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I've noticed on my power bill that there are times during the day when my PV system is selling power back to the grid, while at the same time, I am consuming from the grid (see screenshot below).
That does not make sense to me.
Shouldn't I first use all my PV-generated power before I take it from the grid?
Is there something wrong with my setup/inverter?
...and that... is why we don't use power company hourly data for determining amp safety margins on service equipment :)
It really depends on what your loads are, but they vary all over the map. Your house seems quite small compared to most.
A water heater is kicking on and off at random times based on a bimetal strip thermostat. When it kicks on it pulls 4500W. An A/C on a classic bang-bang thermostat will pull ??? could be anything watts. And these loads can stack. Add dryer or range when you're using them, plus all your 120V loads and whatever odd 240V loads you have (hot tub, EVSE etc). A few times a day a refrigerator will do a defrost cycle and pull 500W instead of its normal ~100W when it's running.
So sometimes your loads are stacking up higher than your solar output. In those moments, the deficit is drawn from the grid.
Thus your smart meter is keeping two different tally sheets: when you are exporting power to the grid, and when you are importing because your loads exceed solar output.
Every hour, the smart meter sends the tally sheet up to the utility. If you had net metering billing, these would cancel out. But power companies don't like offering net metering anymore. People absolutely refuse to make any effort whatsoever at energy storage (hard with electricity, easy with heat). That stubbornness and ignorance, combined with the large installed base of solar, creates what's called the "duck curve". So "net metering considered bad". But this isn't energy policy StackExchange lol.
Anyway, that's how you can be both generating solar and using electricity in the same hour. Electricity doesn't store for an hour.
First, anything you can do to de-peakify your loads helps. If you had a big house with A/C and old bang-bang controls, you might add a 24V coil contactor so when the thermostat calls for A/C by energizing the Y wire, that interrupts the resistive water heater. Pretty easy to do with COTS kit - a RiB would be a UL-listed product for switching the 240V, and the rest is 24 volt stuff.
In your case, your house seems fairly low-power. I always love seeing real world data. I can't speak for every season, but it looks like your house takes far less than the power company and EPA stated national home averages of 1.2kW (~29 kWH per day). Note houses like yours are averaged with typical American large homes with AC, electric range, water heater and dryer and often electric heat.
So again seasons matter, but I sit there looking at that graph and see more on the bottom than the top. Well, that just screams "battery solution". You'd have to crunch the numbers and see how DIYable that might be. Note that a home line-voltage power system will need an outside "render it safe" disconnect for the firemen, right next to your utility meter or outside disconnect if you have one.
Obviously, solutions like Tesla Powerwall will do it "right out of the box". They can even work with microinverter panels.
And here's an interesting point: people keep crashing EVs. There are a lot of intact batteries from wrecks in the 50-100 kWh class (or even the original Leaf's 24 kWh pack) that are in an absolutely beautiful, safe enclosure with onboard balancer, built in contactors, CANbus management... a wreck even comes with a battery charger made for that exact battery and other kit. Even a DC/DC converter to make 12 volts for control voltage. The potential is really there to just use that stuff straight, and if your unmodified Nissan Leaf battery has a problem, you can send it out to anyone who services Nissan Leafs or do a unit swap.
But if you have an EV and it's home during solar generation times, that's another option. EVSE's (the wall unit "chargy thing") real job is to tell the car how much power it can take from the wall. Some EVSE's can talk to power monitors in your panel which have CT's on your service wires... and tell the car to take exactly your solar surplus.
Technology Connections has a great video on buying dirt cheap electricity and storing it as heat. There are lots of ways to do this. Aside from that video, you can direct solar into a water heater. (Tanked water heaters are very dumb resistor heaters and can be regulated as such; so they are favorites of off-grid solar people looking for a "dump load".) The Brits also have "hot rocks" storage heaters which heat up on evening power rates and radiate later. Water, however, is a better thermal storage medium than any substance on earth. And you can't beat the price.
However, I consider it very wasteful to get only 3.41 BTU out of 1 watt-hour of solar. At the very least, do like Technology Connections and apply that electricity to a heat pump, so you're getting 300-600% step-up in most conditions.
Within each of those hours there will be minutes and seconds when you were using more electricity than your solar panels were generating and minutes and seconds when you were using less.
The overall effect within each hour was some electricity bought from the grid those seconds you were using a lot, and some solid at the other times.
If you have 3-phase power, you can be exporting power on one phase, while importing power on another.
