Watt Meter

Read power consumption from watt meter

Read power consumption measurements from watt meter Nets through the Lager CLI. Supports Yocto-Watt, Joulescope JS220, and Nordic PPK2 hardware.

Syntax

lager watt [OPTIONS] NET_NAME

Options

Option	Description
--box BOX	Lagerbox name or IP address
--help	Show help message and exit

Arguments

Argument	Description
NET_NAME	Name of the watt meter net to read (optional if default is set)

Usage

lager watt NET_NAME [--box BOX]

If NET_NAME is omitted and no default is set, lists all available watt meter nets on the box.

Output

Returns power in watts with 3 decimal places:

Power 'POWER_METER': 5.230 W

The reading timeout is 30 seconds. If no reading is received within that time, the command exits with an error suggesting the device may be disconnected or experiencing USB issues.

Supported Hardware

Manufacturer	Model	Identification	Features
Yoctopuce	Yocto-Watt	USB VID:PID 24e0:002a	Real-time power measurement
Joulescope	JS220	USB VID:PID 16d0:10ba	High-precision power, voltage, and current measurement
Nordic Semiconductor	PPK2	USB VID:PID 1915:c00a	Current, voltage, and power measurement via source mode

Hardware Feature Comparison

Feature	Yocto-Watt	Joulescope JS220	Nordic PPK2
Power reading (read)	Yes	Yes	Yes
Voltage reading (read_voltage)	No	Yes (Python API)	Yes (Python API)
Current reading (read_current)	No	Yes (Python API)	Yes (Python API)
Combined reading (read_all)	No	Yes (Python API)	Yes (Python API)
Measurement method	Instantaneous	0.1s averaged	0.3s averaged
Device selection	Channel-based	Serial number-based	Serial number-based

The CLI lager watt command returns power (watts) for all devices. The Joulescope JS220 and Nordic PPK2’s additional measurement capabilities (individual voltage and current readings) are available through the Python API.

Instrument Name Matching

The backend driver is selected based on the instrument name in the net configuration:

Pattern	Driver
Contains joulescope or js220 (case-insensitive)	Joulescope JS220
Contains ppk2, ppk, or nordic (case-insensitive)	Nordic PPK2
All other watt meter instruments	Yocto-Watt

Default Net

To avoid specifying the net name each time:

lager defaults add --watt-net POWER_METER

Then:

lager watt

Examples

# Read power from watt meter
lager watt POWER_METER --box my-box

# Read using default net
lager watt

# List available watt meter nets
lager watt --box my-box

Scripting Examples

Power Threshold Check

#!/bin/bash
# Verify power consumption is within limits
RESULT=$(lager watt POWER --box my-box)
echo "$RESULT"

# Extract numeric value for comparison
POWER=$(echo "$RESULT" | grep -oP '[\d.]+(?= W)')
if (( $(echo "$POWER > 10" | bc -l) )); then
    echo "FAIL: Power consumption too high: ${POWER}W"
    exit 1
fi
echo "PASS: Power within limits"

Power Profiling

#!/bin/bash
# Sample power over time
BOX="my-box"
NET="POWER"

echo "timestamp,power_w"
for i in $(seq 1 10); do
    RESULT=$(lager watt $NET --box $BOX)
    POWER=$(echo "$RESULT" | grep -oP '[\d.]+(?= W)')
    echo "$(date +%s),$POWER"
    sleep 1
done

Troubleshooting

Error	Cause	Fix
Timeout (30s)	Device disconnected or USB issue	Check USB connection; replug device
Connection refused	Box service not running	Check box: lager hello --box <box>
Device not found	Watt meter not detected	Verify device is connected: lager instruments --box <box>

Notes

• Power is returned in watts (W) with 3 decimal places
• Joulescope JS220 averages measurements over 0.1 seconds for higher precision
• Nordic PPK2 averages measurements over 0.3 seconds; operates in source mode (supplies a configurable voltage 0.8–5V and measures current)
• Net names refer to names assigned when setting up your testbed
• Use lager nets to see available watt meter nets
• Use lager instruments --box <box> to verify the device is detected