Load shedding is a fact of life in South Africa, so it’s essential to have a load-shedding solution in place for your home or business.

A backup battery is ideal. But what size battery do you need? Here’s how to work it out.

Steps for determining the right backup battery size

If you’re investing in a battery or batteries as a solution for load shedding, you must decide how much energy storage you need.

The battery storage capacity is based on the energy your home or business uses, or draws, each day. Work this out and then follow some simple calculations to determine the storage capacity you need in a backup battery.

According to Eskom, the national average daily power consumption of a household is 30 kWh. The power your home or business requires may differ considerably from this average.

How long will a 10 kWh backup battery system last?

If you’re thinking about a backup battery as a load-shedding solution, you’ll be wondering what you can power and for how long.

A 10 kWh battery system can power an average household for at least 10 hours. It may last two to three times longer for more conservative energy users.

You need to decide which devices and appliances you plan to use and for how long. This is easier to work out when you know how long your battery can power them. With usable capacity of 10 kWh, you can power a:

  • 300 W TV for 33 hours
  • 200 W refrigerator for 50 hours
  • five 20 W lightbulbs for 100 hours
  • 25 W phone charger for 400 hours
  • or a 6 W Wi-Fi router for 1,600 hours.

Of course, you’ll want to run certain devices at the same time. You’ll want to keep the lights on, the fridge running, the Wi-Fi connected and the security fence/gate operating. These are all possible with a 10 kWh battery.

If you want the battery to last the outage, you won’t be able to keep the geyser on all day or wash loads of dirty laundry and dishes.

Making sense of backup battery specifications

This is complicated for anyone who hasn’t studied electricity since school. If you’re considering a backup battery as a load-shedding solution, you’ll need to know your amps from your watts. Here’s what each term means.

  • Watts (W): watts measure how much power a device uses. It is the voltage (V) multiplied by the number of amps (A).
  • Kilowatts (kW): a kilowatt is 1,000 watts.
  • Watt-hour (Wh): a watt-hour is a unit that measures the amount of electrical energy used in one hour.
  • Kilowatt-hours (kWh): this is 1,000 watt-hours.
  • Amps (amperes): amps are a measurement of electrical current.
  • Amp hours (Ah): battery capacity (how much electricity is stored in a battery) is measured in amp hours (Ah). The capacity of the battery is how long it can steadily supply the necessary current. One amp-hour is one amp for one hour or 10 amps for six minutes (1/10th of an hour).
  • Volts: voltage is to electricity what pressure is to water flow. The higher the pressure, the faster water flows in a pipe. The higher the voltage the higher the unit of current in a battery.

If you want to be able to run power-hungry appliances like a geyser, stove, dishwasher, washing machine or pool pump during loadshedding, you’ll need to investigate backup batteries with usable storage capacities of 5 kWh and higher.

Step 1

Calculate your daily power consumption in kilo watt-hours (kWh) by adding up the power consumption of each electrical device in your home or business. It’s a little time consuming but simple to do.

Start by writing down the devices – from the TV to the kettle, the fridge to the fan heater – that use electricity. For each device, calculate the daily power consumption by multiplying the wattage rating by the number of hours it is used.

Try and stick to full numbers; this will help with marginally over sizing your system to help when “that” street pole breaker trips out as well.

For example, if you have five 100 W lamps that are used for four hours a day, the calculation is:

5 x 100 x 4 = 2 kWh per day

If a device doesn’t show the consumption in watts, multiply the current in amps (A) by the operating voltage (V). (W to kilowatt=W/1000).

Example: 6A × 230V = 1380w/1000 = 1,38 kWh

Then multiply this number by the number of hours the appliance runs for.

Add up the daily power consumption of each device to get the total household consumption.

Step 2

Decide how many days of backup power you want and multiply that number by the total daily power consumption. If you want two days of backup power for your lamps, the total is:

5 × 100 × 4 = 2kWh per day × 2 days = 4kWh

Step 3

As you continue the calculation to work out your required battery capacity, you must consider the fact that all batteries last longer when they’re used according to manufacture recommendations.

You must enquire on what is allowed as a maximum depth of discharge (DoD).

Let’s use 50% as an example in our calculation. And remember that the inverter will also use energy from the batteries when in operation.

Divide the result from step 2 by this percentage (decimalised), so:

50% DoD is 4kWh × 0,5 = 8kWh.

Step 4

The number of batteries you’ll need depends on the voltage of your electrical system. You may need to connect batteries together to create a battery bank at 12, 24 or 48 V. To work out the minimum capacity of the battery/batteries, divide the result from step 4 by the desired voltages. In this case, we’ll use 24 V.

8,000/12= 667 Ah          or         8,000/24=333.33Ah          or         8,000/48=167Ah.

This can be translated into the kWh’s required for the battery pack by multiplying the Ah by the nominal voltage of the battery For Example: 167Ah × 51.2 = 8,55kWh

Step 5

The final calculation is working out how many batteries you need. Try to stay within 5% of the calculated size required, based on the bank voltage and the target Ah capacity. Always try to oversize your battery bank in terms of capacity.

For example, 100 Ah (12 V) LiFePO4 batteries for a 300 Ah 24 V battery bank:

24V = 300/100 x 2 = 6 batteries

If you wanted to create a 300 Ah battery bank at 12 V or 48V, you would need three and 12 batteries respectively.

12 V = 300/100 = 3 batteries
48 V = 300/100 x 4 = 12 batteries

Typical power needed to run household and small office appliances

At the bottom of this blog there’s a list of common appliances and how much power they use on average. This will help you work out the capacity you’ll need for a backup battery.

The REVOV STAR is a 10.2 kWh residential or small business backup power unit. This fully integrated backup system includes pre-programmed batteries, BMS and inverter, making it a quick installation and all-in-one unit.

The complete DC system stores energy, detects grid status and automatically supplies your home when the national grid is compromised. Unlike generators, the REVOV STAR requires no maintenance or fuel, creates no fumes and is very quiet. Power can be stored from solar or from the grid.

How much power does a fridge use?

The fridge is perhaps the one appliance you want to keep working during load-shedding to avoid wasting food.

Fridges vary enormously, depending on size, but the average fridge uses 700 watts. The REVOV STAR could power a 700 W fridge for 14 hours.

How to power TV during load shedding

It will depend on your TV, but if you have a 100 W 50-inch LED, the REVOV STAR could keep you watching your favourite shows for 100 hours during load-shedding.

How to power Wi-Fi during load shedding

Losing Wi-Fi and the ability to work or use the internet is one of the hardest aspects of load-shedding. The REVOV STAR could keep a 6 W router connected for 1,600 hours.

How much power do laptops, PCs and printers use?

You may use as much as 200 W to power your laptop and other office equipment. The REVOV STAR can keep you in business for 50 hours.

Guide to power used by other appliances

The following is a list of common household and office appliances and the typical power they use, in watts.

Ceiling fan 60 Laptop 50
Cellphone battery charger 25 Laser printer 600
Coffee maker 1,000 Microwave 1,000
Common light bulb 75 Oven 2,150
Computer monitor 25 Paper shredder 200
Copy machine 1,600 Pedestal fan 50
Curling iron 1,500 Personal computer (PC) 100
Deep freezer 500 Pressure cooker 700
Desktop computer 100 Sandwich maker 700
Dishwasher 1,500 Security system 500
Electric blanket 200 Side-by-side fridge 800
Electric shaver 15 Smart fridge 500
Espresso coffee machine 1,300 Smart TV 160
Fan heater 2,000 Steam iron 2,200
Fax 70 Straightening iron 75
Food processor/blender 400 Toaster 850
Fridge/freezer 700 Towel heater 60
Garage door opener 875 Tube light 22
Geyser 4,000 Tumble dryer 5,400
Hair dryer 1,250 TV (50-inch LED) 100
Home phone 3 TV (50-inch LCD) 150
Induction hob 1,400 TV (50-inch Plasma) 300
Wi-Fi router 6 Vacuum cleaner 200
Iron 1,200 Washing machine 1,150
Kettle 1,200 Window A/C 3,250

Load-shedding solutions from REVOV

REVOV offers backup batteries that are ideal load-shedding solutions for your home or business. You can charge REVOV backup batteries from the grid or from a solar, wind or hybrid installation to ensure a reliable supply of backup power during outages or for fully off-grid power.

Choose between new LiFe batteries, affordable REVOV 2nd LiFe batteries and our revolutionary, All in One Systems. Contact us for a quote or to discuss your needs.