The Sense Power Meter offers a powerful real-time measurement of your home’s total usage. But the Power Meter is far more sophisticated than most people recognize. Here are a few details:
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The main Power Meter (as differentiated from the Device-level Power Meters) is the most accurate view on what’s going on in your home. Configured properly, it offers a half second resolution real-time view of your usage based directly on Sense Monitor measurements. Many of Sense’s other views are built on the incoming data that flows to the Power Meter, but virtually all other views are dependent on additional factors and calculations making them thirdhand views. The only other nearly direct measurements are the Voltage, Power and Frequency in the Signals view, under Settings and similar data in the Power Quality view within the Sense Labs.
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The Power Meter shows the results of number of calculations, not direct measurements - Even though it seems like a direct measurement, the Power Meter usage numbers are several steps removed from the actual voltage and current measurements sampled a million times a second by the Sense Monitor.
- Sense produces an RMS (root mean square) result based on AC (alternating current power). Your home voltage and current alternate at 60 cycles per second (Hz), so even if the voltage and current stayed the “same”, Sense can’t just sample the raw voltage and current to produce a power number. See the diagram below to see what voltage, current and power look like over 1/60th of a second. Sense has to assess many voltage and current samples over a number of 60Hz cycles, probably 30 (= 1/2 second), to perform an RMS calculation using all the data points to produce an RMS power value for that 1/2 second. The RMS calculation produces a value that is the same as if the power was constant for the whole 1/2 second. In the same vein, your house voltage is really 120V RMS. The waveform actually peaks around +/- 170V. But Sense hides all these calculation so we can work with a single number. So people who look to Sense to determine “peak power usage” will never see the true peak usage - instead, they will see essentially the peak average power over 1/2 second.
- There are multiple components to AC power - Real (or Working) Power, Reactive Power and Apparent Power - Sense outputs Real Power numbers, just like your electric meter and residential utility customers are charged based on Real Power. But if you look at the inner workings of AC power, especially when a large motor is running, the current and the voltage waves can be driven out of phase from one another. That causes less Real Power to be delivered for the same AC current and voltage (the green curve vs. the red dotted curve below), because part of power delivered is diverted into Reactive Power that doesn’t do any useful work in your house. Generally, the motors in our homes are small enough relative to our total usage, that residential usage doesn’t wreak havoc on the grid, but industrial users have to be very aware of how much their loads change the phase relationship, often referred to as the Power Factor. They will often pay increasing amounts for Power Factors that move too far away from 1.0 (current and voltage usage perfectly in phase).
- Sense’s Total Power combines consumption for both phases/legs in your home - Most homes in the US have 120V split-phase power. That means that there are two different supplies, or so-called legs, to your electric meter and breaker panel with the AC voltage 180 degrees out of phase with the other (below). That’s why Sense has 2 CTs (sensors) to monitor home usage current - each measures the current for one of the two legs. So the Power Meter sums the usage on each leg.
- Sense Solar sometimes adds another calculation to the Power meter - If your solar CTs are on a load-side feed-in, your Total Power is calculated by adding Net Usage for the mains CTs and Solar Production. This addition helps explain why Sense Total usage can sometimes partially mirror solar production if there are problems (open CTs, calibration issues, configuration issues, all typically handled by Sense support)
- The only places in the Sense UI where you’ll see the split-phase RMS voltage and power measurements for each individual leg, are in the Signals section and in the Power Quality view.
BTW - Sense has written a bit more about the Split-Phase Power in this article.
- The Main Power Meter in the smartphone/tablet app gives hints about what Sense is actually “seeing” - The Power Meter attaches tags to significant transitions that Sense “sees”. Here are a set of waveforms from when I was descaling our Coffee Maker - you can see a bunch of the tags that quantify the size and direction of the power transition - positive = On and negative = Off.
And if you zoom in you can see even more !
You can see from the Coffee Maker Device Timeline that Sense “saw” and detected all those on/off cycles.
- Devices have their own Power Meters - but accuracy can vary. Device Power Meters for detected devices show Sense’s “estimate” or prediction of the usage pattern, but that is mostly based on the size of the on and off transitions, not any complex usage waveform the device might present, and totally misses any Always On component. Below is the Device Power Meter for my Coffee Maker for that same series above, so it is a Sense AI detected device. What’s interesting is that my normal coffee making pattern is one 1200W large spike followed by a bunch of 850W spikes. Even though Sense is seeing a bunch of 1200W spikes in the tags from the very different descaling cycle, the Device Power Meter gets populated based on what Sense “already knows” about the Coffee Maker’s pattern.
For comparison, here’s a view of a Device Power Meter from a smartplug that powers my laptop, monitor and external hard drive. It does include Always On power and isn’t reliant of on an off transitions.
I’m betting you didn’t know all that complexity was hidden beneath the simple Power Meter.
