Build a cumulative variable/aggregate from raw data

I got a power consumption sensor (kWh) sending data to my TSI Gen2 environment, and it is malfunctioning in a way that it is losing its accumulated measuremente value when it is shut down. I need to create a new aggregate/variable that would "stack" the measurements , never letting it drop to zero, but always adding to the last greatest value.

I thought about creating a dataset with values from differences from right to left over a fixed timespan, if positive, and then I could create a SUM aggregation over the bucket period on top of it. I am clueless on how to do such thing based on the poor official documentation provided by Microsoft. Any Ideas?

Here are a couple of pictures illustrating my problem and What I am trying to accomplish:

You probably need to add something in the middle (before the IoT Hub/Event Hub) to save the last state of the sensor, and do the appropriate sum if if detects the device was rebooted.