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Solar System Sizing - Capacity Factor

2016 Views 45 Replies 14 Participants Last post by  saln
Just wondering if anyone has any real time information on what their solar array produces. Total panels vs average production. If you know your "sun hours" please let me know that also.

Short story long:
I started a renovation and wiring a house for my kid. It's gutted and I am starting from the studs. All new electrical including new incoming/meter/disconnect and a new 200A load center/breaker panel.

The goal at the house being renovated is to put most outlets and lighting to a transfer panel and run off an inverter, solar, and batteries most of the time. Larger 120 Vac loads like microwave, toaster oven, and bathrooms would normally be on grid power (and GFCI), unless there's a grid outage. If grid is down, appliances could be could be moved to "inverter" outlets, and use them selectively (so not to overload the inverter) from battery back up.

If grid is down, HVAC (mini splits), dryer, electric range and electric hot water are also down. Those loads are too big to even consider right now.

Sun hours for solar panels in NW PA is poor. Energy storage via batteries will be a big part of the system. System will not be grid tied.

I am questioning the calculation done with an online calculator. They recommend 4.5KW (4500 watts) of solar panels based on a winter "sun hours" of 2.64 hours/day. In the last 27 days, the sun has come out 6 days. Two of the six days was only for a couple hours in the late afternoon. Essentially four days of sun in the last 27. I know solar panels generate some power even with cloud cover. (10-25%?). A friend has 12,000 watts of solar. On one of the sunny days, they generated 20.2 Kwh.

I think the 2.64 hours/day for my area is very optimistic. My AO is between Cleveland, Buffalo and Pittsburgh which are the #3,4, and 5 gloomiest places in the country.

What they recommend, I assume, is based on a "capacity factor" for my zip code. But I think they are a little low at 4500 watts of panels.

I may also need to rethink my battery back up (go to multiple commercial server rack batteries) and upgrade to 48vDC/120vAC inverter. Assuming no more than 3000 watt peak, and about 6000 watts/day.

Worst case-grid down, cold weather and on batteries:
Hours On per Day​
Watt Hours per Day​
Coffee Machine110000.2200
Fridge - 20 cu. ft. (AC)135341412
Toaster Oven112000.2240
TV - LCD11503450
Freezer - Chest - 15 cu. ft.127041080
Smart Phone- Recharge161060
Video Game Console11501150
Box Fan120061200
LED Bulb - 100 Watt Equivalent92351035
System Calculation Results​
Total watt hours per day6863
Killowatt Hours per Month:206 kWh
Peak Load: (4650 watts .....BUT selective/controlled use of electrical so not to exceed 3000 watts peak on a 3500 watt inverter when on battery power)4,650 watts
Sun Hours:2.64 hours/day
System Size:4.51 kW

I am not affiliated with this company but the information is from:

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Are you trying to cover most of the conveniences that electricity offers? When power is down, you cut back on anything that is not necessary to get thru the day.

A larger bank is good to focus on. This way it covers night time and the off days when sun is not producing.

You can add panels until at a comfortable level of power to recharge. Securing alternative heating sources would seem to be paramount that far north.

Coffee maker and microwave should be considered luxuries when grid is down. Focus on keeping food from going bad and learn to see in the dark. Re-learn how to read real books to pass your time if everything else is under control.

After that last arctic blast before Christmas, I had enough of relying on propane to keep warm. It gets expensive fast. I finally installed a pot belly stove and worked a way to have quasi central heating so the upstairs and downstairs is relatively at the same temperature.

All of my lighting is LED and most of the standard bulbs only draw 4 watts. I carefully watch what is drawing current later in the afternoon until I loose all sunlight for panels. During the day, not so much. The batteries need some load so they get a full charge, instead of in float.

During milder weather and longer days, I might not draw enough from battery bank to take the control charger out of float. Sometimes this has been a inconvenience at night if I dont want to let the voltage drop too much.

There are enough things from your list that will be a constant drain on your battery bank. So at night, your batteries have to be in full charge or you drop below a save margin for longevity.

Hopefully you are not in a large populated area or inner city. Having some acreage or at least access to wood to burn if necessary sometimes. At least some deadfall during emergencies.

My solar array is fairly small, but runs everything I need for now. I have south and west facing panels on the roof. Around 3 pm during the winter months, my panel output drops off substantially for a half hour as the sun transitions between the two rood angles.

It took me a while to even figure this out. During the summer months, not even noticeable. Even with the leaves blocking a lot of the panels. But the winter angle of sun really cuts down on output. It does not take too many branches or limps to block a few cells to drop voltage down a bunch.
Thanks to all who responded so far.

I spent the day at my friend's house looking at his set up and a few hours with a colleague who does solar installs. Of course the solar guy wanted to sell me a system, but he realized quickly I am no where near what his typical customer wants to spend. He does everything from 5KW arrays to 35KW arrays, off grid, hybrid, grid tied and big battery storage systems. He uses higher end stuff (see Sol Ark inverter/charge controllers). He was able to access the inverter/charge controllers and show me the production day to day and month to month for a number of his clients. He has access so he could troubleshoot or answer questions when they call.

Anyway, I learned a lot about solar.....everything from panels with micro-inverters for 2 leg-240vAC grid tied systems, to what I want to do - simple/basic battery back up, not grid tied, for critical loads (power to outlets, fridge, freezer, and lighting for when we lose power).

The major take away is that for my AO, for the worst "sun-hour" months (November-February), I would need about 5000-6000 watts of solar to be able to provide the energy for the loads. That isn't gonna happen right now.

But I can get an inverter/charge controller unit that is big enough for my loads and can handle 5K watts solar for the final build out, if I go there. They are from reputable companies with a good warranty, serviced in the USA, have replacement components available, and aren't much more than cobbling together parts from amazon.

The inverter/charger units can take power from solar panels and from normal grid power for charging the batteries. So if my panels don't produce enough, I can top off batteries overnight via normal grid power. I'll put the charger on a timer.

The most important thing is battery back up tied to an inverter/charger that can handle the critical loads for about 3 days. This assumes having 800-1200 watts of panels producing about 30% due to cloudy conditions for the three days in a row. And during "good" months (June-September) we should get about 5-6 sun hours per day, so it might be able to provide the necessary power.

I'll probably go with a set up from EG4. Their EG4 3000 watt inverter/charger and three EG4 Lifepower 48V batteries.

Notes for people's replies.....correct, NW PA it not a very good place for solar compared to out west like New Mexico. But I like the no noise aspect. If SHTF I can take the stuff to the BoL. and not draw attention when I'm using. (of course light discipline will be used at nighttime).

Since the HVAC will be all electric, the house will get a 500 gallon propane tank for heat back up. I already have a ProComm vent less propane fireplace to install. The house will also get an antique wood burning parlor stove. That's why on my list in the OP I have box fans. I figure I'd have to move that heat around in a power outage.

I have a couple 4k gasoline generators that can tie into a transfer panel if we lose power for longer than the batteries can supply. Or just use the genny to charge the batteries.

And if we lose power, managing power usage from the batteries will be priority.....No microwave popcorn at that point! But coffee is a must.

Thank you colorado and gave the real time info and lit a fire for me to go seek more.
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Today at peak time 1230, my 23 kw solar system is making about 1300 watts or 1.3 kw
its totally cloudy
yesterday was sunny, at this time I was making about 17 kw

clouds suck
Wow, that is a huge delta between installed vs production. That's only 5.6% production rate. After looking at a lot numbers and systems yesterday, I saw cloudy/overcast day production anywhere from 15-30%.

Here's some points of reference for folks to get an "order of magnitude" for your numbers.

This is a 20,000 watt array:

and this is what it can power today while generating...

Solar has piqued my interest as well as another redundant power source. Like Robert said, W PA is a bit gloomy to rely solely on solar power but as a stop gap it has its place. I do not want to spend $20k or anywhere near and do not want all the panel gear on my roof in the event we move in the next few years. I've found the brand in the link below. Looks good for stop gap or for camping etc . Any comments on this line ?

For a point of power consumption, that size jackery would run a typical freezer/fridge using 300 watts for about 1.75 hours...but the motor start will probably exceed the limit of the built in inverter.
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(Running Watts x Hours) x ( 1 / inverter efficiency) = Required Battery Watts Hours
(300W x 1.75H) x (1 / .90) = 583 WH

You’ll have to look at the specs on the storage device because some battery types can’t be depleted below 50% (cycle depth) of total capacity so if you require 600 WH you’ll need a capacity of 1200 WH, so you need to read the specs of a specific unit to see the recommended cycle depth. You’ll also want to determine the starting W required and ensure that you size the inverter to match. Some inverters will list a peak power rating that they can maintain for a few seconds (usually 200% of running). It really depends on how long the starting W of the freezer is maintained to know if you will exceed the peak rating. Another consideration is that inverters also suck energy the entire time they are powered on, even if there is no or little draw from the appliance, so you also need to understand what the idle inverter draw is and add that to the necessary AH required in stored energy.
Yes. True. I was just trying to keep it simple.

We can dive into efficiency of inverters/converters and such and depth of discharge for different types of batteries at a later date with some folks.:)

We really need to create an electrical 101 sticky.

Actually we need a few. One for electrical basics. One for estimating your power needs for what you want to power. One for solar panel electrical generation and storage. One for those "solar generators" (aka battery /inverter packs). And battery types and characteristics.
Pardon my ignorance but as I understand solar, it will store the energy it collects on a sunny day and be ready for use after the fact ?
Not really. the term "solar" is a very broad brush stroke. There are different types of solar systems.

Solar panels themselves can not store it. They only convert available light to DC (direct current) electricity. No power.

Batteries store it.

The basic three types of solar systems are:

Grid Tied - usually bigger solar panel array that makes DC power. The DC power goes to an inverter. The inverter changes DC to AC (alternating current) and connects to your home's electrical system, which is connected to the grid. This is a "two part" system. Panels and Inverter. Obviously it only makes power when there's sunlight. When it's dark you still have grid power. If grid goes have no power. (there's can be more but this is the most simple explanation)

Off Grid - Typically this type of system is for us "preppers". We want to be able to power things when the grid goes down. The amount of panels and the amount of batteries depends on what you want to power. It's a "four part system". You have solar panels which make DC power. The DC power goes to a charge controller. The charge controller "controls " the voltage/power, which will charge the batteries. The batteries supply DC power to an inverter. The inverter changes DC power to AC power. Once inverted we can use it with our "normal" 120VAC things that we plug into our home outlets. The four parts are solar panels, charge controller, batteries and inverter.

Hybrid - combination of the two.
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I just want to comment about the cloudy weather.

I live 100% off grid, full time. This winter here in northern Maine we've had much less sun than we normally have in winter. So far this winter I've had to use my gas generator four or five times to charge the battery bank for my solar-electric system. In contrast, last winter I didn't use the gas generator at all to charge my battery bank.
What size system (panels and battery)? And what kind of daily loads----peak watt, total watt for day?

All 120 or two leg 240?

I assume no big loads are on solar/battery i.e water heater, clothes dryer, electric stove, baseboard heat, HVAC)

Without reference of size and system, it's hard to quantify your post. I can live 100% off grid and only have power needs for a satellite connection for my internet and my phone. So, I would only only need to generate a few hundreds watt per day.

Not trying to be a ****, just trying to understand peoples' systems.
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TexasHomesteader's numbers are fairly realistic. I've been running a 6000 watt system for about 7 years and my numbers are similar. Keep in mind that different people have slightly different definitions of "cloudy weather"

Yes, I've seen close to 6000 watts out of my 6000 watts in panels but 4400 watts is probably closer to the norm on a sunny day and the batteries are low enough to pull hard.
Thank you.

You have 6K in panels....and generate 4400 normally on a sunny day. That is instantaneous value.

But what was your total generation for a day in watt hours?

And yes, cloudy or overcast is subjective
Below is a screen shot from a friend's house. He has 8K of solar panels. It's grid tied (he has no storage). What he makes he uses, and if there's excess it gets fed back to the grid.

This graph is from 28 Jan. The last three days (Jan 29, 30,31) he added another 20 KWH, for a total of 197 KWH for January. He made 226 KWH in December.

I knew the capacity factor of solar was not great, especially in our area (NW PA). I'm a mechanical engineer and I work in power generation. So availability, efficiency, capacity factor are things I am familiar with. But I guess not familiar enough because I'm not a "renewable" guy. I work gas plants.

From the numbers I'm getting from my friend, here on SB, and a colleague, I'm definitely going to need a grid tied charger for batteries. I'm not going to build a solar panel array big enough to keep batteries topped off. Our worst case is 5-7 days with only 25% (avg) being generated of installed capacity.

But again my goal is to use what I can, have back up for power outages lasting a couple/few days and in the event SHTF, I'll have something long after the gas and propane is gone.

I don't plan on having a good return on investment for my system. 1200-1800 watts solar, 15KWH of battery storage and a quality 3k Watt or higher inverter/charge controller. My estimates, based on current cost per KWH, it will break even in 17 years... I'll consider it the same as my food stores, bang stick food, car insurance, life insurance, etc. It's better to have it and not need it, than need it and not have it.

Product Azure Rectangle Font Material property
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Keep in mind, most grid connected panel arrangements don't actually feed the house. They feed the grid and the house consumes from the grid. It's subtle, but important difference because when the electricity grid goes down, you will not be able to power the house from the panels. There are systems that integrate a transfer switch to allow this, but it is something specific that you will need to design for to ensure that your house can consume solar power in the case that the grid goes down.

My friend's system is tied to his panel. 2 Hots - 240Vac. I removed the covers to inspect what they did. Very cool (and expensive) system. It's been up and running since mid November. I don't think he's too impressed with it. I didn't want to rain on his parade and calculate the return on investment.

And yes, you are correct, he can't generate from solar panels if the grid is down. They need a frequency reference to synch to.

The system uses Enphase IQ8A microinverters on each 385 watt panel.

The transfer switch.(hybrid inverter) that is approved by our utility was $4000 according to him. And then he needed the battery bank, which was another $6-12K (depending on how much battery storage he wanted). This was the installed costs. He's going to show me the paperwork this weekend. I bet I get sticker shock!!

He can do the off grid/hybrid for less himself, but he has to jump through some hoops with the power company.
When I started this thread I was looking at "how much do I need?"

That was not only for the size of the solar panel array. I was also calculating the battery back needed up considering we live in a cloudy area and inverter sizing for basic lighting and 120V electrical outlets.

I know I will not put up enough solar to always keep the batteries fully charged. We go 3-5 days in a row where there's cloud cover and the array is only producing 15-20%. The return on investment sucks for solar in NW PA.

The most important element is to have power if the grid goes down. Typically we don't see grid down events often around here. So I figured I need 3 days of battery storage for the basic electrical usage/load. There will be no high load 120V or 240 if grid is down. I am going to go with the EG4 3K inverter tied in to a 30A 10 circuit transfer switch and three EG4 Lifepower 48V -100AH batteries in a nice rack. This should cover essential 120V power for 3 days.

Then I'm at a Power Generation conference in Orlando and see this, and my heart stops..532 KWH of power!!!!!

Disk array Server Gas Machine Electronic device
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