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Going Solar- Need Some Help!

Hello All,

I am in the designing stage and have been doing a lot of research. I understand this is the time to ask good questions. I was designed for A 25 panel 10KW System that should have an Estimated energy produced: 8,971 kWh /yr.** A little low if you ask me ** However on pvwatt calculator, my area based on a 10kw system should generate 14,171kwh a yr.

I need help with asking the right questions and getting opinion on what to ask for before we commit. I opted out for Storage because we have 2 tesla's and don't think we would be able to really store anything of value based on the consumption . Thoughts?

What questions should i ask or what should i request for in order for me to get an effective system. People complain of the Inverter size and clipping.

Please advise.
 
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WannabeOwner

Well-Known Member
Nov 2, 2015
7,281
4,031
Suffolk, UK
I opted out for Storage because we have 2 tesla's and don't think we would be able to really store anything of value based on the consumption

My thoughts are:

Cars have to be at home, and have spare battery "space" to charge from PV. They can't (yet) supply energy to the house, so you have to drive somewhere to have space in the battery.

If you are "away" then no car at home to charge, and house probably using minimal power, so likely that Solar will exceed house usage - and a static battery could be used.

I'm in UK (the South most point in the UK is basically level with Canadian border, so we are a long way North of USA). In mid-Summer my PV generates a peak of about 15kW. I have 2x EV, 2x wall charger (max 7kW) and 2x PowerWall (max charge around 10kW I think). House "background" is around 1.8kW.

So at Max PV of 15kW I need to be charging both cars, or at least both PowerWalls plus one car. We don't have net-metering in the UK, but if you do then maybe exporting would work for you.

I find it a challenge to charge a car every sunny day in Summer - I don't consistently drive far enough every day! and some days are cloudy, and solar reduced. During the Summer our excess PV generates about 1,000 miles a month into the cars

In Summer my Solar production drops below house usage at about 7PM ("PV Dusk"), and starts exceeding house usage between 6AM and 7AM ("PV Dawn"). My PowerWalls are able to provide sufficient power to run the house from 7PM to 7AM - but, basically, I'm asleep during that time, so the house usage is at "minimum".

In Winter I have Off Peak pricing from midnight to 7AM. There is no way that my 2x PowerWalls will provide for the house for then whole 7AM to Midnight Peak Period in winter - we are cooking, washing, and so on during the day. The Solar we get, at UK latitude is much reduced (mid winter is 90% less than mid Summer), and we have lots of overcast / rainy days (compared to Summer). Any solar I get, in Winter, means my powerwalls just run out a bit later in the day! The PowerWalls will be at 100% at 7AM (when off peak ends) and on days when there is strong sun the PV will exceed house usage, and charge battery, around 10AM ... my PowerWalls have lost around 20% by then - very unlikely there will be enough sun to make that back up to 100%, so basically I don't have any excess PV, in Winter, to charge the car.

In Winter I get good benefit from time-shifting my off-peak to the peak period, but at your latitude I expect you will do more like my Summer all year round - but your day length will be shorter than my Summer days ... so, unless you have net metering, you will be buying off the grid overnight; if you have PowerWall / similar you could store excess daytime PV and use overnight ... but you would need enough PowerWall kWh to run your house from Dusk to Dawn (or only plan to satisfy a part of that period, and buy from grid from the rest).

The way I look at it my PowerWalls mean I no longer pay 90% of my Grid electricity in Summer, and in Winter I buy about 75% of my power at Off Peak price, 10% at Peak, and the rest is from PV in winter. I'm not interested in "payback", but rather "No longer paying X% of my electricity bill" - regardless of what the price might rise to. In UK electricity price rose 54% in April 2022, a further 27% in October, and expecting 20% in April 2023. Actually, my Off Peak price has gone down - because UK is installing huge amounts of Wind around the UK coast, and they want people to use that up "overnight", so only my Peak usage has gone up, and that is now only 10% of what it used to be, and that is only during the Winter months.

So, for my Man Maths, that all looks very rosy to me :)

Sorry, I don't know how much of this translates to your USA location.
 
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Hello All,

I am in the designing stage and have been doing a lot of research. I understand this is the time to ask good questions. I was designed for A 25 panel 10KW System that should have an Estimated energy produced: 8,971 kWh /yr.** A little low if you ask me ** However on pvwatt calculator, my area based on a 10kw system should generate 14,171kwh a yr.

I need help with asking the right questions and getting opinion on what to ask for before we commit. I opted out for Storage because we have 2 tesla's and don't think we would be able to really store anything of value based on the consumption . Thoughts?

What questions should i ask or what should i request for in order for me to get an effective system. People complain of the Inverter size and clipping.

Please advise.
I think the biggest question is what size the inverter is. If you only have a much less than 10 kW inverter, then that could be limiting the expected production during peak sun. If so, then find the cost to get a bigger inverter and compare the extra cost with the extra production.
 
I think the biggest question is what size the inverter is. If you only have a much less than 10 kW inverter, then that could be limiting the expected production during peak sun. If so, then find the cost to get a bigger inverter and compare the extra cost with the extra production.
The inverter is 7.5Kw. And I did ask but Tesla said the inverter cannot be bigger the solar system.. it would shut off
 

K5TRX

Member
Sep 16, 2018
174
216
USA
@Jimmydade
Congratulations on doing the solar!

The PVWatts Calculator factors in your location but there are other important factors. So far we know that you:
  • Are looking at a 10kW system with a 7.5 kW Invertor
  • Live in Atlanta
  • Opted out of storage
  • Have two Tesla's
  • Tesla estimates 8,971 kWh /yr which sounds like some shade on the solar panels or not facing south?
  • PVWatts is estimating 14,171kwh a yr which sound like facing due south and no shade?
I hope we hear from you but let me try my psychic powers. I am starting with these general assumptions:
  • You get the most usable sun from 8 AM to 4 PM.
  • Most people use the most electricity from 4 PM to 10 PM.
  • The average Electricity Usage in Georgia is 1,559 kWh per month and 18,708 kWh a year.
  • Georgia averages 14¢/kWh.
  • Georgia Power is anti-solar with no real Net Metering.
  • The average car drives 12,000 miles a year.
  • A Tesla might use 300 W/mile or 3,600 kWh per car per year.
  • You are putting the solar panels on a south facing roof with no shade.
  • You set your thermostat back when you are out to save energy.
In order to give you the right questions to ask, we need to ask:
  1. How many kW of Electricity do you average a month?
  2. Will you use a south facing roof? Will it be shaded?
  3. How many hours a day is each car parked at home every day from 8 AM to 4 PM?
  4. How many miles a day (or a year) do you average for each car?
  5. What do you pay per kWh?
Scenario 1: 10 kW Solar - If you are retired or both work from home so the cars can charge during the day.
Then 10 kW of solar panels could charge both cars and power the house from 8 AM to 4 PM.
A small system with no storage would have a fast payback period.
But what is your plan for the other 16 hours of the day?

Scenario 2: 6 kW Solar - If you only want solar power for when you are gone from 8 AM to 5 PM.
Even in the Atlanta heat of 95° and 95% humidity, your house will be happy, sitting there all by its lonesome.
Better yet, get no solar and just turn off the A/C and the lights when you leave. Let the thermostat turn the A/C back on at 3.

Scenario 3: 12 kW Solar + Powerwalls - If you want to store solar for when you are home from 5 PM - 8 AM.
Then you need a battery to store the solar energy from 8 AM - 4 PM when you are gone for 5 PM until 8 AM when you re home.
In the summer, say your thermostat automatically sets to 84° at 8 AM when everyone leaves and then back down before you come home.
You might need 30 kWh for the house from 5 PM - 8 AM and 9 kWh per car for the average 30 miles a day?
Georgia's solar cap is 10 kWh but only if you sell power back but they pay so little there seems to be no point.
12 kW Solar could provide energy form 8 AM to 4 PM and charge one 13.5 kWh Powerwall for 5 PM to 8 AM.
14 kW Solar could charge two Powerwall+ for 27 kWh for over-night.
16 kW Solar could charge three Powerwalls for 40.5 kWh.

As to Inverter, just ask for a larger Inverter. Tell them you will pay the difference.
The reason why the first Powerwall+ gets a discount as it includes a 7.5 kW inverter.
 
Um. Speaking as a person who's had a 9.02 kW system on the roof since 2008..

Total inverter size in my system is 7.8 kW. There's actually two inverters: One at 4.8 kW, the other at 3.0 kW. As it happens, my roof happens to be aimed nearly dead south (200 degrees); angle of the roof is 25 degrees. That works out to actually give a 90 degree (perpendicular) to the Sun angle around noon around twice a year. As a result, and this shouldn't be a surprise, the inverters would max out for about ten or twenty minutes around noon, assuming (a) no haze and (b) no clouds. No surprise, gain: That was very rare. Over a year's time I'd say that the inverters weren't operating in their max'd out level 99% of the time, if not more.

You guys got it easy these days. Back then, normal operation was to put strings of panels in series, then put strings in parallel with other stings. For example, the 3kW inverter has hooked up to it 16 panels in two strings of 8 panels each. This lead, inevitably, to losses and inefficiencies:
  1. Take those 8 panels in each string. At a given sun angle and all, there's manufacturing variations that says that each panel has a different voltage than another. Sum up the 8 panels on one string and the voltage/current that gives one maximum power won't be the same as the voltage/current on the other eight. Figure about five percent loss that way. The other group of 27 panels are arranged in three parallel strings of nine each; same issue.
  2. As Ye Sun Goes Up And Down In The Sky, the power (combination of voltage and current) for a given panel varies madly (which is mentioned in #1, above), but this also affects the inverters. In my system, the voltage into a given inverter can range between a minimum that's durn near zero and a maximum around 650V or so. The more variation like this, the harder it is for the designers to max power out of the system and another couple percent of loss and inefficiency.
  3. On top of all that, back when the panels got put in, they were amorphous silicon. My panels look mottled; modern ones are a nice, solid, black/blue color and have several more percent efficiency than these olden guys. My neighbor's system has those nice crystalline silicon panels and, as a result, has fewer panels for the same energy output.
There's been multiple runs by various over the years to improve matters:
  1. Everybody's using crystalline cells nowadays. Better manufacturing processes and the cost advantage of amorphous silicon went out the window.
  2. For a time, installers were putting 220 VAC, 60 Hz DC to AC inverters on each and every panel; all the panels would run in parallel, each panel would max its output power as ye Sun moved around. But, as it happens, 60 Hz tends to be a bear to work with. The lower in frequency one needs to go, the bigger and more problematic the inductors and capacitors around those 60 Hz inverters tend to be. So, yeah, more efficient than stringing panels together in series and parallel, but extra losses in each inverter on each panel..
  3. Current technology eschews 60 Hz except at the Main Inverter by unleashing the Gods of Electronic Controllers on the electronics on each panel. The following is a bit tricky, so, follow closely here:
    1. Each panel has a DC-DC converter on it that switches at $DIETY's own speed, maybe 200 kHz to a MHz or something. This means tiny, low-loss ferritic inductors and small low-loss capacitors; these guys can hit 98%+ efficiency going from the power off the silicon cells to the DC output power of the converter.
    2. Say one has a series string of ten of these panels. The voltage across all ten of these panels, in series, is set to a fixed value, 300V. So, nominally, each panel would have 30 V across it (10x30). But, at a given sun angle, any given panel might have a different power level. Say one is putting out 190W, another is putting out 210W, others are all over the map.
    3. So, the software control system rigs it so the high power panels get more than 30V across them; the low power panels get less than 30V; and, because the panels are all in series, they all have the same current on the outputs of their DC-DC converters, but different voltages. This is called a Degree of Freedom in control theory parlance, and it means that each and every panel is operating at its maximum possible power, all the time, with a fixed voltage across the entire string of 300V.
    4. Want more power: Add more strings in parallel. All these parallel strings will be running at 300V, and all the panels will be running at their maximum power, thank you control theory.
    5. Finally: The DC to city power AC is done at the Main Inverter, which gets a fixed 300V, easy for designers to optimize for (as compared to what I'm running).
    6. Extra Bonus: Add a Battery, whose nominal charge/discharge voltage is 300V. Play significantly fun games with the actual rectifiers and one can disconnect from city power and run ye house off the battery.
Having said all that: At the time of installation I did a fair amount of calculating. As a EE, it comes natural.

Most of the losses were in the wires. The inverters are ~100 feet from the roof up there: I^2-R losses are a Thing. 300V DC systems or no, big, thick copper wires cost serious $$, so there's a significant trade-off between using honking big wires on the interconnect or using more normal wires but losing energy in the wire losses.

Finally: It's a 9.03 kW panel system, no trees, a bit of shading from a chimney in the afternoon, and decent angles to the sun. It was calculated to make 10 MW-hr of energy per year at my latitude in NJ. In reality, it's been making 12 MW-hr, a bit better than expected.

Don't know how much of the above helps, but there it is.
 
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I'm a fan of some storage even if no ROI. I'm in the camp of don't expect any ROI on storage, but in CA with our super high energy rates, it's easier to justfiy the cost due to a larger gap between on-peak and off-peak rates.


This scenario has always been in the back of my mind too:

Some folks shot up the power substation. This can happen anywhere honestly.

Some of the longest outages in our area has been user error as well. Remember, no power from grid means no power for you neither even with solar normally.

Energy infrastructure is probably notoriously (outside of Nuclear) soft easy targets for crooks and any crazy yahoo can probably take out the grid for tens of thousands of people anywhere.

Outside of them targeting your house directly, (less impact for crooks so less likely probably), we could possibly see copycat criminals doing this.
 

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