Rapid SCADA: Calculating Period Energy Differences

A cumulative energy channel reports the meter total since reset, but period consumption is the difference between two boundary readings. If the meter reads 82000 kWh at one daily boundary and 83000 kWh at the next, the completed-day consumption is 1000 kWh. The key design choice is whether the channel must contain completed-period values for archives and trends, a live period-to-date value for a mimic, or both.

Separate the meter total from period consumption

Do not add the cumulative source value to an hourly total. The source already represents all energy accumulated since the meter was reset; adding it repeatedly would not calculate consumption for the active period.

Choose the required channel behavior

The available approaches do not produce the same result:

One channel therefore cannot safely be assumed to provide both a live period-to-date display and an archive containing only completed-period points. The evidence does not establish a mechanism that changes the current value without affecting archived minute data or the behavior of PrevVal and PrevData.

Capture and calculate a completed-period difference

The demonstrated script design uses three channels: a period-start trigger, a saved meter reading, and a calculated difference. In the example, channel 327 is the cumulative energy source.

1. Generate a boundary indication with HourStarted() or DayStarted(). MonthStarted() was proposed for monthly operation, but its availability must be verified in the installed environment.
2. When the boundary starts, copy the cumulative reading into the saved-value channel. If the calculated channel has no valid status after server startup, mark the captured value with status 4, meaning that the value is defined but unreliable.
3. Subtract the previous saved reading from the new saved reading and assign the previous saved status to the result.

public CnlData IfStart(int started, int curVal)
{
    if (Stat() < 1)
        return NewData(Val(curVal), 4);
    if (Val(started) > 0)
        return NewData(Val(curVal), Stat(curVal));
    return Data();
}

public CnlData DivIfStart(int started, int lastVal)
{
    if (Val(started) > 0)
    {
        double div = Val(lastVal) - PrevVal(lastVal);
        return NewData(div, PrevStat(lastVal));
    }
    return Data();
}

Configure the saved-value channel with IfStart(X, 327) and the result channel with DivIfStart(X, X+1), where X is the trigger channel. This records the difference only when the selected period starts.

Handle server startup and incomplete periods

After a server restart during an hour, day, or month, the system may lack a trustworthy reading from the true period boundary. The supplied logic returns the current source reading with status 4 when Stat() is less than 1. Treat the resulting period as incomplete until two valid boundary snapshots are available.

A live period-to-date calculation also requires the reading captured at the active period boundary. If that snapshot was not retained through restart, the full consumption since the actual boundary cannot be reconstructed from the current cumulative reading alone.

Verify calculation and trend behavior

1. At a known boundary, confirm that the saved-value channel equals the cumulative source reading and carries the source status.
2. At the next boundary, confirm that the result equals the new snapshot minus the previous snapshot. Readings of 82000 kWh and 83000 kWh must produce 1000 kWh.
3. Restart the server inside a period and confirm that an unavailable boundary context is identified with status 4 rather than presented as a complete period.
4. Inspect minute and boundary archives separately. A completed-period channel should change at boundaries; a live period-to-date channel will change inside the period.
5. If Graph Pro does not connect hourly difference points, do not assume that changing GapBetweenPoints alone solves the issue. Values of 3600, 3700, and 5400 were reported as ineffective, so verify the actual timestamps and graph data-selection behavior before changing that setting again.

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