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If you own or are planning to own your own solar power system, you’ve probably heard of an Inverter.    Although a vital part of any solar electric system, inverter choices can easily be overlooked in the shadow of seemingly more important decisions such as installer and solar panel options.   I mean, doesn’t an inverter just change the solar energy (DC) to house energy (AC)?  How complicated can it be?  Contrary to what your salesman or inverter manufacturers may have you believe, there is no “one size fits all” inverter.   As with many electronic devices; design, cost, warranty, time-testedness and monitoring capabilities are variables designed to offer as many choices as the different homes, businesses and climates they inhabit.  For example, you wouldn’t want to pay a premium for an inverter designed to deal with partial shading issues – if you have zero shading.  At a minimum, you’d at least appreciate knowing how the “other” inverter would affect the contract price.  So, in this spirit – let’s take a preliminary tour of the Inverter (or optional, microinverters).  Inverters have three primary functions:

1. Inversion of the incoming DC energy (direct current) into home and appliance-friendly compatible AC energy (alternating current).  This is its main function.
2. An MPPT (maximum power point tracking) function ensures that the inverter receives the maximum amount of power from the solar panel by adjusting the voltage (load) to match what the inverter wants; managing the complex relationship between sunlight and other varying conditions.
3. Various electronic monitoring sensors that will, for example, detect when there is a fault in the grid-tied solar panel or on the grid and shut down power so the solar system cannot injure power-line workers during a grid outage.

The most traditional form of Inverter is a String Inverter, so-called because it is connected to a line or “string” of solar PV panels.  One solar power system, depending on the size/number of strings in the system, may have one or several string inverters which sit in parallel and convert the arriving DC current into an AC output.

These six string inverters service a large commercial solar electric (PV) system. Each inverter services forty solar panels, on four strings of 10 for a total of 240 panels.

Micro-inverters and Power Optimizers

Another type of inverter is the Micro-Inverter.  As the name suggests, this inverter is a smaller version of a string inverter, but rated to handle the output of each individual panel.  Therefore an inverter is located on the back of every solar panel unlike a string inverter that is often located on the side of the building/ground level.  Micro-inverters have been around almost as long as string inverters.  Historically, the manufacturing costs involved with making a dozen “mini” inverters couldn’t compare to the cost of one large string inverter.  However, as the solar industry has grown  – so has the demand for the micro-inverter; driving up demand while reducing production and distribution costs.

In response to the popularity of the micro inverter, Tigo, followed by String inverter giants SMA, developed a device called the Power Optimizer that works in conjunction with a string inverter.  As with micro-inverters, power optimizers are placed on the back of each panel and provide an MPPT function to each panel – limiting individual shading and orientation effects to the panel they’re attached to as opposed to the whole string.  However, because they are designed to operate alongside a string inverter they do not require individual transformers and are therefore much smaller, lighter and most importantly, cheaper than micro-inverters.

Solar design expert, Rich Schroeher says there is a time and a place for all three of these inverters:

Q:  Aren’t micro inverters better when the solar arrays are facing different directions or have different roof pitches?

Rich:  With micro-inverters, in essence, each solar panel is it’s own solar system, independent of the other panel/ inverter combinations, so various orientations of panels are easily accomodated.

With string inverters, a different inverter must be used for an array that is facing a different direction, or at a different pitch than the other arrays.

One way around this is string inverters that contain 2 MPPT inputs. These are, essentially, 2 inverters in one. They also have the added benefit of being able to handle more of the array than their rating. For example, let’s say you have two arrays facing different directions: one South facing and one East facing.  The entire system would use a 5000 watt inverter divided into two channels of 4000 watts for a total of 8000 watts.  The inverter will still only supply 5000 watts but when the south roof starts getting sun, the east roof is losing it. If at anytime more than 5000watts is being fed to the inverter, it just limits it to 5K.

 

Q:  Some homes have partial shading on the roof. How do you know when to use a string inverter – possibly with dual MPPT, or a power optimizer – or stick with a microinverters that will ensure a shaded panel won’t negatively affect the whole array’s output?

Rich:  It depends on the situation. Microinverters and optimizers can help with small areas of shading but are not a cure-all. They work best for an application where a small % of the roof is shaded at various times during the day, such as a chimney shading a few collectors where the shadow moves during the day.

Q:  Are there any longevity differences between string and micro inverters?

Rich: Opinions vary.  A solar system with 24 micro inverters, as opposed to 1 string inverter, has 23 more points of failure.  Plus the extreme temperatures that microinverters are subjected to can be an issue; electronics generally don’t like heat.  Microinverters are also located on the roof behind each panel, as opposed to the string inverter which is usually conveniently located close to the main electrical supply on the ground level.

Q:   Enphase microinverters claim to get greater efficiency, up to 16%, than their string counterparts due to the fact that the maximum power point is matched to the solar panel as opposed to the inverter, which limits the available output.

Rich:  In a Partial shading situation this could be true. If there is no shading at all or very early or late in the day when production is very low anyway it wouldn’t be near that high.

There are only two notable benefits that microinverters offer in a zero shade situation: they eliminate the string inverter losses from module mismatch (very small differences between modules), around 2 – 3%.  They also help with DC wiring losses.  Combined those two losses may be 5% overall.  This all assumes that the module attached is not too large (powerful) as to have any appreciable amount of clipping – meaning the full power of the module is limited by the inverter.

Q:  Microinverters, are more expensive than string inverters, at $.70/watt with a twenty-five year warranty.  Although String inverters are as much as 50% less, they often come with only a ten year warranty.  Doesn’t the microinverter’s longer warranty cover the inflated cost?

Rich:  In the long run probably no. Most string inverter companies’ offer extended warranties up to 20 years. Some include the cost of labor to replace. The labor cost to replace is about equal for a string inverter and a microinverter or perhaps a bit lower for string inverters depending on where in the array a failed micro is located. Say you have a system with 20 panels and the labor cost for inverter replacement is $150.00

      String inverter cost over lifetime $150.00

      Microinverter cost over lifetime  as much as $3,000.00 Plus 19 more scheduled trips and possible inconvenience to the owner. 

Q:  SMA’s new inverters include a “Secure Power Supply” capability.  What is this?

Rich:  This is a small (15amp) receptacle that is powered up when the grid goes down. As long as the sun is shining it will provide 120 volts of AC electricity. My first thought was to power a refrigerator or freezer to help carry a system owner through the outage without food spoiling.  It can’t be used at night but it will let you use some of the system during outages.

As is often the case, there are no cut and dry answers here. Variables such as system size, shading, orientation, panel layout and budget will determine whether a String, MPPT optimizer or micro-inverter may or may not be the best choice for your roof.  Most installers/designers worth their salt will consider all of these options and come up with the best solution for your house.

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What Does the Future Hold?

Solar Renewable Energy Credits (SRECs) have played a large part in the financing of solar energy systems in Maryland since the RPS (Renewable Portfolio Standard) was enacted in 2005. These market-based, tradable credits are the property of the solar system owner to resell, typically to brokers who bundle them for final resale to competitive energy suppliers in the interest of meeting their solar compliance goals. In Maryland (as well as Washington DC), these credits are generated by both solar electric (PV) and solar water heating systems.

The price of SRECs is supposed to reflect the over or under supply of these credits in the marketplace. Both Maryland and DC have very aggressive solar goals (2% by 2020 in MD and 2.5% by 2023 in DC) with steep adoption curves so we need lots of SRECs to meet compliance.

Maryland:

That said, the solar industry boomed for several years recently and we are currently going into an oversupply phase in Maryland. This has the effect of pushing down prices on SRECs in the near term.   There are many contributors to the oversupply and the industry and legislators are frequently working hard to promote policies that help to smooth out the supply, but in the end, SRECs are a market mechanism that is subject to “animal spirits.”

As solar prices decline it is fitting that SREC prices are declining too – after all, we should need less incentives as solar costs come down to “grid parity.” When Maryland’s SREC market was conceived, the designers planned for a declining value as more solar got on to the grid. In fact, the Alternative Compliance Payment (ACP) schedule – the amount energy suppliers have to pay if they cannot buy SRECs – is designed to decrease over time. The ACP is considered to be the maximum that an SREC would cost in a rational market. Recently SRECs have traded on the order of 35% of the ACP, but as high as 75% a few years ago. In Maryland, the ACP is scheduled to drop from $400 to $350 in 2015 and then down to $200 in 2017, $150 in 2019, and so on.

DC

Washington DC is a different market and one that is much better insulated from the shocks of large utility scale systems that flood SRECs onto the market. The sheer geography in DC does not lend itself to 10 MW solar farms and thus the SREC supply curve is a little smoother due to the requirement being fulfilled primarily with many smaller systems. As a result, DC SRECs have shown more consistency and maintained a higher price, benefitting system owners and prospective system owners.

What now?

Regardless of the trends for solar return on investment (ROI), we all want to maximize our incentives for our own benefit. SRECs are no different. While there are many more new solar customers every day, there are also many solar system owners now approaching the end of 3 or 5 year SREC contracts (aka “strips”) and they too need to decide how to proceed going forward. Do I want to sign up for another strip (3 or 5 year term contract) and accept a large discount on my SREC price for that price security or do I want to maybe float with the market for a while? I’ve got no crystal ball, but I do know that there are many efforts underway in Maryland, some legislative and some not, to help to smooth the SREC supply and thus maintain a reasonable value for SRECs to continue to help incentivize solar. For that reason, I believe we will see some recovery of SREC prices in Maryland in the next year or two and thus maybe it is better to hold off on a term contract. In DC, I would personally opt for more surety and take a term contract with the discount price, but that is my risk averse nature. Others might like to bear more risk and float in hopes of higher SREC values in the future.

Either way, we are lucky to have these incentives in Maryland and DC. They are working to increase solar installations and jobs and they are also helping to drive down the installed price of solar in our region.

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SES gives the skinny on going sunny

1. 1. Cut your Energy Losses
   In most homes, heating and cooling accounts for the most energy usage. Regardless of where your energy is coming from; gas, coal, solar, electric, wind, etc. a homeowner’s first consideration is to reduce the loss of incoming energy:
   • Replace old windows and doors with Energy Star products. This can reduce energy consumption by 7 – 15%.
   • Seal up any air leaks throughout the house, attic and basement.
   • Insulate! This blocks air loss in the winter and preserves cool air in the summer.
2. Consider those Big Appliances
   Refrigerator: Keep the condenser coils clean. Keep refrigerator away from oven/washer/dryer and other heat producing appliances. Place with consideration to air circulation.
   Stove: Remember that gas is generally a more cost-effective choice than electric.
   Attic Fan: An excellent way of pushing hot air outside in the day, and drawing cool air at night.
3. Need a new roof?
   If you think your roof may need repairing or replacing within the next 5 or so years –it may be worth replacing before the solar panels are installed. If you’re on the fence about replacing your roof prematurely, remember that installers such as SES do provide a Remove and Reinstall service where, for a reasonable fee, the panels will be safely removed then re-installed after re-roofing.
4. Small Bite or Full Monty?
   Just about every solar energy company sells and installs Solar Electric (Photovoltaic) systems. However, solar water heaters are an entirely different type of solar panel and not every solar company installs them. Thus, not every solar company is likely to tout the cost-effectiveness of this mature technology. Solar water heaters require only a fraction of the roof space (and often a third of the cost) that a solar PV system requires. For a family of four, currently using electric to heat their home’s water – solar water heating may provide the biggest bang for your solar buck, providing around 75% of a home’s annual hot water. However, a family of two heating their water with gas may consider investing in a solar electric (PV) system sized for their usage. Insofar as solar electric (PV), many homeowners cover 60 – 100% of their electrical needs from solar. Some choose to offset as little as 25% and opt to add panels as their budget allows.
5. READ your Energy Bill
   For electricity, your energy bill should tell you how many kilowatt hours you use per year. This is an important number as you generally do not want to install a system that will exceed your annual consumption. Experienced installers can help you assess this, but a good rule of thumb is to divide that number by 1200kwH and that will tell you the approximate max sized PV system you should consider. That will be represented in kW – probably something like 8 to 10 kW if you are an average consumer. If you heat your water with electric, then solar water heating should absolutely be considered first.
6. Best way to pay for your solar system
   The beauty of an increased popularity in solar is that there are a number of ways a homeowner can finance their project. Without question, whether immediately or over time, Solar Power is far cheaper than your Utility bill.

Payment plans range from No-Money-Down leasing options to initial investments ranging from $2000 – $50,000 depending on your energy usage and type of solar system. Thanks to current financial incentives, many Marylanders receive over 50% of their upfront investment back the first year. Some of our customers have transferred funds from low yield CDs and even borrowed from 401ks in order to fund their higher-return solar investment. Other homeowners have no interest in ROI’s and fund-juggling and simply want a lowered electric bill/carbon footprint. Thankfully there is a wide variety of financing options available.

• Know YOUR solar energy tax credits, grants and incentives
  Any solar salesperson worth his/her salt will visit your home with full knowledge of the available financial incentives for your state and county. For example, an Anne Arundel County resident has the good fortune of FOUR different financial incentives (for both PV AND solar water heating) that combine to offer a huge discount on their system. Acclimating yourself to current incentives for your county will help the sales process be much more informative for you. Every solar homeowner is eligible for a 30% Federal Tax Credit. Then SRECs (solar renewable energy credits), state grants, county tax credits differ across the region.
• Solar Panel Placement
  Whether solar water heating or solar electric (PV), a south facing shade-free roof provides optimum conditions for solar panel placement. However, any shade-free roof – including East and West orientations provide excellent rooftops for solar panels. The number of panels installed depends upon system size, roof space and budget. Homeowners with a good amount of spare land may also consider ground-mounted systems, some of which offer solar tracking options where panels are mounted to motorized pole that literally follow the sun, improving solar efficiency by 40 – 45%.
• How many solar panels on the roof?
  Insofar as solar water heating, a shade-free area of 64 square feet is typical to house two 4′ x 8′ solar thermal panels (sized to provide a family of four with around 75% of their annual hot water load). Most solar electric (PV) panels measure 3′ 6″ x 5′ 6″. So, a roof that is 30′ wide by 14′ from gutter to ridge will fit two rows of 9 panels. An experienced solar designer will get precise roof measurements and ensure the panels are placed in the most space-efficient and visually appealing design.
• Choosing a solar energy installer
  Of course, the author of this blog would like to gleefully exclaim, “Choose Solar Energy Services!”. However, most educated consumers know that the best way of finding the best fit for an installer comes from shopping around for the popular 3 proposals/estimates. There are a lot of solar installers our there in today’s market. Some considerations:

 

Time in the Business: This matters. Obviously the more systems a licensed installer has installed – the more experience they’ve gained – and the more they’ve refined their installation technique. Just as important, however, is the question of whether the installer will be around in 5, 10 or 15 yrs when perhaps the incentives for solar have declined. An older company is likely to have deeper roots in the industry regardless of current solar trends and incentives. You want your installer to be around for the life of your system, 25 – 35 yrs plus. This narrows the field considerably.

Service Department: Solar Water Heaters require a 3 – 5 yr simple service visit. Does the installer provide this service?

All Options on the Table: A company who offers both leasing AND upfront purchase will likely lay ALL your options out on the table. This also goes for installers who offer both solar water heating AND solar electric.

There should not be a charge associated with a site evaluation of your home for solar energy. This is a necessary part of the sales process and should take place before the cost estimate/proposal is presented.

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“If my collector is at 136 degrees, my TST (bottom of tank temperature) is at 161 degrees and my S3 (top of the tank temperature) is at 154 degrees – why is my circulator pump still running? Won’t this cool the bottom of the tank?”

SES says: The pump runs intermittently for a minute periodically when the collectors reach 240 degrees. This keeps the collectors from overheating. Your maximum tank temperature is set to 160. If the bottom sensor, the TST reading, exceeds 160 degrees, the controller will run the pump briefly in the evening to bring the tank temperature back down to 160.

It has been our experience that setting the max temp higher than 160 leads to overheating problems. Yes, you are losing a little heat from the tank when the cooling feature comes on, but at this time of year it is excess heat. You are producing much more hot water than you are using at this time. This is the summer solstice; the collector and tank temperatures will moderate in a few months. Some customers add an extra loop to their solar tank and use that excess heat to heat a swimming pool or hot tub. Most just use control features to manage the excess heat.

Don’t worry about the power consumption. The Grundfos Alpha pump only uses about 5 Watts whether heating or cooling the tank.

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Crosstown Properties, LLC – Multifamily Solar Water Heating (Completed July 2010) – Washington DC

SES was engaged by a third party solar project developer to install a 32 panel (1280 SF) solar water heating system for an occupied apartment building in Washington D.C. The system included the installation and integration of 1600 gallons of additional solar storage in non-pressurized tanks. The system is designed to offset approximately 70% of the water heating load for this 45 unit apartment building. The solar developer provided all of the capital to install the system at no cost to the building owner. The building owner receives clean energy at a discount to their traditional natural gas, achieving a monthly savings immediately upon commissioning of the system.

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