> ## Documentation Index
> Fetch the complete documentation index at: https://docs.lagerdata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# 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

```bash theme={null}
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

```bash theme={null}
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](/reference/python/watt).

### 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:

```bash theme={null}
lager defaults add --watt-net POWER_METER
```

Then:

```bash theme={null}
lager watt
```

## Examples

```bash theme={null}
# 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

```bash theme={null}
#!/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

```bash theme={null}
#!/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