Journey to Solar - Part 2 - Understanding Panels, Inverters & Gateways
In part 1 of the Journey to Solar - I discussed getting a good understanding of what your energy needs really are and where you should make investments in efficiency to change your consumption model and potentially reduce the size of your solar solution.
For Part 2 let's assume you've done your homework and made your investments to improve your efficiency and your ready to start designing your system.
Where do you start ? How do I size my system ? Do I want Storage ?
So let's start with an example set of criteria. You added up your total annual kWh usage is it's around 20,000 kWh. You've also determined that your maximum kWh consumption during any one hour in the year is 10kWh and that your daily maximum total consumption is about 100kWh and your maximum monthly usage is about 3000kWh - with a range of 1500 -> 3000 during any one month. Finally let's say for that 100kWh day - 60kWh is during daylight hrs and 40kWh is evenings/overnight.
So your thinking oh - okay - if I buy a 10kW solution that covers my maximum hour usage I'll be good - right ? Chances are the answer will be no....
There are a lot of factors that can impact a solar design and therefore the usable output of a solar system. They range from things like panel Azimuth (location), roof angle, shading, electrical cabling, temperature and DC->AC conversion. All of these factors can introduce loss into a system.
Let's start first with the PV panels themselves and what's called STC or "Standard Test Conditions". STC is a laboratory measurement of a panels output in Watts and what the panel manufacturer then markets the panel as. So a manufacture will market a 325W panel and that's it STC.
So kind of like EPA mileage ratings on a car - "your mileage may vary" is a reality. Start looking at is what's called the PTC or "PV USA Testing Conditions". These are a more real world measurement of what any one panel may produce. For example a 325W STC Panel may actually have a 300W PTC output. A great resource is from the state of CA which lists thousands of panels.
A change in the market place you can consider is choosing 72 cell panels vs 60 cell. 72 cell panels are 12" longer but the same width as 60 cell - so you can use the same mounting hardware. 72 Cell units come in outputs upwards of 450W vs the 350 - 375 available with 60 cell panels. Yes you will pay a premium for 72 cell panels - but I'm a big believer in trying to keep your roof top footprint as small as possible for the total kW you need. Depending on your requirements - when you start getting into large kW installs (so 16 -> 22kW) the panel count can start to get really high and so anything you can do to reduce that footprint helps..
But that's just the start..
Another great tool is called PVWatts published by the National Reusable Energy Lab (NREL). https://pvwatts.nrel.gov/pvwatts.php This tool will allow you to put in the size of the system you're thinking about and you can either A) take a number of default loss factors or B) customize your factors (like the ones I mention above) and it will tell you based on your installation address - what you should expect as output of that solution for any one month and what the savings are based on standard electricity rates for your area. By default the PVWatts tool calculates a 14% loss from the the system capacity of your panels to the actual AC output that you will consume.
I recommend customizing the factors to try and match your "real world" installation as closely as you can. This is especially true is you need to install a system that isn't going to be located at true south or will have shade for example.
Using our 10kW solution as an example at a location in Scottsdale, AZ - the system outputs a low of 1059kWh of output in December to a high of 1800kWh in May. So notice if we say our low usage for a month is 1500kWh - this system won't cover it all - you'll be some 441kWh short and need to draw from the grid to cover it. Similar in summer - that 1800 maximum in May is some 1200kWh short of our 3000kWh example.
But remember a solar system by itself only produces energy during specific hrs in the day. In fact one of the very cool things about the PVWatts tool is it can actually spit out a report that estimates your output during any one hour of any given day of the year. Again using our 10kW example - on Jan 1 - that system will produce energy between 8A and 4P and the panels would produce approximately 43.4kWh in total ranging from a low of about 2.3kWh at 8A to a high of about 6.5kWh at 12P.
In Summer - so let's say July 1 - the system produces energy from 6A to 7P and for a total of 56.4kWh - with a range of output from as little as 0.75kWh @ 6A to 6.8kWh at 1P. So again notice this system will NOT cover our highest estimate hour usage of 10kWh nor the daylight total of 60kWh nor the monthly of 3000kWh. So with this size of a system you'll pay the local utility for between 43 -> 57kWh every day. So obviously a reduction - but not complete independence.
So you have a choice. Increase the size of the PV solution to more closely meet your actual requirement or potentially look at storage. A good Solar Installation company will typically size a system as close as possible to 100% of your annual usage
The goal with storage systems like Tesla's Powerwall is to use your solar system to charge the batteries during generating hrs and use them instead of the utility during peak periods, overnight or for blackouts. To me storage is the potential game changer in residential energy alternative systems. But it certainly effects your design, costs and there are some Utility & IRS rules you need to be aware of.
To manage solar/storage systems a device that Tesla for example calls a Gateway is used. The gateway ties together your utility feed, solar feed and battery feed and uses intelligence and rules you set to determine your energy "flow" - so what all you're drawing energy from.
One caveat with batteries though - in some states - you can NOT use the utility feed to charge your batteries 100% at most they allow around 25%. This is sometimes enforced via the Apps that manage the system. But it does vary - so again make sure you've done your homework to insure the system meets your needs.
With Storage - the design approach is run your home via solar and then use excess generation from the system to charge your batteries. But as you've seen from our example - while adding storage may provide you the ability to avoid "time of day" or Peak Hr charges or run your home during non-generation hrs - your Solar Generation system must be powerful enough to support this. And often that means installing a much larger solar system.
The other big use case is blackout coverage. But then you really need to understand what all you would run during a blackout and how long you may need to run. In most cases the model is 24 hrs.
So again - let's use our 100kWh daily total example with 60kWh daylight and 40kWh overnight. To support that you would need 3 x Tesla Powerwalls - at 13.5kWh each that's 40.5kWh. So now your system needs to be able to not only support your daylight usage - but also generate enough excess output to keep the batteries charged.
This where designs can get tricky and you have to try and make the best decision that meets your goals. It's very easy to get carried away in the math of trying to make all the kWh numbers work only to find out you just designed a $100K system that won't fit on your roof.
Another consideration is a term commonly called "net metering". Most residential solar systems are what are called "grid tied" which means they are installed in conjunction with your utility feed and you typically need to have signed an "interconnect" agreement with the utility.
With Net metering you and the utility have an agreement that states they will credit you for any excess energy your system creates at what is often called an export rate. Net metering rules vary across the country - some states have a statewide standard that all utilities must comply with whereas other states leave it up to the individual utilities to support this concept. Some utilities require that you be on a special "solar" plan while others allow you to use any plan. So do your homework. When you introduce storage - the goal is to not really focus on net metering - but more on battery charging.
And finally you need to ask yourself an important question - is the cost/complexity of adding storage going to offset the additional utility costs. Just like everything else in this market, prices are trending downward, but for example - if your primary goal is blackout coverage and you have a good alternative fuel source - like natural gas - a generator is much more cost effective than solar storage and can run for days if needed.
Another factor is that "export rate" from your utility (if they even have one). If the export rate is close to your purchase rate - then often the ROI model on batteries for things like "time of day" shifting, etc, doesn't pay over the typical 10 year warranty period of most batteries.
Let's shift gears a little and talk about an important part of your solar installation - a device called an Inverter. PV Solar panels generate their power as DC and for it to be usable in your home - it needs to be "inverted" to AC. That's the role of these devices.
Today there are 2 primary forms of inverters - String & Micro. String inverters are used when all your panels are all connected together and a single output line and sent to a central inverter. In installations where there is little variance in things like shade or electrical loss, etc - they work great and are what is installed in most Solar installations. One challenge though with String inverters is if one panel is suddenly outputting less than the others - they all drop to that lower output. Some companies are adding what are called power optimizers to each panel to reduce that exposure.
Micro inverters are individual devices attached to each PV panel. This allows each panel to operate independently which is great when you need to account for factors like shading or east-west installations. But they are typically more expensive and if something happens to one - your now going to your roof to fix it. In addition - microinverters are not the best when thinking about a solar+storage solution. But the industry is changing all the time and it's best to keep abreast with the latest products. In fact some PV Panels manufacturers are now integrating microinverters into the panels themselves.
The one recommendation I would make is do NOT Cheap out on your inverters. When compared to Panels - that typically have a 25 year warranty - inverters may be have 10 year - so make sure to buy the highest quality you can afford.
The same is true about your installer. Today - Solar installation companies are everywhere - at least out here in AZ at last count there were 130+. And they're kind of like HVAC companies in that they have aligned with certain OEMs for Panels, Inverters, Mounting Hardware, etc. And just like any company like this - some are good - some are not so good. There are a lot of steps in properly installing a Solar system - permits, inspections, utility agreements, etc. So perform your due diligence - get multiple quotes and understand what your buying and who your buying from. This is especially true when understanding their financing models.
There are several firms that focus on the DIY market. Can you do a DIY Solar installation - yes. Should you ? - the answer may be no.
Finally a word on Tax Incentives and what all qualifies. For 2020 the IRS Incentive Tax Credit (ITC) is 26% on a solar systems for 2021 that drops to 22% and then the program ends - meaning no incentives in 2022 and beyond - at least for now. The one interesting question right now is - as solar system components continue to drop in price - do you execute now to get the best IRS ITC you can or do you wait and follow the price drops down...
Any storage you include in your system also qualifies - but only if it is being charged from solar generation by at least 75%. Here is a good document from NREL on this subject https://www.nrel.gov/docs/fy18osti/70384.pdf The rules can get complex so make sure you've done your due diligence.
Bottom line is this - can you reduce your reliance on your local utility by installing a solar system - absolutely. Can you reduce your utility charges to zero ? Yes - but it may take a much larger investment than you think. Should I include Storage ? - I would definitely look at it - but understand if it really makes sense for you.
Comments
Post a Comment