverter capacity?
Motors and compressors can draw a short start-up surge above their normal running power. The inverter must tolerate that surge.
Should a geyser be connected to a small backup system?
A conventional electric geyser is a high-energy load. It normally requires a larger system, deliberate scheduling and professional design.
Related buying guides
Sources consulted
- NERSA SSEG registration clarification - Registration depends on grid connection and installed capacity.