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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?

enter image description here

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    \$\begingroup\$ those bars do not indicate that the two events happened simultaneously \$\endgroup\$
    – jsotola
    May 31, 2023 at 0:03
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    \$\begingroup\$ The bars each represent one hour - if the sun was shining for 30 minutes out of that hour but then a cloud obscured it for 30 minutes then you could get exactly that sort of chart. During that hour you could be exporting and importing but not simultaneously (which is impossible by the way as there is only one set of wires). \$\endgroup\$ May 31, 2023 at 0:47
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    \$\begingroup\$ Well, what's that 0.8KWh spike that happens daily afternoons? If you knew what it was, then you'd know how long it actually lasts (must to be lots less than a full hour.) \$\endgroup\$
    – wbeaty
    May 31, 2023 at 8:47
  • \$\begingroup\$ Andre - Hi, You posted an answer to say thank you. Although that's appreciated, Stack Exchange has different rules from typical forums (please see the tour & help center). An answer cannot be used in that way, so it was deleted, sorry. Please see the article in the help center called: "What should I do when someone answers my question?" & follow that guidance. If you need clarification on an answer, you can comment below it. Or if your question has been solved, consider upvoting useful answers & consider áccepting your choice of best answer. Thanks. \$\endgroup\$
    – SamGibson
    Jun 1, 2023 at 0:05
  • \$\begingroup\$ If you don't have "net metering" and have bi-directional or dual metering, then your PV power to the grid and your consumption from the grid are logged separately so that the generation and consumption cancel out on the utility side rather than on your side of your meters. If this is how your system is set up, then within the hour, that generation and consumption may be simultaneous or not--there is just no way to tell. \$\endgroup\$
    – pdb5627
    Jun 7, 2023 at 19:03

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...and that... is why we don't use power company hourly data for determining amp safety margins on service equipment :)

  • Electricity does not store, even for a second.
  • House power demands are extremely intermittent and spiky (not that way though).

Power is spiky

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.

Electricity does not store. Not even for a second.

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.

What to do about it, if anything

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.

But heat does store.

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.

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    \$\begingroup\$ To be nitpicky, every SMPS in the house is storing electricity for time periods on the order of microseconds to milliseconds. Thats the fundamental operating principle of SMPS regulation. Along with the bulk filter caps on all those supplies. But those are the relatively small loads in comparison, and they don't extend beyond the hundreds of milliseconds. \$\endgroup\$ May 31, 2023 at 8:24
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    \$\begingroup\$ At the millisecond and below time scales, we just don't call it storage, we call it regulation, decoupling and filtering. \$\endgroup\$ May 31, 2023 at 8:30
  • \$\begingroup\$ It is interesting that the three giant positive spikes are at, well, weird times in most households (3am, 1pm, 11pm), and are much larger than the 'busy' evening times (cook dinner, dishwaser, whatnot). So one thing to do is try and figure out what those giant spikes are... \$\endgroup\$
    – Jon Custer
    May 31, 2023 at 13:01
  • \$\begingroup\$ @Jon yes, absolutely. They might be 800W for 1 hour - but if they were, they'd almost always be spanning across two intervals. The fact that they're always in 1 interval suggests a much shorter load - e.g. 3200W for 15 minutes or 4800W for 10 minutes. Turning that into a 600W load that runs during solar hours would help avoid that sell/buyback problem. \$\endgroup\$ May 31, 2023 at 19:03
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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.

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  • \$\begingroup\$ What's worth noting is the two diagrams are separate representations of two different entities and are not arithmetically summed up. Then having 800 Wh from the grid and 700 Wh to the grid doesn't equate to 100 Wh flow from the grid - the last is true only for accounting for the total energy of a one-hour period \$\endgroup\$
    – LuC
    May 31, 2023 at 12:39
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If you have 3-phase power, you can be exporting power on one phase, while importing power on another.

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