Free tool
How big a backup do you actually need?
Most South African businesses either buy nothing, or over-buy a ride-through system they never needed. Enter your load and how long you want to survive a slot — get the inverter and battery size, and what that load costs to run in rand.
1.50 kWh
Usable energy needed
2.4 kWh
Battery (recommended)
1 kVA
Inverter (recommended)
R1 226
Energy cost / month
Over three years this load costs about R44 126 to run — before a single unplanned shutdown. One hard cut mid-write on a database is a restore, not a reboot.
Sizing is a minimum after inverter losses and battery depth-of-discharge. Size up for start-up surge and future load. It is guidance, not a quote.
Methodology
How the sizing works, so you can argue with it.
Usable energy is simply your load in kilowatts multiplied by the hours you want to ride through. A 600 W rack over a 2.5-hour slot needs 1.5 kWh at the load.
A battery has to deliver more than that. We divide by an inverter efficiency of about 90%, then by a usable depth-of-discharge of about 90% for lithium — so the nameplate battery is larger than the energy you actually use. The inverter is sized to the load with a power-factor allowance and 25% headroom, then rounded up to a common size.
The running cost multiplies the load by its hours and a rand-per-kWh tariff you can change — a published utility rate, never our cost. Every figure is a starting point; a real design accounts for surge, growth and your specific site.
Questions
Load-shedding, sized.
How much battery do I need to survive load-shedding?
Enough usable energy to cover your load for the length of the slot, plus headroom for inverter losses and battery depth-of-discharge. For a 500 W load over a 2.5-hour Stage 4 slot that is about 1.25 kWh usable, which after losses means roughly a 2.4 kWh lithium battery and a 1 kVA inverter as a minimum — size up for start-up surge and future load.
Do I need to keep everything running, or just shut down safely?
Often the cheaper, sufficient answer is a graceful-shutdown UPS rather than a full ride-through system. You do not always need to keep working through a slot; you need to not stop badly. A hard cut mid-write on a database is a restore, not a reboot.
What does the load actually cost to run?
The calculator multiplies your load by the hours it runs and a rand-per-kWh tariff you can adjust. That is the electricity cost only — it is a published utility rate, not a hardware price. An always-on 600 W rack is over R1,000 a month in energy alone.
Is this a quote?
No. It is engineering guidance to get you to the right size before you shop. When you are ready, we will size and quote a system properly for your site.
Ready to size it for your site? Talk to us or browse backup power.