What battery backup actually covers
Battery backup exists for one job: keeping essential appliances running when the grid goes down. Fans, lights, a fridge and phone chargers during load shedding — that is the problem it solves.
It is not the same as a full solar system. A backup system stores electricity and releases it during outages; a solar system generates electricity from sunlight. The two can work together, but backup alone needs no panels at all.
Because the job is narrow, the sizing is straightforward. List what must stay on, how much power it draws, and for how long — and the system size follows from those three answers.
How backup sizing works
Backup sizing comes down to energy, and energy is watts multiplied by hours. A 100-watt load running for 3 hours needs 300 watt-hours of stored energy; a 500-watt load for the same 3 hours needs five times as much.
That is why the calculator asks for both appliances and hours. Watts alone tell you the inverter size, but only watts times hours tell you the battery size.
The battery bank stores the energy and the inverter converts it to the alternating current your appliances use. Both halves have to match your load, or the system either trips or runs out early.
- Energy needed = total watts × backup hours.
- Watts decide the inverter size; energy decides the battery size.
- Both halves must match your actual load.
How to use the calculator
Start by listing the appliances you want on backup and their watts — fans, lights, TV, fridge, chargers. If you are unsure of a wattage, check the label on the appliance itself.
Then set how many hours of backup you need. Be honest here: two hours of real load shedding needs a bigger bank than one, and sizing for the worst evening matters more than sizing for the average one.
The calculator then shows the inverter size your load needs and the battery bank size that covers your hours. Adjust the appliance list or the hours and watch both numbers move.
- List your backup appliances with their watts.
- Set the backup hours you actually need.
- Read the inverter size and battery bank size together.
Inverter and battery: different jobs
The inverter and the battery do different jobs, and confusing them is the most common sizing mistake. The inverter handles power — how many watts can run at once — while the batteries handle energy — how long those watts can keep running.
An oversized inverter on a small battery bank will run everything for a short time and then stop. A big battery bank on a small inverter will last for hours but trip the moment you switch on one heavy appliance.
That is why the calculator shows both numbers side by side. Size the inverter to your peak load and the batteries to your hours, and the system works as one unit instead of two mismatched halves.
Before you buy
The calculator deliberately lists no prices, because battery and inverter prices vary by brand, capacity and vendor — any number printed here would be wrong somewhere within days. Get your prices from vendors, not from a blog post.
Ask for itemised quotes: inverter, batteries, wiring and installation as separate lines, so you can compare like with like across installers.
Battery types also differ in how they behave — some need regular maintenance and ventilation, others are sealed and need less attention. Ask the vendor what type they are quoting and what its warranty covers, and check the installer's track record before paying.
- No prices here by design — get itemised vendor quotes.
- Compare inverter, batteries, wiring and installation separately.
- Ask about battery type, maintenance and warranty before buying.
