kW

[niveknow]

I've seen/read several discussions on this forum, but wanted a very basic point of view without high-jacking anyone else's thread.

In simple terms, help me understand something. Assuming a 12.24 kW system. Solar radiation, roof position, etc. All these variables give us an actual production. Also factoring efficiently, let's be conservative at 85% and call it 10 kW. Am I correct by saying this is peak capability on a perfect solar day. I did a PV Watt and Smart Meter (thanks, Bob) on a very hot Houston afternoon. I see the following chart:

See the peak at 2kW. In basic terms.. does that mean I'll be capable of producing 4-5x my consumption? In otherwords.. my system is 4-5 x oversized and thus power should be going somewhere like net-meter to grid or battery. I live in a new home which is generally more efficient and green where possible and running a 26k gallon pool as well on a 90+ degree Houston day and I'm peaking 2kW with AC running. With that last statement, I can't imagine many people need much more than the smallest Tesla system.

Seems too simple. What am I missing?

(I have 2 PWs coming which hoping to carry me through the night)

 

[power.saver]

First, your graph is in kWh, not kW. kWh is energy, kW is power.

This graph is in 15-minute increments, your hourly consumption in kWh is 4 times that shown, or more accurately the sum of the four 15-minute intervals in each hour. So around 7 kWh from 3-4pm. A 12 kW system would cover about 75% of that at full production, leaving the rest to go to the grid or into the PWs. A small system would not be able to cover your peak demand or charge the PWs. Also, production is not uniform throughout the day, and is significantly reduced during the early morning and late afternoon time.

You need to look at daily consumption versus your daily production to determine if the size of your solar system will cover your usage.

 

[Mardak]

See the peak at 2kW.

The graph is showing kWh at 15 minute intervals. So for the whole hour, the 2kWh is more like 8kWh.

 

[ra88it]

With that last statement, I can't imagine many people need much more than the smallest Tesla system.

Seems too simple. What am I missing?

I wish it was that simple. Some of us require above and beyond their largest residential system and as such have had to order two different ones.

Summer, modest home, 150kWh of EV usage.... ~200kWh of daily usage

 

[Southpasfan]

What makes it less than simple is that, as stated "kwh" is like saying "gallons" -- so, if one was measuring gasoline, one could say "its takes ten gallons of gas per day in the summer to run my house."

However, how many "gallons" a stated kwh system produces during a day is another question.

Its easy enough to figure out how many Kwh you are using during the day, just look at the meter. Its easy enough to figure out how many average Kwh you use during the day, look at your electric bills or bills.

Its somewhat more difficult to predict what a system will produce, since it varies by location. I am lucky in that my nextdoor neighbor just installed a system and lets me see the data, but its very close to what PVwatts predicts.

So, for example, his 7.2kwh system is producing about 40kwh per day. That's where the 85% of the system for a perfect solar hour comes from. Its a graph where I can see he gets that 85% (5+kwh) a couple of hours a day, with less before and after.

So the next question is how much does he use in a day? Some days less than 40kwh.

In my case, since I am right next door, I can easily figure out what the system would produce now.

Then, it gets more complicated by figuring out powerwalls. Since each powerwall is 13.5kwh, if you have two that's 26kwh to cover, from, say, these days, 6:00 p.m. to 6:00 a.m.? But the powerwalls might come on sooner as Panel production drops around 4:00 p.m.

 

[niveknow]

Ahhhhh.. Just had the Uh-Duh moment with the 15min interval. That makes more sense now when I x4 the values to get an hour.. which is in line with a 12 kWH (~10 kWH actual) taking the hottest point of the day. Given this I should be right at fully maximizing sun power and little to zero going to PW/Grid. Which also means the rest of the day light hours I will be charging PW and net-metering.

My full 24 hour run based on the same report tells me I'll be running about 30kWH from sundown to sunrise which barely covers me with 2 pW capacity (13.5 kWH x 2) storage. Options here to add a 3rd PW or bank on net-metering to level me out.

I took a stab at the daily consumption. Green is positive where I'll net meter to grid. Red I'm drawing from grid. I'll owe the grid $285 in this scenario which does not factor in PW. This surprisingly works out on a 4100 sq home consuming 21k kWH annually, but of course throws all this off when an EV or two comes into the picture and will have to bake in charge demands overnight without the sun.

 

[ra88it]

Another solution to look at might be the EVTV solution for a full model 3 LR pack setup much like a powerwall.

 

[vickh]

Another solution to look at might be the EVTV solution for a full model 3 LR pack setup much like a powerwall.

EVTV solution? EVTV Motor Verks - Custom Electric Car Conversion Instructional Videos

 

[miimura]

EVTV solution?

I don't recommend this solution, but this is where to look: EVTV Motor Verks Store:

 

[jboy210]

Ahhhhh.. Just had the Uh-Duh moment with the 15min interval. That makes more sense now when I x4 the values to get an hour.. which is in line with a 12 kWH (~10 kWH actual) taking the hottest point of the day. Given this I should be right at fully maximizing sun power and little to zero going to PW/Grid. Which also means the rest of the day light hours I will be charging PW and net-metering.

My full 24 hour run based on the same report tells me I'll be running about 30kWH from sundown to sunrise which barely covers me with 2 pW capacity (13.5 kWH x 2) storage. Options here to add a 3rd PW or bank on net-metering to level me out.

I took a stab at the daily consumption. Green is positive where I'll net meter to grid. Red I'm drawing from grid. I'll owe the grid $285 in this scenario which does not factor in PW. This surprisingly works out on a 4100 sq home consuming 21k kWH annually, but of course throws all this off when an EV or two comes into the picture and will have to bake in charge demands overnight without the sun.

The number of PWs you need depends upon how much you want to spend to be energy independence and how often you would use that capability. Remember, you can always load shed and reduce your consumption. Turning the A/C up a few degrees, shutting off the pool pump, and not charging an EV can save dramatic amounts of power.

I live in a part of California that is annually subject to Public Safety Power Shutoffs (PSPS) because of wildfires and utility infrastructure. Last year we had 2 incidents that shutdown power to my home for 4 days. We work from home, so that meant minimal work and impacted income. This year we have a Solar Roof and 2 Powerwalls, but still plan on instituting a load shedding plan in the event of a PSPS. For us this means turning up the thermostat on the A/C, putting off drying clothes, disconnecting our Teslas from garage charger and using Superchargers as needed, driving less, and minimizing oven usage by cooking meals that use the gas range.

Now, we could buy more Powerwalls, but by doing these load shedding steps we can easily meet our needs. You might want to see how your consumption is impacted by testings some of the steps.

 

[bob_p]

Some actual data from our system - in Houston. For the last 6 months, actual solar power generation from our 15.4 KW solar panels (excluding EV charging), in KWh:

• June: home 3063, solar 2038, from grid 1209, to grid 89
• May: home 3266, solar 2107, from grid 1254, to grid 30
• April: home 1858, solar 1607, from grid 464, to grid 130
• March: home 1752, solar 1294, from grid 536, to grid 10
• February: home 1448, solar 1062, from grid 507, to grid 36
• January: home 1530, solar 856, from grid 760, to grid 14

The "to grid" indicates excess solar power during the day when the PowerWalls were full. Since we aren't on a net metering plan, any power sent to the grid is uncompensated, so I've adjusted usage (primarily pool pumps) and reserve % to reduce the amount of wasted power we sending back to the grid.

Due to the way our roof is designed, the solar panels are not all facing ideal directions, so we'll have a few panels with little or no power early in the day and another set getting little power late in the day. If we had ideal locations for all of our panels, we'd generate more solar power.

The amount of solar power can also be reduced if we encounter days with considerable cloud cover. On the average, during May and June, we're generating around 500 KWh of solar power each week. But during the week of Jun 22-28 when there were a lot of storms, we only generated 294 KWh (though because it was cooler, our house also used less power).

When I pull down a spreadsheet (using Export My Report) from the Smart Meter Texas website, there are 3 columns of interest:

• USAGE_START_TIME: time of day of the beginning of the 15 minute period
• USAGE_KWH: KWh used during that 15 minute period
• CONSUMPTION_SURPLUSGENERATION: indicates if power drawn from the grid (CONSUMPTION) or excess power sent to the grid (SURPLUS GENERATION)

The smart meter data can be useful both for planning a new system, and once the system is installed monitoring actual use and then using that to determine which electric plans would result in the lowest electricity costs.

 

[niveknow]

@bob_p was the one that pointed me to the Smart Meter Texas for any Texans looking to details. Very useful data there.

Hey Bob. Some interested data in your figures. You said less than ideal facing for your solar, yet you're producing close to the 15.4 kW system with 2000+ Kwh in June and May. Which direction are they facing now? That's pretty good for less than ideal. I'll be facing southwest on a 12 kW system.

The other interesting bit from your data is that your production numbers earlier in the spring. Given the amount of data you're working with that you've done the PVWatt. How close are you those earlier estimates? I heard they're fairly accurate and that's where I plugged in my SW facing and got my production estimates.

And BTW. You said you were not doing a net-metering provider and utilizing a free evening plan. M2 Energy has a plan as well which has free EV charging for 3 hours/nightly. Maybe that'll give you the best of both worlds with buyback plus free EV charging in evenings.