Question for the home solar power experts...my sample house

I have a house in the Rockford, IL area. It has a flat 30' x 20' south facing roof. I use around 11,000 kWh of electricity per year in my home, at a cost of around 13 cents per kWh total.

I have always thought solar power is pretty cool, but really haven't looked much into it. Just out of curiosity, what would a solar system for my house "look like?" Not in a visual sense, but considering my circumstances, how many panels would fit on my roof? How much electricity could they theoretically produce per year in my area, considering our climate and days of sunshine? What would the major components generally be and how much would they roughly cost? In other words, if you were doing a "back of the envelope" solar project for my home, what components would it have and how much would it cost, roughly? I've always sorta assumed solar in northern IL is a waste of time considering how many cloudy days we have and how far north we are, but I am just curious what some rough numbers would look like.

Yes, I realize I could spend a few hours on the internet looking all this stuff up myself, but I would bet that some of you could throw something together in about 5 minutes, knowing what you know about home solar just off the top of your head.

This might help get you started:

http://www.bdbatteries.com/panelcalculator.php

There are lots of online resources for this sort of question.

My solar system is 5.16kw and I make about 10,000kw a year offseting 90% of my power.

PVwatts is the best calculator to use - it's calculations based on actual data.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Illinois/Rockford.html

To completely offset your 11 MWh of electricity usage a year, you'd need approximately a 9 kW (STC rated) system tilted up between 30-45* facing south in Rockford, IL.

Assuming a panel rating of appx 225W, that'd be about 40 panels. But your roof isn't big enough to fit 40 typical panels. A typical 225 panel is about 17 sq/ft and you have 600 sq/ft so that would give you room for about 35 panels laying flat.

But to maximize output at your latitude you lose about half your roof space to avoid panels in front from shading panels in the back (would have to run calcs to determine actual minimum spacing requirements), so I'd expect you to be able to fit about 18 panels or so for appx a 4 kW system.

An excellent source for 'how they did it' info.

http://homepower.com/basics/solar/
http://homepower.com/search/advanced/results/?search=&resourceid=5&energyapplicationid=4

drees said:

PVwatts is the best calculator to use - it's calculations based on actual data.
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Illinois/Rockford.html
To completely offset your 11 MWh of electricity usage a year, you'd need approximately a 9 kW (STC rated) system tilted up between 30-45* facing south in Rockford, IL.
Assuming a panel rating of appx 225W, that'd be about 40 panels.

Click to expand...

So he could buy larger watt panels and not have to use as many. The new SunPower panels are 19% efficient and if he chose the 305W, he would only need about 26 panels.

LEAFfan said:

drees said:

PVwatts is the best calculator to use - it's calculations based on actual data.
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Illinois/Rockford.html
To completely offset your 11 MWh of electricity usage a year, you'd need approximately a 9 kW (STC rated) system tilted up between 30-45* facing south in Rockford, IL.
Assuming a panel rating of appx 225W, that'd be about 40 panels.

Click to expand...

So he could buy larger watt panels and not have to use as many. The new SunPower panels are 19% efficient and if he chose the 305W, he would only need about 26 panels.

Click to expand...

All through LESFfan.....I realize there are online resources, but I just wanted some back of the envelope stuff.

So regarding those SunPower panels, and the other ones previously mentioned a few questions. What do these panels cost per panel? And when you say a panel is rated at 225W or 305W, what does that mean? How much electricity they produce in a day or something? What is the math that relates a 9kW system to 11MWh of production per year?

And how do inverters come into play? Do you need x amount of inverters per x number of panels or similar? What do inverters cost, and what makes one inverter better than another?

Again just looking for rough estimates and judgements. I will review those links above as well once I get a basic idea from you all as to what I should be thinking about.

LEAFfan said:

So he could buy larger watt panels and not have to use as many. The new SunPower panels are 19% efficient and if he chose the 305W, he would only need about 26 panels.

Click to expand...

Size of the panel doesn't matter - only efficiency (power per sq/ft) matters. SunPower panels are the only ones on the market that are substantially more efficient than your typical panel (~19% vs ~14-15%) - but you will pay a premium for them. If you are lacking room for the panels, then the premium may be worth it.

Ualdriver said:

So regarding those SunPower panels, and the other ones previously mentioned a few questions. What do these panels cost per panel?

Click to expand...

You'd have to get a quote from a Sunpower installer - you can't get these panels by themself. Retail prices for regular solar panels range from $2-$4/watt these days.

Ualdriver said:

And when you say a panel is rated at 225W or 305W, what does that mean?

Click to expand...

That is how much power the panel produces under lab conditions - it's pretty much the maximum amount of power you'd expect to get out of a panel when the sun is perpendicular to the panels and temperatures are moderate.

Ualdriver said:

What is the math that relates a 9kW system to 11MWh of production per year?

Click to expand...

The amount of solar irradiance that hits the panels on average over a year for a particular location adjusted for which direction the panels are facing.

Ualdriver said:

And how do inverters come into play? Do you need x amount of inverters per x number of panels or similar? What do inverters cost, and what makes one inverter better than another?

Click to expand...

The design of the system will usually dictate the inverter that is used. There are central (1 bit inverter for whole system) and micro inverters (1 inverter per panel). Each has it's pro's and cons. As far as central inverters go, SMA is the #1 brand. Micro-inverters - Enphase is #1 for that market. Cost for inverters range between $0.25-$0.75/watt.

Ualdriver said:

LEAFfan said:

drees said:

PVwatts is the best calculator to use - it's calculations based on actual data.
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Illinois/Rockford.html
To completely offset your 11 MWh of electricity usage a year, you'd need approximately a 9 kW (STC rated) system tilted up between 30-45* facing south in Rockford, IL.
Assuming a panel rating of appx 225W, that'd be about 40 panels.

Click to expand...

So he could buy larger watt panels and not have to use as many. The new SunPower panels are 19% efficient and if he chose the 305W, he would only need about 26 panels.

Click to expand...

All through LESFfan.....I realize there are online resources, but I just wanted some back of the envelope stuff.

So regarding those SunPower panels, and the other ones previously mentioned a few questions. What do these panels cost per panel? And when you say a panel is rated at 225W or 305W, what does that mean? How much electricity they produce in a day or something? What is the math that relates a 9kW system to 11MWh of production per year?

And how do inverters come into play? Do you need x amount of inverters per x number of panels or similar? What do inverters cost, and what makes one inverter better than another?

Again just looking for rough estimates and judgements. I will review those links above as well once I get a basic idea from you all as to what I should be thinking about.

Click to expand...

If you've got a lot of roof, or can/will ground-mount, efficiency in terms of power/sq foot may not be as important as power/$. Space limitations push one toward smaller more efficient panels (and higher panel cost).

Inverters can be per-panel (like Enphase) or whole system (like Outback or Sunnyboy or ...everyone else).

Panel rating uses a standard light lab test. This allows direct comparison among different panels. The STC rating will likely be used to determine the amount of state or federal rebates as well. But mounting location and angle, tracking or stationary, temperature, etc. will affect - most likely lower - the actual energy output.

Do yourself a favor and hit the Homepower link. It's an overview of solar and equipment with links to more detail on each of the components. It's also the best source I know for panel and inverter comparison shopping. Do that education and due diligence first before you go out for bids.

Good luck!

I know it's bad form to post advertisements, but Mike is one of us.

I purchased Mike Boxwells 'Solar Electricity Handbook, 2011 Edition'.

It describes in great detail all the questions and answers being posted here.

It is available at all bookstores and online sites. For me, it was a great addition.

I am also ANXIOUSLY awaiting his 2011 Nissan LEAF Handbook, as well.

I highly recommend the Solar book for all folks interested in the topic (pre or post install).

SilverLeaf said:

I know it's bad form to post advertisements, but Mike is one of us.

I purchased Mike Boxwells 'Solar Electricity Handbook, 2011 Edition'.

It describes in great detail all the questions and answers being posted here.

It is available at all bookstores and online sites. For me, it was a great addition.

Click to expand...

OK, I bought his book and read it last week. It was a really good book.

So as I follow along in this book (sort of) step by step......

I looked at my electric bill and I use the most electricity in July where I use 40,800 watt-hours per day on average.

I looked around on line, and saw that SunPower makes 315W panels.
I looked in the charts in the back of the book and figured I'd use a year round tilt optimization, which gives me a solar insolation of 5.52.

So my high quality SunPower panels should produce 5.52 x 315W = 1739 watt-hours per day.

I figure the roof is hot so I'll have a 10% loss due to heat. Now I'm at 1565 watt-hours per day.

I need 40,800 watt-hours, so I need 26 panels (40,800 watt-hours needed/1565 watt-hours per panel)

Are those calculations above correct so far?

Ualdriver said:

Are those calculations above correct so far?

Click to expand...

Easiest thing to do is to take a year's worth of bills and figure out what your typical annual kWh usage.

Then go to PVwatts to get an idea of how many DC watts of panels you'll need to generate that power. PVwatts will take care of all the deratings for your system - and you can tweak the default settings if needed. I would aim for ~90% of your annual usage at the most. PVwatts tends to underestimate generation by about 10% at least initially - and you'll only get paid a fraction of the retail price for any over-generation.

drees said:

Ualdriver said:

Are those calculations above correct so far?

Click to expand...

Easiest thing to do is to take a year's worth of bills and figure out what your typical annual kWh usage.

Then go to PVwatts to get an idea of how many DC watts of panels you'll need to generate that power. PVwatts will take care of all the deratings for your system - and you can tweak the default settings if needed. I would aim for ~90% of your annual usage at the most. PVwatts tends to underestimate generation by about 10% at least initially - and you'll only get paid a fraction of the retail price for any over-generation.

Click to expand...

Cool, I'll check out the site but I was trying to "design" a theoretical system myself to understand what a system would look like.

Ualdriver, you might also consider doing an audit of your existing electric bills in terms of conservation before you calculate generation. Perhaps you've already made your house as energy efficient as possible, but if you haven't a "rule of thumb" is that for every dollar spent on conservation saves two dollars spent on generation. We made changes in appliances as we needed them, window coverings, ultra high efficient heat pumps, insulation and other areas (e.g. EVs) to lower our energy use without lowering our standard of living. For example, we keep the house at 68 degrees in the winter and 74 degrees during the summer and still generate excess electricity that is sent to the grid. Anyway, just a thought before you settle on your final design.

ERG4ALL said:

Ualdriver, you might also consider doing an audit of your existing electric bills in terms of conservation before you calculate generation. Perhaps you've already made your house as energy efficient as possible, but if you haven't a "rule of thumb" is that for every dollar spent on conservation saves two dollars spent on generation. We made changes in appliances as we needed them, window coverings, ultra high efficient heat pumps, insulation and other areas (e.g. EVs) to lower our energy use without lowering our standard of living. For example, we keep the house at 68 degrees in the winter and 74 degrees during the summer and still generate excess electricity that is sent to the grid. Anyway, just a thought before you settle on your final design.

Click to expand...

I'm not putting solar on my house.....or at least not in the up and coming years. I'm just talking theoretical so I can understand how to build and design one. I was just trying to build a theoretical system using parts I'm finding on the internet so I can LEARN about solar. That's it. That's why I was sort of building a system here to get input as to what I am forgetting, what would be better, etc. I'm just trying to figure out what a really cool set-up on my house would cost me in parts and such.

Ualdriver said:

ERG4ALL said:

Ualdriver, you might also consider doing an audit of your existing electric bills in terms of conservation before you calculate generation. Perhaps you've already made your house as energy efficient as possible, but if you haven't a "rule of thumb" is that for every dollar spent on conservation saves two dollars spent on generation. We made changes in appliances as we needed them, window coverings, ultra high efficient heat pumps, insulation and other areas (e.g. EVs) to lower our energy use without lowering our standard of living. For example, we keep the house at 68 degrees in the winter and 74 degrees during the summer and still generate excess electricity that is sent to the grid. Anyway, just a thought before you settle on your final design.

Click to expand...

I'm not putting solar on my house.....or at least not in the up and coming years. I'm just talking theoretical so I can understand how to build and design one. I was just trying to build a theoretical system using parts I'm finding on the internet so I can LEARN about solar. That's it. That's why I was sort of building a system here to get input as to what I am forgetting, what would be better, etc. I'm just trying to figure out what a really cool set-up on my house would cost me in parts and such.

Click to expand...

The point still stands. You should include new appliances and other upgrades where needed in your budget, even if theoretical.