19 graphs that explain solar panels for home owners (+maps)

Every week or two I get an email asking for information about going solar.
Although I’ve had solar panels myself for three years, and have a bit of solar info on the site, I generally refer people off to other resources in the full knowledge they aren’t ideal.
Today I’m going to fix that. But rather than write a giant piece full of wordiness I’m going to try to do a punchy graphical summary of some questions you should ask yourself before going solar.
Now let me warn you.  This will eventually get a little nerdy, but that is the whole point.  If you want to get a good solar installation, at a decent price, you need to be able to grill the solar companies you get quotes from.
It’s all part of the fun!!
1) How does solar work?

Think of this from the sun down.  First the rays hit the solar panels themselves where the solar energy is turned into direct current electricity.  Then it goes to the inverter which converts the power to alternating current.  Next you’ve normally got a solar meter, followed by an isolation switch (with a trip), then it hits the consumer unit where it is ready to be used or exported to the grid via a net meter (when in excess).
2) Is it sunny where you live?
I’ve always found it rather bizarre that Germany lead the world in solar.  I mean I know they have great engineering, high electricity prices and a series of green governments, but it just isn’t that sunny.  The sunnier it is where you live the more energy you will generate.  Also in places like California and Australia a solar system will produce more electricity on hot days when air conditioning demand is high.  It just makes more sense there.
3) Do you have a good roof for solar?
Solar panels need to face towards the suns rotation during the day.  In the UK example above you can see that most energy is generated facing directly South, although anything within 30° is not too bad, particularly if it is slightly West as you’ll have more afternoon energy when people are at home.  In the Southern hemisphere this is reversed (face them North) and on the equator they can be flat. If someone selling solar tries to tell you it doesn’t matter which bit of roof they use you need to close the door in their face!!
4) Is the roof tilt at a decent angle?
Depending on their distance from the equator solar panels should have a different tilt to maximise their production.  Generally you just have to hope your roof is in the ballpark.  A quick estimate of the ideal is 85% of your latitude.  So in LA where the latitude is about 34° N the ideal tilt is roughly 29° off horizontal.  A little geeky, but kind of useful!
5) What type of ownership is best?
As someone who has an extremely valuable solar system sitting on my roof I do find the idea of not owning your system rather odd.  But to be fair, in America leasing your system became the dominant ownership model in 2011. If you’re an American and would like a good objective comparison of different ownership models check out the calculator at the Institute for Local Self-Reliance.
6) How much will it cost? (US edition)
People get very excited by the beautiful crash in solar module prices, but the market place lives and dies by the install price.  In the chart above you can see why solar has grown so quickly in the US since 2010.  These days you’d be hoping to get near $4/W installed.  So a typical 5kW system could be $20k, dropping to something like $12k after rebates (location dependent).  You’ll need a few quotes to test the water.
7) How much will it cost? (UK edition)
The 3.3 kW system on my roof cost me over £12k back in 2011, today you can get a similar system for half the price.  The chart above gives you an idea of the cost of a 4kW system in the UK.  Because feed in tariffs have also dropped dramatically the payback time remains roughly 10 years.  Again you’ll need to get a quote to know, and why not sort your energy bills out while your at it.
8) How much will it cost? (AUS edition)
Around 1.4 million homes have solar in Australia.  Which doesn’t sound impressive until you realise we only have 9 million households and far fewer roofs.  Despite the chaotic policy environment we (I’m an Aussie) do have three things that made this happen. Lots of sun, expensive electricity and a very competitive solar market (see above).  In fact in Australia a 5kW system is just $8k.  In some areas the 10kW systems are close to crashing through the fabled $1/W mark. Sadly, our Prime Minister isn’t too impressed by all this.
9) Will you be eligible for a Feed-In-Tariff?
Fit in tariff policy around the world is a political punching bag.  The left hand chart from Germany probably explains why they had so much success growing solar to begin with. A stable Feed In Tariff climate. As a potential solar owner you really want to understand if you will be eligible for a feed in tariff and if so for how long?  Australia serves as a great warning to other countries as most tariffs are now set by suppliers, rather than state governments.
10) Are there tax credits available?
Depending on where you live in the world their may be tax credits or other subsidies that reduce your out-of-pocket expenses.  For Americans a nice place to start looking is the Database of State Incentives for Renewables & Efficiency.
11) How much do you pay for power?
power prices
The more that you pay for electricity the more you’ll save when you use your own solar energy instead.  When you look at the combination of feed in tariffs and high power prices in Europe you can see how such a cloudy region of the world got ahead in solar.
12) Where are the panels made?
People often sneer at Chinese panels, but the truth is these days they make a lot of very good panels at an incredibly low prices.  As you can see above seven of the ten biggest panel manufacturers in the world produce primarily in China.
13) Who makes the best panels?
Ok, this is damn nerdy but here goes.  If you really want to know how good a panel is goes to this website and divide the PTC (PVUSA Test Conditions) rating of a panel by its nameplate wattage. This is what can be called the performance ratio (graphed above), which essentially tells you if you are getting what is says on the tin based on the excellent independent Californian laboratory testing of panels.  Above 90 is a good panel.
14) Who makes the best inverters?
By far the most likely component to fail in your system is the inverter.  So if you are going to pay a bit more for some quality, spend it on the inverter. Top quality inverters include SMA, PowerOne and Xantrex while good quality budget options come from Sungrow and Growatt.  Again you can look at their testing on the Californian site where efficiency should be above 95%.
15) Is you roof shaded during the day?
Something people often don’t realise about the traditional ‘string’ set up of solar panels is that just a small bit of shading on one panel will reduce production across the whole string.  If you have any shading issues at all (antennas, branches, chimneys) consider asking for micro-inverters that can limit this problem to the shaded panels.
16) How much of the power will you use?
When you do get a quote make sure you check if the assumptions about how much you’ll use are sensible.  I sent one salesman here backing just because he was sure I’d use over 50%.  (We actually use under 25% because nothing is on during the weekday periods)
17) Are they assuming prices will rise?
You also need to keep an eye on the assumptions about power prices.  Being too bullish about potential prices rises makes solar look a better deal than it is.  Solar is generally a decent investment, one shouldn’t need shoddy assumptions about sky-rocketing power prices to make a hard sell.
18) Did they include the cost of capital?
If you don’t spend money on solar you can use it elsewhere.  So if you want to be rigorous about payback calculations you should consider the cost of capital.  Again omitting this can makes payback look a little rosy.  You can tell a solar comparison calculator is really rigorous if they include options for this figure.
19) How much carbon will it save?
If you are going solar to reduce emissions then the result really depends on where you live.  If you are displacing coal fired electricity is saves a huge amount whereas you live on a grid dominated by hydro or nuclear it will have no real impact.
20) Will the solar panels look good?
Ugly solar has the potential to devalue your house.  Why people put absurd looking arrays on period homes is beyond.  There are much cheaper ways to cut carbon.  In contrast attractive solar adds value.
21) Make sure you keep the paperwork!!
That’s me!!  Holding all my important solar paperwork.  For the UK, where I live, that includes my original quote, the payment receipts, my panels warranty, my inverter warranty, the MCS certificate and my feed-in-tariff registration.  If anything goes wrong, or we want to sell this house, I will need these.
Good luck on going solar!!!!!!
Source: Shrink That Footprint. Reproduced with permission.

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