The government is giving 100 free electricity units to domestic consumers in rural areas. How many appliances can I use for free?
— Tshomo, Dagana
I think you are right. No one explained what these energy units are and what they can be used for. Let’s take a look at how these units are computed. The story starts with watts. Each electrical appliance has a power rating in watts (named after the famous Scottish inventor James Watt). These ratings are printed on the appliance, often at the back. Watts are often abbreviated simply as W. You can find this number on any item that uses electricity, from light bulbs (ranging from around 20W to 100W) to boilers, water heaters and radiators, which are serious power hogs and can consume up to 3000W.
The amount of energy that an appliance consumes depends on its power ratings and also on how long it’s been turned on. If a 40W power bulb is switched on for two hours, it consumes double the amount of energy compared to a 40W bulb switched on for one hour. To calculate the energy used, you simply multiply the power ratings (in watts) by the time the appliance is working (in hours). The resulting number is the amount of energy units, called - how surprising it that? - watt-hours. For example, say you have a 40W light bulb switched on for a total of 10 hours. It consumed 40W x 10 hours of energy, or 400 watt-hours. Often kilowatt-hours are used instead of watt-hours; one kilowatt-hour is 1000 watt-hours.
Back to electricity units. Each month, domestic users in rural areas will receive 100 units of energy for free. Now here is the trick: Each unit is simply 1 kilowatt-hour (1000 watt-hours). For non-rural domestic users, by the way, the first 100 units are about Nu 1 each (and the next two hundred units are about Nu 2 each for both rural and non-rural consumers). Here are a few examples of typical usage. Let’s start with the ubiquitous mobile phone charger, which uses about 3W. If you charge your phone for two hours, it will use about six watt-hours, which are 0.006 units. If charged daily, this amounts to a total of 0.18 units of energy in a month. Another popular appliance is a rice cooker: A 600W rice cooker working for 20 minutes (⅓ of an hour) will consume 200 watt-hours, or 0.2 units per use. In contrast, a 1000W spiral heater working for 5 hours per day will use 5 units a day, or 150 units a month. To figure out the total units consumed by multiple appliances (mobile charging, rice cooker usage, etc.) simply add up the units.
Time per use
Units per use
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