Electrical Fundamentals and Principals Behind Solar Designs

Electrical Fundamentals (Volts, Amps & Watts)

Electricity and plumbing have a lot in common.  Instead of flowing water, electricity deals with flowing electrons.  Instead of pipes, electrons flow through wires.  Instead of reservoirs, electricity can be stored in batteries.

To further visualize these similarities consider the diagram below showing a water tank (or a battery.)

At the bottom of a full tank of water there will be a certain pressure caused by the weight of the water that would push water out of the tank of an opening were to form.  In a fully charged battery there will be certain voltage (similar to pressure) caused by a chemical process that would cause electrons to flow if a path (or circuit) was created.

When an opening is formed at the bottom of a tank, the water flows out at a certain rate depending on the resistance of the pipes and pressure behind the water.  Similarly, if an electrical circuit is formed, electrons will flow at a certain rate depending on the voltage and resistance of the load.  The flow rate of electrons is referred to as current also known as Amps, A, or in some cases the letter “I”.  The resistance of the load is expressed in Ohms or with the symbol “Ω”.

Energy is the flow rate times the pressure, or volts times amps.  Energy is expressed in units of Watts.
Useful formulas:

Volts = Amps x Ohms,  V=IR
Watts = Amps x Volts, W=AV, W=RI2
Kilowatts = 1000 x Watts

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POWER VS. ENERGY (KWH VS. WATTS)

It is very common for people to confuse Watt hours (Wh) and Watts (W).  The “h” isn’t just a longhand way of expressing Watts, it is very significant.  A Watt hour is a unit of energy and a Watt is a unit of power.  Watt hours equals Watts times Time.

For example, a 65W light bulb has an instantaneous electrical consumption of 65W of energy.  If you run that light bulb for 2 hours it will have consumed 130Wh of power (2h x 65W = 130Wh).

Another way to highlight differences between Watts and Watt hours is to compare them to Speed vs. Distance.  Watts are to Speed as Distance is to Watt hours.  If someone drove down a highway at a constant speed of 65mph for 2 hours they will have travelled 130 miles.

In terms of calculus, if you were to plot Watts over Time the area under the curve would be the Watt Hours.

In a solar system, the number of panels is proportional to the kWh used over a given period (so is your electric bill).  The inverter in an off-grid system is sized based on the maximum instantaneous Wattage needed by the user.

Series and Parallel Connections

Solar panels and batteries are often connected in parallel, series or combinations of the two to achieve a desired voltage or current.

If both series and parallel will work, series is preferred because line losses will be less and parallel connections can be expensive.

Terminology:
Strings – Refers to the number of panels in series.  If someone were to describe an array configuration as having three strings of twelve modules, they are saying that the system has three parallel strings of twelve panels in series.

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HOW SOLAR WORKS

A thorough explanation of how solar works involves some very complicated physics.

Light, which consists of photons, strikes the silicon atoms on a solar cell.  When this happens, energy is added to the atom which causes the electrons orbiting the nucleus of that atom to move into higher energy shells (orbits).  Solar cells are made of two types of silicon layered one on top of the other.  One layer has a higher electron density than the other.  Because of this layering, electrons from the top layer can be captured by the bottom layer when they are excited into higher energy shells.  Another feature of the layering is that the captured electrons cannot move back to the layer they originally came from.  This surplus of electrons on one layer and deficit on the other creates a voltage (force) of electrons being attracted back to the top layer.  If wires are attached to each layer (one positive and one negative), connected to devices that run on electricity, completing a circuit, the electrons will flow down those wires, power electrical devices along the way and return to the top silicon layer.  This process repeats continually as long as photons strike the silicon atoms.

Calculating Solar Production

Converting the Watts of a solar array to actual kWh that reduces your power bill requires a computer simulation that takes into account latitude and climate factors that would control how much sun exposure your panels get.
http://pvwatts.nrel.gov/

AC vs. DC, 120V, 240V, Single Phase vs. Three Phase

Electrical current is transmitted in two main ways, Direct Current DC and Alternating Current AC.  Direct Current is when the current maintains a relatively constant flow without changing directions.  DC current is produced by solar panels and stored in batteries.

Alternating Current changes directions in a sinusoidal pattern several times per second.  In the United States, the frequency is 60hz, meaning that it goes from positive to negative 60 times per second.  In Europe and many other countries the frequency is 50hz.  Alternating Current is what the utility grid distributes and what inverters produce from DC sources.

When transmitting current over long distances Alternating Current is preferred because of the more dramatic voltage losses that take place when using Direct Current.  The reason for this is that with DC, each electron has to travel from the source to the destination.  AC on the other hand, is just electrons vibrating into each other.

The standard voltage of AC power coming out of an electrical outlet is 120V, but when it is transmitted over the grid it is at a much higher voltage, at least 240V depending the particular section of the grid.  The higher voltage is preferred because it reduces line loses.

In single phase AC there are two conductors and an optional ground.  These two conductors oscillate opposite of each other.  Since one full oscillation is 360 degrees, the two conductors would be 180 degrees out of phase.  In three phase power, there are three conductors and an optional ground.  Each conductor is 120 degrees out of phase from the other two conductors.  Three phase is commonly used for large electric motors.

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The Equipment and Engineering Behind Our Custom Systems

Aside from our portable products and accessories, Grape Solar has three main categories of custom designed systems. These categories include:

Grid-Tied systems feed their output directly to the utility grid through an inverter. The power produced by these systems does not power someone’s home or business, but rather feeds the grid, which reduces net consumption, resulting in a lower utility bill. Grid-tied systems do not utilize any form of power storage or energy buffer (i.e. batteries) and only consist of panels, an inverter system, mounting and possibly combiner boxes for cable management. Grid-tied systems usually produce the best return on investment and are therefore the most common multiple panel systems. The main weakness of Grid-Tied systems is that they cannot function without a utility grid being present and will not provide power during a grid outage

Off-Grid systems can be anything from a panel on an RV to a small solar generator for a gate opener, to a standalone residential back-up system. Off-grid systems do not feed power to the utility grid and therefore must be designed very carefully so that their output matches the electrical consumption. In order to design a cost effective and functional off-grid system, Grape Solar engineers would need to know the wattage of each device that is to be used on the system and the average runtime of each device over a given period of time. One of the biggest challenges in designing off-grid systems is sizing the inverter to handle the startup current of the various loads. Off-grid systems typically cost 150% to 200% per Watt more than grid-tied systems because they require batteries, charge controllers and more complex inverters.

Grid-Interactive systems take the power storage feature of the off-grid systems and combine it with the grid-feeding ability of grid-tied systems. These are most commonly used as back-up systems for critical appliances in areas where utility outages are frequent. A grid-interactive system functions the same way that an off-grid system does, except that the inverter draws power from the grid to charge the batteries when they are low and feeds power to the grid when the batteries are full. When the batteries are full and the grid is active, the system feeds its output to the utility grid. When the grid is down, the system draws from the batteries, which are being charged by the panels, to supply power to a critical load on a home or business. Grid-interactive systems have a cost comparable to off-grid systems, about 150% to 200% that of strictly grid-tied systems.

Each type of system above uses a variety of components. By talking with each customer we are able to assess power consumption goals, budget expectations, space requirements and the likelihood for future expansion. With this information, we select components like putting together a puzzle, choosing the pieces that combine to form a system that best meets the customer’s goals. These components include:

Panels

Solar panels come in a variety of types, voltages and sizes.

Monocrystalline (mono) cells are made from an ingot with a uniform crystalline structure. These cells are dark blue, almost black and have beveled corners, giving them an octagonal shape. Polycrystalline (poly) cells are made from an ingot with an irregular crystalline structure which gives them a more bluish tint. Poly cells do not have the beveled corners and are rectangular in shape. In the early days of solar there was a large difference in performance and price between mono and poly, with mono being more efficient (generating more watts per square foot) and more expensive. But, recent advancements in manufacturing have reduced mono manufacturing costs and increased poly efficiencies, making them nearly identical in price and performance. A more efficient panel is not necessarily a better or longer lasting panel, it just means that it has a smaller surface area than a less efficient panel of the same wattage.

The voltages of panels vary depending on the number of cells (or number of pieces of cells) in a panel. Panels with an open circuit voltage of between 16Voc and 25Voc are ideal for small off-grid applications where a low cost PWM style charge controller is preferred. Higher voltage panels can only be used in grid-tied applications, or in conjunction with MPPT charge controllers that have transformer circuitry to bring their voltage down to what can safely be fed onto a battery bank.

Sometimes we get requests for “the biggest panel” we have, because they want to produce a large amount of power. Typically, multiple panels would be a better way of producing large amounts of power because they can be produced, shipped and installed easier. Panel size is typically limited by inverter or charge controller specifications and space limitations.

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Incentives, Rebates and Credits

Incentives, rebates and tax credits can come from the federal government, the state government, the utility company and even the city. This means that each town may have a different program. To add to the complexity, these programs are constantly changing. Because of this, it doesn’t make sense for Grape Solar, a company with a customer base spanning continents, to become experts on the various programs. If you need information on a specific area we can connect you with one of our expert local installers. In the meantime, this website has comprehensive information for the United States:

http://dsireusa.org/

The IV Curve

A graph of solar panel current vs. voltage is referred to as the IV Curve.  The optimum combination of voltage and current for producing the most energy from a solar panel is at the point along the curve where the slope equals -1A/V.  The current and voltage at this point are referred to as Impp and Vmpp respectively.  The “mmp” stands for Maximum Power Point.

When a solar panel gets warmer the curve shifts to the left, producing less voltage.

When a solar panel gets exposed to more light the curve shifts upward producing more current.

Mounting

Solar panels can be mounted in a variety ways and Grape Solar can provide several options depending on the particular installation. For customers in the Northern hemisphere with fixed tilt racking, we recommend pointing the array to the South at a tilt angle about 5 degrees less than the latitude of the installation.

Roof mounted solar is the most common. In designing a roof mounted system we would need to know the type of roofing material, such as: Asphalt Shingle, Flat Tile, Curved Tile, Corrugated Metal, Standing Seam Metal, Shake, etc. For flat roofs we offer ballasted and penetrating racking options. In situations where extra tilt is required, we can supply tilt legs. Racking systems are designed to last 10 years and not harm a roof or cause leaks. Typically Quickmount provides our roof attachments, and rails and panel clips are provided by Haticon. For flat roofs we use either UniRac Rapid-Rac or PanelClaw.

Ground mount arrays are designed with four and sometimes five rows of panels in landscape orientation. Ground mount arrays are slightly more expensive than roof mount arrays because of the support structure required. Our most common types of ground mount racking include UniRac and IronRidge.

Pole mounting is the simplest way to mount solar panels and ideal for small projects like gate openers, lighting and electric fences. Pole mounted systems are best for just a couple of panels and get very expensive when large numbers of panels are added to the array. We typically recommend DPW or IronRidge for pole mounted systems.

Vehicle, like RV or Boat mounting, is very popular with our small off-grid kits. Depending on the budget, vehicle mounting can be accomplished with do it yourself kits utilizing 3M double sided tape, “Z” feet, or the deluxe adjustable tilt systems designed by AM Solar.

Do It Yourself racking is a very viable option for small off-grid systems. Our panels have “C” channel aluminum frames with mounting holes. Pressure treated wood, angle iron or “C” channel can be used to construct a panel mounting system. For design inspiration do an internet search for “solar panel mounts”.

Tracking and Adjustable Tilt racking were much more popular when solar panels were at about twice the price that they currently are. For example, when a tracking system may add \$1.50/W to the cost of an installation and increase performance by 20% and that same \$1.50/W could just be used to buy more panels and increase performance by 100%, tracking becomes less attractive. The cost of these systems and the maintenance required to keep them functional makes them practical only in very rare situations where space is limited. Similar to tracking, Adjustable Tilt Racking does not produce enough extra power to justify the added costs. Grape Solar does not currently offer any tracking or adjustable tilt racking solutions.

System Design

System Design: Charge Controllers

Charge controllers go between panels and batteries. Their purpose is to take power from panels and feed it to batteries without over charging the batteries. When selecting a charge controller or determining what panels to put on a charge controller there are four rules that should be followed.

1.   Make sure the voltage of the solar panel array going into the charge controller is at least a couple volts higher than the battery bank voltage over a wide range of temperatures and charge levels. For example, a 12.0V panel (if there was such a thing) would not charge a 12V battery because there would no “pressure” differential that would cause current to flow. The voltage of the panel would have to be at least 14.0V to charge a 12V battery bank. Panels that are commonly referred to as “12 volt” panels typically have an actual voltage output of 17-22 volts.

2.   Make sure the open circuit voltage of the solar array is not above the voltage limit of the charge controller. For example some Sunforce charge controllers have a voltage limit of 25V. Because of this, you would not be able to use our 250W panel because it has an open circuit voltage of 37.7V. Also, Outback charge controllers have a voltage limit of 150V. Because of this you would not want to put more than three250W panels in each series string because on a cold day (panel voltage increases as temperature decreases) their combined voltage would exceed 150V.

3.   Make sure the current going from the charge controller to the battery bank does not exceed the charge controller current rating. The calculations for current have nothing to do with the current coming off the panel array, since current coming into a charge controller and going out of a charge controller are not necessarily equal. To calculate current, take the total wattage of the solar array, multiply it 90% to take into account losses and real world solar irradiance, then divide it by the voltage of the battery bank. For example, two 250W panels on a 12V battery bank would produce about 37.5A (2 x 250W x 0.90 / 12V = 37.5A). This would not work on a charge controller rated for 15A.

4.   Use MPPT when there is a large differential between the panel voltage and the battery bank voltage. There are two types of charge controllers, PWM and MPPT. PWM charge controllers feed the batteries with a direct connection to the solar panels. MPPT charge controllers utilize a transformer to step down the voltage and increase the current before being fed onto a battery bank. For example, even though the Xantrex C35 charge controller can handle the voltage of a 250W panel and is compatible with 12V systems, it uses PWM and would force the panel to operate at 12V instead of its more efficient 30.7V. An MPPT charge controller would be a better choice and result in much better production.

System Design: Inverters

The inverter is the component that converts DC power into the AC that runs most electronics. The two main types of inverter we deal with are Grid-tied and Off-grid. Grid-tied inverters take power directly from the solar array and feed it onto the utility grid. Off-grid inverters take power from a battery bank and power devices.

Grid-tied inverters are selected based on the wattage of the solar array. The panels are organized in combinations of series and parallel to match the voltage and current handling characteristics of the inverters. For example a system consisting of twenty 250W panels for a total of 5000W would use a KACO 5002xi inverter. If all the panels were in series, the voltage would be too high. If all the panels were in parallel, the voltage would be too low. By using string sizing software we can determine that there should be two strings of ten panels to operate efficiently with the inverter.

Another type of grid-tied inverter is the micro inverter. This is when each panel has its own inverter and inverters are connected to each other in parallel. For these inverters, only certain panels can be used that match the voltage and current requirements of the inverter. Since grid-tied inverters only feed power to the grid and not to certain devices, the size of your load does not matter in system design.

Off-grid inverters draw their power from the battery bank, therefore it doesn’t matter how many or what type of panels are used in the system. Off-grid systems are designed around the total wattage of all the devices they would power. For example, if you have a 1200W microwave and 800W of lights, you wouldn’t want to use an inverter with an output of less than 2000W.

Off-grid inverters come in two main varieties - modified sine and pure sine. Grid power (what comes out of a typical electrical outlet) is pure sine, meaning that its oscillations from negative to positive happen in smooth arcs. Modified sine, on the other hand is a square wave pattern. Modified sine inverters are cheaper, but may affect long term life for motors or compressors, are not compatible with LED lighting, and may not function with all types of chargers or complex circuits. Inverters also have to be sized to match the load of the system. Sometimes pumps or other electrical motors have a startup current that is up to nine times that of their steady state current, and systems have to be designed carefully to take this into account. Some of our small off-grid kits use Xantrex modified sine inverters. For pure sine inverters we use Xantrex or Outback. Outback also makes the pure sine inverters for our grid-interactive kits.

System Design: Battery Banks

A battery serves as an energy buffer between the power producing panels and the power consuming load. Since grid-tied systems feed directly to the grid and do not provide backup power, no battery is needed. But, off-grid and grid-interactive systems do require batteries. A common question is “Do I really need batteries if I only want to work during the day?” The answer, 99% of the time is yes. In some rare cases special DC pumps can be connected directly to panels, so can lighting. The reason for needing batteries is that without them the current being produced would have to exactly match the current being consumed, which is not possible with small scale conventional solar power systems.

As a general rule of thumb, we recommend that the Ah rating of a battery bank should be at least half of the panel wattage rating. For example, a 200W solar kit should have a 100Ah battery bank. On an average day, a 200W system will take a 100Ah battery from emptyto full. If a smaller battery bank was used, the battery would reach full charge and the panels would be disconnected from the battery by the charge controller before sunset, leaving expensive panels doing nothing.

A bigger battery bank is always better. Bigger battery banks provide more backup time and the lifespan of the batteries is also improved. A system with a large battery bank would likely be less deeply discharged than a system with a small battery bank. If a battery is only discharged 20% each day it will last longer than a battery discharged 50% each day.

To convert battery Amp hours (Ah) to usable Watt hours (Wh) take the nominal voltage of the battery, multiply it by the Ah rating, and multiply that by 50%. For example, a 100Ah 12V battery has a usable capacity of 600Wh (12V x 100Ah x 50% = 600Wh.)

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Combiners, Cables, and Connectors

Combiner boxes, cables and connectors are included in some of our kits. Each Grape Solar panel comes with two leads, approximately 3’ foot long each, one for positive and one for negative. To connect panels in series (positive to negative) the leads built into each panel will be long enough if the panels are next to each other. Each lead has a durable, weather resistant MC4 connector on it. We advise that customers do not remove these connectors because that will void their warranty and potentially shorten the life the panel. To connect to our panels we recommend using MC4 cables that can be purchased through our Accessories Page (linked). A single MC4 cable can be used for both the positive and negative leads by cutting the cable in half. The ends with connectors go to the panel and the bare ends can be stripped and sent directly to the combiner box or charge controller, or can have longer cables spliced onto them.

When making a parallel connection it is best to use an MC4 T-branch or a combiner box. A parallel connection is when all the positive leads are grouped together, separate from all the negative leads which are also grouped together. For higher current systems, or systems with more than five parallel connections, a combiner is recommended.

Fuses, Breakers, and Grounding

Some of our kits come with fuses, breakers and grounding hardware. Fuses are typically used within the combiner boxes of grid-tied systems between the panels and the inverters because breakers aren’t designed to handle the higher voltages that would be present in these types of systems. In off-grid systems breakers can be used in the same manner. Fuses are also used occasionally between charge controllers and battery banks to prevent the possibility of a current overload that could damage the charge controller. On our smaller off-grid kits, fuses and breakers are not required due to the relatively low power and low likelihood of a surge, but customers may add them if they wish. Most string inverters and large off-grid or grid-interactive inverters come with built in breakers.

The mounting kits for Grid-tied systems come with grounding hardware as per code requirements. The smaller kits have grounding instructions included. Panels, mounting rails, charge controllers and batteries can all be grounded with 10AWG cable connected to a metal stake 2’ into the ground. None of our panels require positive grounding. We recommend that inverters be grounded as per the manufacturer's instructions.