Home IoT Hub Series · 13

[Part 13] Why is the power bill so high? Make consumption visible with ACS712

Measure current with ACS712 while keeping mains-voltage work safely separated.

This part measures power usage with ACS712. Treat it as a low-voltage learning project unless you are qualified to work with mains wiring.

For real AC mains measurement, use an enclosed current-sensing module or get help from a qualified technician. The tutorial structure is for understanding the data path.

Parts and cost

Pin wiring

Full code

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>

const char* ssid     = "IoT_Hub_Net";
const char* password = "iothub1234";
const char* hubURL   = "http://192.168.1.1";

#define CURRENT_PIN  A0
#define SLOT_NO      15
#define VOLTAGE      220.0 // Korean household voltage
#define INTERVAL     15000

void setup() {
  Serial.begin(115200);
  pinMode(CURRENT_PIN, INPUT);
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  Serial.print("Connecting to IoT_Hub_Net");
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println("\nConnected!");
}

void loop() {
  // 50note
  float sum = 0;
  for (int i = 0; i < 50; i++) {
    sum += analogRead(CURRENT_PIN);
    delay(1);
  }
  float avg = sum / 50.0;

  // ESP8266 ADC: 0~1023 → 0~1.0V
  float voltage = avg / 1023.0;
  // ACS712 20A: 2.5V at 0A, 100mV/A
  // modulenote
  float current = abs(voltage - 0.512) / 0.1; // note
  float power = current * VOLTAGE;
  if (power < 5) power = 0; // note

  Serial.print("Power: "); Serial.print(power, 0); Serial.println(" W");

  WiFiClient client; HTTPClient http;
  String url = String(hubURL) + "/set?no15=" + String((int)power);
  http.begin(client, url); http.GET(); http.end();
  Serial.println("HTTP -> " + url);

  delay(INTERVAL);
}

Upload and check

Connect the Wemos D1 Mini to the PC with a USB data cable, select the ESP8266 board and port in the Arduino IDE, upload the sketch, then open Tools → Serial Monitor and set the baud rate to 115200.

Serial Monitor

After uploading, open Tools → Serial Monitor in the Arduino IDE and set the baud rate to 115200. The following is an example Serial Monitor flow for checking normal operation. This is not a real screenshot. It shows zero-offset calibration and power updates to slot 15.

[BOOT] Home IoT Node #13 - ACS712 power usage
[CAL] zero_offset=512.8, samples=200
[SENSOR] current=0.42A voltage=220V power=92.4W
[FILTER] noise_window=stable
[HTTP] GET /set?no15=92.4
[HUB] 200 OK - slot no15 updated
[LOOP] next power sample in 10000 ms

• If Connected or connected appears, the Wi-Fi step passed.
• If HTTP 200 or 200 OK appears, the value reached the brain server.
• If the value does not update, check both the slot number in the URL and the sensor wiring.

Build notes

• Check the zero-current value first. Then apply a known load and confirm the measured value changes in the expected direction.
• Power on the hub brain first so the node can join IoT_Hub_Net.
• Use the same slot number written in the source code.
• Open the serial monitor at 115200 baud and confirm HTTP 200.
• If the value does not appear on the hub, recheck wiring, GPIO number, and Wi-Fi connection.

Peeking at Your Home's Power Usage with the ACS712 Current Sensor

The ACS712 is a hall-effect sensor that measures the current (A) flowing through a wire. Multiply that by the household voltage of 220V and you get a rough estimate of power draw (W). Wired into your 'Home IoT Hub', it lets you actually see how much electricity each appliance consumes.

Troubleshooting checkpoints

The node code can look short, but real home installation is affected by placement, power, Wi-Fi quality, and sensor noise. Check these points before letting the node become a decision input for the brain server.

Use Cases

• Per-appliance power monitoring: A refrigerator's current swings sharply as its compressor cycles on and off. An air conditioner spikes at startup then settles, and a washing machine shows a different waveform for each wash and spin stage. Logging these reveals each appliance's operating pattern.
• Standby power check: Find devices like set-top boxes, microwaves, and chargers that seem off but still draw a trickle of current. Even 0.02-0.05A adds up to real waste over a month.
• Anomaly detection: If a fridge compressor runs longer than usual or current stays high, suspect a bad door seal, refrigerant issue, or aging unit. Have the hub send an alert when a threshold is crossed.
• Bill estimation: Compute kWh from (average current × 220V × hours) to gauge in advance whether you're entering a higher tiered-rate bracket.

Recommended Installation Spots

• Individual circuits at the distribution panel: To monitor by circuit (kitchen, living room, etc.), install behind the breaker. This gives a coarse breakdown of total usage, but the work must be done by a licensed electrician.
• At the outlet level: To watch just one appliance, building the sensor into a power strip or dedicated metering adapter is far safer and more accessible. Strongly recommended for beginner makers.
• Choosing what to measure: Start with high-draw, frequently-running appliances like the fridge and air conditioner for the most interesting data and biggest savings.

Practical Tips / Cautions

• ⚠️ 220V shock hazard: The ACS712 requires passing the target wire directly through it, meaning you handle live wiring. Electric shock can be fatal. Do not perform distribution-panel or direct-wiring work as a non-professional; always use a licensed electrician.
• Zero-point calibration: With no load, the sensor output may not sit exactly at mid-value. Save the no-load output as an offset and subtract it to improve accuracy.
• Noise handling: Instantaneous readings jitter, so use an average (or RMS) over many samples.
• Module selection: The 5A module suits small devices, 20A general appliances, and 30A large loads like ACs and induction cooktops. Pick with headroom above expected current, but an oversized module loses resolution at low currents.
• Power-factor error: 'Current × 220V' is closer to apparent power (VA). Appliances with motors or compressors actually consume less real power (W) due to power factor. Account for this error in precise bill calculations.

Frequently Asked Questions

Q. Can I just plug it into an outlet?
A. No. The ACS712 needs a single wire threaded through the sensor. To use it safely you must build it into an insulated metering adapter or power strip, and live wiring carries a shock hazard.

Q. Will the measured power match my electricity bill exactly?
A. It's a rough estimate. Power factor, voltage fluctuation, and sensor error make it differ from the utility meter. Use it for comparing appliances and spotting trends rather than absolute values.

Q. I'm nervous about wiring directly into the panel. Any alternative?
A. Yes. Outlet-level metering is the answer. Build an insulated power-metering adapter, or if you're still uneasy, start with a commercial smart plug and expand to the ACS712 once comfortable. Safety comes first.