Watt-hour Calculator

Calculate energy consumption and electricity cost for any appliance.

Eskom tariffs have climbed year after year, and most South African households have no real idea which appliances are quietly draining their wallets. A geyser left on overnight, an old fridge ticking away in the kitchen, a pool pump running longer than it needs to — these add up fast.

This Watt-hour Calculator gives you a simple way to convert any appliance’s wattage into actual energy use (in Wh and kWh) and translate that into a daily and monthly cost in rand. Enter the wattage on the appliance label, how long you use it, and your electricity rate — the calculator does the rest.

What Is a Watt-hour?

A watt-hour (Wh) is a unit of energy. It tells you how much electricity an appliance uses over time, not just how powerful it is at any single moment.

Here’s the difference in plain terms:

• A watt (W) is the rate at which an appliance draws power.
• A watt-hour (Wh) is what you get when an appliance running at that rate uses electricity for an hour.

So a 100-watt light bulb left on for 1 hour uses 100 watt-hours of energy. Run that same bulb for 10 hours and you’ve used 1,000 Wh — which is the same as 1 kilowatt-hour (kWh).

Eskom and your municipality bill you in kilowatt-hours, so understanding how watts, hours and kWh fit together is the first step in taking control of your power bill.

The Watt-hour Formula

The maths behind this calculator is straightforward:

Energy (Wh) = Power (Watts) × Time (Hours)

To convert that into kilowatt-hours, simply divide by 1,000:

Energy (kWh) = (Watts × Hours) ÷ 1,000

And to work out the cost in rand:

Cost (R) = kWh × Electricity Rate

A Practical Example

Let’s say you run a 2,000W kettle for 15 minutes (0.25 hours) a day:

• Daily energy use: 2,000 × 0.25 = 500 Wh (0.5 kWh)
• Monthly use (30 days): 0.5 × 30 = 15 kWh
• At R3.20/kWh: 15 × R3.20 = R48 per month

Doesn’t sound like much on its own — but multiply that by every appliance in the home and the picture changes quickly.

How to Use This Watt-hour Calculator

The calculator only needs four inputs:

1. Appliance Power Rating (Watts) — Look on the back, base or label of the appliance, or check the manual. You’ll usually see a number followed by “W” (e.g. 1500W). Some labels show amps and volts instead — multiply them together to get watts.
2. Hours Used Per Day — Estimate how long the appliance actually runs. For things like fridges and geysers, the compressor or element only runs part of the time, so use the duty cycle (more on that below).
3. Days Used Per Month — Most appliances run daily (30 days), but seasonal items like heaters, pool pumps or aircons may only run on certain days.
4. Electricity Rate (R/kWh) — Use your latest municipal or Eskom tariff. As of 2025, residential rates in South Africa generally fall between R2.50 and R4.50 per kWh depending on your municipality and consumption block.

Click Calculate and you’ll see:

• Daily consumption in watt-hours and kilowatt-hours
• Total monthly consumption in kWh
• Daily and monthly cost in rand

Typical Power Ratings for Common South African Appliances

Wattages vary by model, but these averages give you a useful starting point if your appliance label is missing or unreadable:

Appliance	Typical Wattage	Notes
Electric geyser (150L)	3,000W	Element only runs intermittently
Kettle	2,000W – 2,400W	High wattage, short run time
Tumble dryer	2,500W – 3,000W	One of the biggest energy users
Iron	1,500W – 2,400W
Microwave	800W – 1,200W
Oven (electric)	2,000W – 3,000W
Stovetop plate	1,500W – 2,500W	Per plate
Dishwasher	1,200W – 1,800W
Washing machine	500W – 1,500W	Higher with heated water
Fridge/freezer combo	100W – 250W	Compressor cycles on and off
Air conditioner (split)	1,000W – 2,500W
Pool pump	750W – 1,500W
Plasma/LED TV	50W – 200W
Desktop computer	200W – 500W
Laptop	30W – 90W
LED light bulb	5W – 12W
Old incandescent bulb	40W – 100W	Worth replacing
WiFi router	5W – 15W	Runs 24/7
Phone charger	5W – 20W

Understanding Duty Cycles

Some appliances don’t run continuously even when they’re switched on. A fridge, for example, has a compressor that cycles on and off to maintain temperature. A geyser’s element heats water until the thermostat cuts out, then waits until the temperature drops before kicking in again.

This is called a duty cycle, and it matters because the appliance only draws its full wattage during the active part of the cycle.

As a rough guide:

• Fridges and freezers: roughly 30–50% duty cycle (so a 200W fridge effectively averages 60–100W over 24 hours)
• Geysers: depends heavily on usage, insulation and thermostat settings, but a typical household geyser runs 3–5 hours per day in total
• Air conditioners: vary based on ambient temperature and the unit’s inverter technology

When entering hours into the calculator for these appliances, use the effective run time rather than the time the appliance is plugged in or switched on.

Why This Matters for South African Households

With load shedding, rising tariffs, and growing interest in solar, knowing your appliance energy use isn’t just academic — it has real practical value:

Sizing a backup or solar system. If you’re investing in an inverter, battery or solar setup, you need to know your kWh consumption to size the system correctly. Undersize it and you’ll be in the dark; oversize it and you’ll waste money.

Identifying the biggest culprits. Geysers and tumble dryers typically account for the largest share of household electricity bills. Quantifying their cost makes the case for geyser timers, solar geysers, or hanging laundry on the line a lot more compelling.

Comparing appliances before buying. A cheaper appliance that uses 30% more electricity may end up costing you more over its lifetime. Use the calculator to model the lifetime cost difference before you commit.

Budgeting for prepaid electricity. If you’re on a prepaid meter, this calculator helps you predict how long a recharge will last and avoid running out at the worst moment.

Tips to Reduce Your Watt-hour Consumption

A few changes can make a noticeable dent in your monthly bill:

• Install a geyser timer so the element only heats water during the hours you actually need hot water.
• Lower the geyser thermostat to 55–60°C. It’s enough for showering and bathing, and reduces standing losses.
• Switch to LED bulbs. A 9W LED produces the same light as a 60W incandescent — that’s an 85% reduction.
• Use cold water for laundry where possible. Heating water accounts for most of a washing machine’s energy use.
• Hang laundry instead of using the tumble dryer when weather allows. Dryers are among the most expensive appliances to run.
• Unplug devices that draw standby power. TVs, set-top boxes, microwaves and chargers can collectively consume 5–10% of your monthly electricity even when “off”.
• Defrost the freezer regularly. Ice build-up makes the compressor work harder.
• Service air conditioners so they run efficiently.

Frequently Asked Questions

How do I find out the wattage of my appliance? Check the rating plate or sticker on the back, bottom or inside the appliance. It will usually list a wattage (e.g. “1500W”). If only voltage and amperage are shown, multiply them: Watts = Volts × Amps.

Is 1 kWh the same as 1,000 watts? Not quite. 1 kWh is 1,000 watts of power used for one hour. So a 1,000W heater running for 60 minutes uses 1 kWh. The same heater running for 30 minutes uses 0.5 kWh.

Why is my actual bill different from what the calculator predicts? Real-world consumption depends on duty cycles, seasonal variation, tiered tariff blocks, fixed service charges, and additional appliances. This calculator gives you an estimate per appliance — your full bill includes everything in the home plus fees and taxes.

What electricity rate should I use? Use the rate from your most recent municipal or Eskom bill. Many South African tariffs are tiered (Inclining Block Tariffs), meaning the price per kWh increases the more you use. If you’re unsure, use your average effective rate (total bill ÷ kWh used).

Does this calculator account for inverter and battery losses? No — it calculates appliance-level consumption only. If you’re sizing a backup system, add roughly 10–20% to account for inverter inefficiency and battery losses.