Next: We Have Solar at Home Part 8.
Previously: We Have Solar at Home Part 6.

Question	Estimate from April 2022	Actual for 2023¹
How much electricity will we use between 2023 and 2032?	Between 3217 and 5835 kW·h per year	904 from PG&E plus an unknown amount of solar used on site
How much will PG&E charge for that electricity?	Between $1287 and $2334, increasing 7.6 percent per year	?$, increasing 10.4% in calendar 2023²
How much will the system cost to install and operate?	$17,000 or $20,000	?
How much electricity will the system generate?	3915 kW·h or 6934 kW·h, decreasing 1% per year.	?

How much did the system cost to install and operate?

We expected a 24% non-refundable tax credit, but the Inflation Reduction Act, signed in August 2022, bumped that up to 30%. So that’s a total cost of $14,357. Arguably the $500 (pre-credit) for the extra 240V power outlet shouldn’t count, but we’ll leave it to be consistent with the conservative (pro-status-quo, anti-solar) bias we’re using to guard against excess optimism.

How much electricity did the system generate in 2023?

The monitoring package we’re using—meaning, the one the vendor provided—is called SolarEdge, which is another company. There’s a box in the garage, next to the inverter, that reads the data and sends it out (via cellular, I think) to SolarEdge, which collects it and provides a UI. I don’t actually know what our business relationship is with SolarEdge. Did we—meaning our vendor—buy the monitoring unit and therefore get lifetime monitoring service? That might be good to know. Anyway, a SolarEdge report shows us what we generated in 2023³.

2023 Solar Production by month

The system produced 7.7 MW·h³.

The warranty

Simply Solar warrants that in the first 12 months of operation, the PV solar system will generate a minimum of 90% of the quoted kilowatt-hours production. The quoted annual production of the solar system is 7418 kWh .

7.7 MW·h is more than 7.4 MW·h, and certainly more than 90% of 7.4. Of course that’s Jan–Dec 2023 and the first 12 months was Sep 2022 to Aug 2023, but I’m pretty sure it was more than enough either way. So that’s a solid pass.

How much electricity did we use in 2023, 2nd Attempt

Knowing the total generation in 2023, we can now calculate how much energy we actually used, which is the solar output plus the net grid usage: 8600 kW·h. If that didn’t make sense right away, consider this graph combining total solar production in yellow and net usage in black. There’s one trick: if the net usage is negative, a surplus, there’s no black bar for that period, and the yellow bar is displaced downward by that amount.⁴

A stacked bar chart, with the X axis Billing Period Start from 2022-03-19 to 2024-03-22 and the Y axis kW·h from -1000 to 1000. A double-hill of black bars rises from near zero at the start to perhaps 50 in 2022-09, then back up as high as 600 before dropping and then disappearing in 2023-03. Another hill rises from 2023-09 to 2024-02, peaking around 600 in the middle. Yellow bars on top of the black bars start in 2022-09, and range from about 500 high to almost 1000 high. In periods without black bars, the yellow bars start below the X axis to varying degrees, down to almost -400. — Net usage in black combined with total solar generation in yellow.

We can see a few patterns in this chart. Before solar, we used a very consistent 400 kW·h per month plus or minus 30. In 2022-08, with solar physically present but not hooked up for NEM, usage dropped considerably. From there on, solar generation follows the seasons fairly closely, except that summer 2023 lacks an expected peak. Total usage runs 6 months out of phase: highest in the winter, lowest in the summer; never as low as before solar, and often twice as much. It’s not immediately clear why usage went up so much, but getting the electric dryer working around the same time must be related. We also stopped using the gas furnaces in 2023 and switched to electric space heaters⁵, but winter 23/24 isn’t really higher than winter 22/23.

We still haven’t worked out how much PG&E charged for those 904 kW·h spread over various times and rates. Next week, we’ll delve into the mysteries of the PG&E bill. Meanwhile, good night and good luck.

Next: We Have Solar at Home Part 8.

For reasons, all references to 2023 mean a PG&E billing year, rather than a calendar year, so: 2022-12-22 to 2023-12-20. ↩︎
which is ten days off of the Billing 2023 that we’re using. But we also want to know the actual dollar figures, taking into account peak and off-peak, and NEM, so we’ll come back to this. ↩︎
That’s calendar year, and we’ve been using the PG&E billing year. So that answer’s not precise. However, the difference is only ten or eleven days, and ten or eleven days around the winter solstice is so little production that, so if we—meaning SolarEdge’s webpage— are rounding to a tenth of a megawatt, it won’t matter. At the month to month level, though, it means these numbers cannot be compared directly to PG&E billing periods^ב. ↩︎ ↩︎
Before we had solar, all of our power consumption was from PG&E imports. The first full billing cycle with NEM started 2022-09-22. The net import was 51 kW·h; that’s the black bar from 0 to 51. Total solar production in the same time period was 477 kW·h; that’s the yellow bar that goes from 52 to a bit over 500. The top of that bar, ~528, is our total consumption.

To convince yourself this is correct, consider: The PG&E Export as -340 kW·h, so we used 137 kW·h of solar and the rest was surplus at the time it was generated^א. The Import over the same time was 392 kW·h; we used 100% of that because there was nowhere for imported energy to go except our consumption. So our total usage was 137 + 392 = 529 kW·h for that billing period. Which, with rounding errors, is the same number.

Look at a lighter month, like the billing period starting 2023-03-23. That month had 984 kW·h of total solar generation, and a net usage of -341, i.e., a surplus. So the yellow bar starts at -341, and rises 984 to 643. The height above 0, 643, is the actual usage for that period. ↩︎
Which gives me the idea to convert gas usage from BTUs to kW·hs and add it to the chart. Not today. ↩︎

Remember that power is instantaneous. If the panels are producing 2 kW and one hairdryer is plugged in and using 1 kW, then there is a surplus of 1 kW. If this state of affairs continues for an hour, that's an export of 1 kW·h. This 337 kW·h of surplus probably came mostly in the time around noon, and instead of solid hour blocks, it might be 10 minutes here, then parity, then a net import for a few minutes as we all dry our hair at once, then the fuse blows and we're back to exporting 2 kW for a while.
SolarEdge provides a "Billing Cycle" chart. Like the Monthly Chart, it breaks production down to the day, but you can set the start/end date to match your particular billing cycle. This is the easiest way to get a monthly total.

SolarEdge Billing Cycle Settings