Watt Meter

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

Syntax

lager watt [NET_NAME] [COMMAND] [OPTIONS]

With no COMMAND, lager watt NET_NAME reads power (watts). The current, voltage, and all subcommands read the other quantities (Joulescope JS220 and Nordic PPK2 only).

Commands

Command	Description
(none) / `power`	Read power in watts
`current`	Read current in amps
`voltage`	Read voltage in volts
`all`	Read current, voltage, and power together

Options

These options are available on each read subcommand (power/current/voltage/all). --box is also accepted directly after lager watt NET_NAME for the default power read.

Option	Description
`--box BOX`	Lager Box name or IP address
`-d, --duration FLOAT`	Averaging window in seconds (default `0.1`). Longer windows average more samples for a lower-noise, higher-resolution reading.
`--json`	Emit a machine-readable JSON object instead of formatted text
`--help`	Show help message and exit

Arguments

Argument	Description
`NET_NAME`	Name of the watt meter net to read (optional if a default is set). Must appear before the subcommand.

Usage

lager watt NET_NAME [--box BOX]                  # power
lager watt NET_NAME current [--box BOX]          # current
lager watt NET_NAME voltage [--box BOX]          # voltage
lager watt NET_NAME all [--box BOX]              # current + voltage + power

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

Output

Lager prints each reading in its base unit (W, A or V) with up to six significant digits. A small reading appears in scientific notation, so a 52.34 µW load prints as 5.234e-05 W (#837):

Power 'POWER_METER': 5.234e-05 W
Current 'POWER_METER': 0.012 A
Voltage 'POWER_METER': 3.3 V

all prints all three quantities:

Measurements 'POWER_METER' (0.1s):
  Current: 0.012 A
  Voltage: 3.3 V
  Power:   0.0396 W

With --json, output is a single JSON object (units are base SI — amps, volts, watts):

lager watt POWER_METER all --json --box my-lager-box
{"netname": "POWER_METER", "current": 0.012, "voltage": 3.3, "power": 0.0396, "duration_s": 0.1}

The default reading timeout is 30 seconds (scaled up for long --duration windows). If no reading arrives within that time, the command exits with an error. The error names the two likely causes: a disconnected device, or a USB fault.

Increasing resolution

Two levers improve a power/current reading:

Display — output keeps six significant digits in the base unit. A sub-milliwatt or sub-milliamp reading appears in scientific notation (e.g. 5.234e-05 W), not as zero. A nonzero reading is never lost to rounding.
Averaging window — pass --duration to average over a longer capture. A longer window reduces noise on the mean, giving a steadier, higher-effective-resolution value:

lager watt POWER_METER current --duration 1.0 --box my-lager-box

Long averaging windows

--duration also works for long windows — e.g. the average current over a minute:

lager watt POWER_METER current --duration 60 --box my-lager-box

On a Joulescope JS220, a window longer than ~10 s uses the instrument’s on-device charge accumulator (average current = Δcharge ÷ Δt). The JS220 does not buffer raw samples for such a window. That approach is gapless: it captures every transient, not just the sampled fraction. It also uses constant memory, so it scales to a window of any length — a minute, ten minutes, longer. A short window continues to use direct sampling.

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 (`power`)	Yes	Yes	Yes
Voltage reading (`voltage`)	No	Yes	Yes (configured source voltage)
Current reading (`current`)	No	Yes	Yes
Combined reading (`all`)	No	Yes	Yes
Configurable averaging (`--duration`)	No (instantaneous)	Yes	Yes
Device selection	Channel-based	Serial number-based	Serial number-based

The current, voltage, and all subcommands require a Joulescope JS220 or Nordic PPK2. On a Yocto-Watt (power only) they exit with a clear “not supported” message — use lager watt NET_NAME for power instead. The same readings are also 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

lager defaults add --watt-meter-net saves the key net_watt_meter, but lager watt reads net_watt. A default set this way is therefore not used yet (#836). Give the net name in each command:

lager defaults add --watt-meter-net POWER_METER

Then still name the net:

lager watt POWER_METER

Examples

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

# Read current / voltage (Joulescope JS220 or Nordic PPK2)
lager watt POWER_METER current --box my-lager-box
lager watt POWER_METER voltage --box my-lager-box

# Read current, voltage, and power together
lager watt POWER_METER all --box my-lager-box

# Average over 1 second for a lower-noise reading
lager watt POWER_METER current --duration 1.0 --box my-lager-box

# Machine-readable output for scripts
lager watt POWER_METER all --json --box my-lager-box

# Read using default net
lager watt

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

Scripting Examples

Power Threshold Check (JSON)

#!/bin/bash
# Verify power consumption is within limits using JSON output
POWER=$(lager watt POWER all --json --box my-lager-box | python3 -c 'import sys,json; print(json.load(sys.stdin)["power"])')

if (( $(echo "$POWER > 10" | bc -l) )); then
    echo "FAIL: Power consumption too high: ${POWER}W"
    exit 1
fi
echo "PASS: Power within limits (${POWER}W)"

Current Profiling (JSON)

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

echo "timestamp,current_a"
for i in $(seq 1 10); do
    CURRENT=$(lager watt $NET current --json --box $BOX | python3 -c 'import sys,json; print(json.load(sys.stdin)["current"])')
    echo "$(date +%s),$CURRENT"
    sleep 1
done

Troubleshooting

Error	Cause	Fix
`does not support reading current/voltage`	Net is backed by a Yocto-Watt (power only)	Use `lager watt NET_NAME` for power, or move the net to a Joulescope/PPK2
Timeout	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

Readings print in base units (W, A, V) with up to six significant digits; a small value appears in scientific notation instead of 0. --json emits the same base units.
--duration sets the averaging window; the Joulescope JS220 and Nordic PPK2 honor it, while the Yocto-Watt returns an instantaneous value and ignores it. On the JS220, windows longer than ~10 s are measured gaplessly with the on-device charge accumulator (constant memory, any length).
The Nordic PPK2 operates in source mode (supplies a configurable voltage 0.8–5V and measures current); its voltage reading is the configured source voltage.
The current, voltage, and all subcommands must follow the net name: lager watt NET_NAME current.
Net names refer to names assigned when setting up your testbed.
Use lager nets to see available watt meter nets, and lager instruments --box <box> to verify the device is detected.

Lager Box Management

Configuration

Power & Simulation

Measurement

I/O & Communication

Security

Development

Utilities

Syntax

Commands

Options

Arguments

Usage

Output

Increasing resolution

Long averaging windows

Supported Hardware

Hardware Feature Comparison

Instrument Name Matching

Default Net

Examples

Scripting Examples

Power Threshold Check (JSON)

Current Profiling (JSON)

Troubleshooting

Notes

​Syntax

​Commands

​Options

​Arguments

​Usage

​Output

​Increasing resolution

​Long averaging windows

​Supported Hardware

​Hardware Feature Comparison

​Instrument Name Matching

​Default Net

​Examples

​Scripting Examples

​Power Threshold Check (JSON)

​Current Profiling (JSON)

​Troubleshooting

​Notes

Syntax

Commands

Options

Arguments

Usage

Output

Increasing resolution

Long averaging windows

Supported Hardware

Hardware Feature Comparison

Instrument Name Matching

Default Net

Examples

Scripting Examples

Power Threshold Check (JSON)

Current Profiling (JSON)

Troubleshooting

Notes