My Rambling Thoughts

Taking guesswork out of electrical usage


Tuya Smart Plug

Get the Tuya Smart Plug with Power Monitor feature.

First thing first. This is an IoT device and requires Internet connection (phones home to Tuya Cloud). Its security is suspect. By default, it is controlled using a mobile app. It can be controlled remotely.

(Tuya devices can be controlled without Internet access, but it requires some setup.)

The 20A model can be gotten at S$7+ on Lazada. 16A should be ample — the UK plug is rated 13A only. I don't see how it can be used to measure 20A appliances.

It consumes some power. It uses 15W/day in on state, <5W/day in off state. If you go wild and make all your appliances "smart", you will add several kWh usage every month! (e.g. an extra 9.2 kWh/month for 20 appliances.)

To get accurate power usage, the plug should be kept on (up to an hour after last use). It takes time to upload readings to Cloud. It is not real-time. Even so, these devices are considered "chatty".

It does not show devices drawing <3.5W, but it still accumulates their usage.

Devices that use <0.5W draw 10W/day. I have 10 to 15 such devices around. Collectively, they use 4.6 kWh per month.

Appliances with soft power button still draw power when off. Appliances with hard power button may be truly powered off.

The fridge uses ~2 kWh daily. This is ~65% my estimate, but is twice its rated power consumption (0.98 kWh/day).

After measuring several devices in soft-off or sleep mode, my takeaway is:

  • If it uses <0.5W, can keep it on 24/7 (0.37 kWh/month)
  • If it uses >1.5W (1.1 kWh/month) and is rarely used, e.g. once every few days, it is better to turn it off

Power usage by appliance

AppliancePower (W)Est usageMthly kWh
Air-con 1149010h/day223.5
Dehumidifiers2008h/day48.8
Dryer21002h/week16.8
Fridge 226024/795.2
Hair dryer16005m/day4.1
Iron240030m/mth1.2
Kettle220016m/day17.9
Kitchen hood?00
Microwave12103m/week0.3
Oven?00
Toaster9804m/day2.0
Vacuum cleaner 3105060m/mth1.1
Washing machine 420004h/week3.2
Water dispenser 5200024/7?
Water heater450015m/day34.3
 
Fans10018h/day54.9
Lights4712h/day17.1
Air purifiers1124/78.1
Air quality monitors1.2524/73.7
 
PC2224/716.1
Notebook1324/79.5
Modem4.224/73.1
Routers1124/78.1
Chargers156h/day5.5

1 With one cooling unit at 50% duty cycle

2 I'm assuming the fridge uses 130W to maintain coldness

3 It takes a lot of power to create suction

4 When using warm or hot water. I estimate it uses 200W with cold water

5 It is in sleep mode when not dispensing water

It takes a lot of energy to produce instant heat.

The top 3 energy consumers are air-con (223.5 kWh), fridge (95.2 kWh) and fans (54.9 kWh), followed closely by dehumidifiers (48.8 kWh).

Fans use a lot of energy, surprisingly.

How to calculate power usage

If an appliance uses 10W, it uses 10 Wh in one hour. It'll use 240 Wh in a day and 7.32 kWh in a month (if 24/7).

Another example. If an appliance uses 1000W but is only used for 10 minutes a day, it'll use 166.7 Wh in a day and 5.08 kWh in a month.

The electrical tariff is currently S$32.47 cents/kWh (including GST).

1 kWh is actually quite cheap. The trick is to use less of appliances that use >10 kWh per month.

Top energy guzzlers

Don't need to think. The top 3 are: air-con, fridge and water heater. The question is, how much do they consume?

Unfortunately, we are not able to measure the power consumption of air-con and water heater directly, so we can only estimate.

My fridge is rated to use 358 kWh/year (3 energy star rating). Is this true? In contrast, my dryer is rated at 683 kWh/year.

Estimating air-con

My air-con is MA56EV1 (compressor) with four FT25DVM FCUs (Fan Coil Unit). It is non-inverter.

#FCUUnit capacity (kW)Total power (kW)
12.561.49
22.151.52
31.781.60
41.451.64

(The full table is more complicated cos there can be a mix of 2.5, 3.5, 5.0 kW units.)

Running four 2.5 kW units just about max out the capacity (5.80 vs 5.94 kW).

It does not take much more energy to run 2 or 3 units.

Assuming one air-con is turned on for 10 hours a day and the compressor runs for 5 hours, the energy consumed is 223.5 kWh/month.

Estimating water heater

My water heater is Bennington C600. It is rated to use 4.5 to 5.3 kW. It requires 20A plug. A standard 13A plug allows max 3 kW.

Assuming 15 minutes usage daily, the energy consumed is 34.3 kWh/month.

Estimating lights

LED lights do not use much power, but they still add up over time.

I estimate I turn on 6 10W LED lights for 4 hours at night and 4 of them for 8 hours in the day — my living room is rather dark. It works out to be 17.1 kWh/month.

Estimating ceiling fan

I cannot find the power rating for my ceiling fan. I normally set it to lowish medium speed. My guess is 60W. It is kept on for a long time, sometimes even for the entire day. If it's 12 hours on average, it works out to be 22.0 kWh/month.

Electric bill 2024

YearMthTariff (cents/kWh)Usage (kWh)AvgEff
Jun485446261
May* 587504302
Apr29.79626479265
Mar* 406399252
Feb375348212
2024Jan29.89* 464444272
Dec310558301
Nov* 546425260
Oct28.70512428258
Sep* 634475284
Aug543453268
Jul27.74* 705506291
Jun701473254
May* 495423-
Apr27.43628352-
Mar* 467--
Feb512--
2023Jan28.95* 486--
Dec360--
Nov* 581--
2022Oct29.74?--

* Estimated.

My electrical usage is around 550 to 600 kWh, although there are surges to 650 kWh. It remains mostly the same from two years ago.

I can tell the efficient neighbours don't use air-con. It is difficult to match them, but is it possible to catch up to average neighbours?

(Efficient refers to the average of the lower 50% of neighbours.)

Who are the big power users? Air-con alone contributes 150 to 200 kWh. The fridge is always on 24/7. There are several 24/7 devices too.

Revisit QuickBASIC?

QuickBASIC 4.5 holds a lot of nostalgia for me. But I am unlikely to revisit it. The language has too many issues and limitations.

  • Limited operator precedence
  • No short-circuit logical operators (VB added AndAlso and OrElse)
  • SUBs and FUNCTIONs are separate
  • Parameters are passed by reference (BYVAL was added later)
  • No data pointers (not needed for most part)
  • No function pointers (limits flexibility)
  • TYPEs (structures) cannot contain arrays

Some QuickBASIC enthusiasts have brought it to modern times: QB64 and FreeBASIC.