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The Role of an Inverter in a Solar Electric System

Everything you wanted to know about inverters

The inverter is one of the most important and most complex components of an independent system. Luckily, you don't have to understand the inner workings of an inverter, but you should understand some basic functions, capabilities and limitations.

This seminar gives you the basic information so that you can choose the right invert and use it wisely.

Why you need an inverter

An independent electric power system is one that is untethered from the electrical utility grid. Such systems vary in size from tiny yard lights to remote site homes, village, national park, medical and military facilities. They also include mobile, portable, and emergency backup systems. Their common bond is the storage battery, which absorbs and releases power in the form of direct current (DC). In contrast, the utility grid supplies consumers with alternating current (AC) power. AC is the standard form of electricity for anything that "plugs in" to utility power (it is more practical for long distance transmission).

The inverter converts DC to AC, and also changes the voltage. In other words, it is a power adapter. It can allow a battery-based independent power system to run conventional appliances through conventional home wiring. There are many ways to use DC power directly, but if your electrical needs are beyond the simplest "cabin" level, you will need an inverter for many, if not all of your loads (devices that use power).

DC flows in a single direction. AC alternates its direction many times per second. The standard DC voltages for home- size systems are 12, 24 and 48 volts. The standard for AC utility service in USA is 120 and 240 volts at 60 Hertz (cycles per second). In Europe and some countries in Latin America, Asia and Africa, it's 220V at 50 Hertz. The inverter is used to reconcile these differences.
Not a simple device

Outwardly, an inverter looks like a box with one or two switches on it, but inside is a small universe of dynamic activity. A modern home inverter must cope with input voltage that varies as much as 35% (with varying battery state and activity), and also with huge variations in output demand (from a single night-light to a big surge required to start a well pump or a power tool). Through all, it must regulate its output quality within narrow constraints, with a minimum of power loss. This is no easy task. In addition, some inverters provide battery backup charging, and can even feed excess power into the grid.
Characteristics of Inverters

Here is a list of the important characteristics of inverters. These are the factors to consider when selecting an inverter.

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Application environment

Where is the inverter to be used ?

  • Home
  • Recreational vehicle
  • Marine
  • Portable
  • Emergency backup

Electrical standards

  • DC input voltage
  • AC output voltage and frequency

Power capacity (watts) (How much will it put out?)

  • Continuous rating
  • Limited duration ratings
  • Surge rating (for starting motors/pumps)
  • Expandability (modularity, stackability)

Power quality (waveform)

Some inverters produce "cleaner" power than others.

  • Sine wave inverters
    • Ideal, smoothly alternating AC (like swing of a pendulum)
    • Equivalent (or superior) to grid power relatively expensive
  • Modified sine wave inverters
    • Inferior waveform, choppy alternation (like pendulum forced by hammers)
    • Inexpensive
    • Adequate for many homes with simple needs, but about 5% of loads malfunction
    • May confuse digital timing devices in some appliances
    • May overheat power converters in some appliances/computers
    • May overheat surge protectors (don't use them) causes some devices to buzz (some fluorescent lights, ceiling fans, transformers)
    • Reduces energy efficiency of motors and transformers by 10% or more, causes motors and transformers to run hotter
    • Generally reduces the reliability of appliances

Inductive load capability

Some loads accept the AC wave with a slight time delay. These are call inductive loads. Motors are the most severely inductive loads.

  • Starting large motors (well pump, washing machine, power tools, etc.)

Physical attributes

There are two ways that inverters are built:

  • Transformer type inverters
    • Heavy, expensive
    • High surge capacity
    • Historically the most reliable
    • Makes buzzing noise
  • High frequency switching type inverters
    • Light weight, inexpensive
    • Less reliable in cases of cheap consumer units
    • No audible buzz

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It is not possible to convert power without losing some of it (think of "friction"). Efficiency is the ratio of power out to power in, expressed as a percent. If the efficiency is 90%, that means 10% of the power is lost in the inverter. Lost power manifests as heat. Efficiency of an inverter varies with the load. Typically, it will be highest at about 2/3 of the inverter's capacity. This is called its "peak efficiency". The inverter requires some power just to run itself, so the efficiency of a large inverter may be low when running very small loads.

In a typical home, there are many hours of the day when electrical load is very low. Under these conditions, an inverter's efficiency may be around 50% or far lower. The full story is told by a graph of efficiency vs. load, as published by the inverter manufacturer. This is called the "efficiency curve". Watch out. Some manufacturers cheat by drawing the curve only down to 100 watts or so, not down to zero!

Because the efficiency varies with load, don't assume that an inverter with 93% peak efficiency is better than one with 85% peak efficiency. The 85% efficient unit may be more efficient at low power levels, for example.

Automatic on/off

As stated above, an inverter takes some power just to run itself. This "idling" can be a substantial load on a small power system. Cheap portable inverters usually have a manual on/off switch. If you forget to turn the inverter off, you may surprised by a discharged battery bank after a few days. Most inverters made for home power systems have an automatic load-sensing system. The inverter puts out a brief pulse of power about every second (more or less). When you switch on an AC load, it senses the current draw and turns itself on. Manufacturers have various names for this feature, like "load demand", "sleep mode", "power saver", or "standby".

This feature can make life a bit awkward because a tiny load may not trigger the inverter to turn on. For example, you start your washing machine and after the first cycle, it pauses with only the timer running. The timer may draw less than 10 watts.

The inverter's turn-on "threshold" may be 10 or 15 watts. The inverter shuts off and doesn't come back on until it sees additional load from some other appliance. Some people solve this problem by leaving a small light on while running the washer.

Some system users cannot adapt to this situation. Therefore, inverters with automatic on/off also have an "always on" setting. That way, you can run your low- power night lights (they won't flash on/off) and your clocks and other tiny loads without losing continuity. A good system designer will then add the inverter's idle current into the load calculation (24 hours per day), and the cost of the power system will be correspondingly higher.

Battery charging features

Some inverters have a built-in battery charger that will recharge the battery bank whenever power is applied from an AC generator or from the utility grid (if the batteries are not already charged). This function is essential to most renewable energy systems because there are likely to be occasions when the energy supply is insufficient. It also makes an inverter into a complete emergency backup system for on-grid power needs (just add batteries).

Here is a list of specifications that relate to battery charger function:

* Maximum charging rate (amps)
* Generator size and voltage requirements
* Charge control features, including accommodation of different battery types (flooded or sealed), temperature compensation, and other refinements

Be careful when sizing a generator to meet the requirements of an inverter/charger. Some inverters require that the generator be oversized. Be sure to get experienced advice on this, or you may be disappointed by the result.

Expansion options

* Some inverters can be "stacked" to expand their capacity

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Laboratory Certification

Inverters should be certified by an independent testing laboratory such as UL, ETL, CSA, etc., and stamped accordingly. There are different design and rating standards for various applications, such as use in buildings, vehicles, boats, etc. These also vary from one nation to another. An inverter used for a home power system must be appropriately rated for the system to pass an electrical inspection.

Phantom loads

High tech consumers are stuck with gadgets that draw power all of the time that they are plugged in. These little demons are called "phantom loads" because their power draw is unexpected, unseen, and easily forgotten. An example is a TV with remote control. Its electric eye is on all the time, watching for your signal to turn the screen on. Other examples include any devices with an external wall-plug transformer or a built-in clock, plus smoke detectors, alarm systems, motion detector lights, fax machines, answering machines, and all cordless (rechargeable) appliances. Central heating systems have a transformer in their thermostat circuit that stays on all the time. How many phantom loads do you have?

There are several ways to cope with phantom loads. (1) You can avoid them (easy for a small cabin or other simple- living situation). (2) You can minimize their presence and disconnect them when not needed, using external switches. (3) You can work around them by modifying certain equipment to shut off completely. (4) You can substitute devices that use DC power instead of AC. (5) You can pay the additional cost for a large enough power system to handle the extra loads plus the inverter's idle current (often over $1000 added). Be very careful and honest when considering avoiding all phantom loads.

You cannot always anticipate future needs or human behavior. All it takes is one phantom load to mess up your perfect plan.

Powering a water well or pressure pump

At a remote site, a water supply pump is often the largest electrical load. It warrants special consideration for several reasons. (1) Most pumps draw a very high surge of current during startup. The inverter must have sufficient surge capacity to handle it while running any other loads that may be on. (2) Most pumps are used for automatic pressurizing. In that case, the pump will start unexpectedly, several times per day. (3) In North America, most pumps (especially submersibles) run on 230 volt power while smaller appliances and lights use the 115 volt standard. (4) AC water pumps are not very energy-efficient.

The power system (as well as the inverter) may need to be substantially larger to handle the load.

It is important to size an inverter sufficiently, especially to handle the starting surge. Oversize it still further if you want it to start the pump without causing lights to dim or blink. Ask us for help doing this because inverter manufacturers have not been supplying sufficient data for sizing in relation to pumps. To obtain 230 volts from a 115 volt inverter, either use two inverters "stacked" (if they are designed for that) or use a transformer to step up the voltage. (The pressure switch should be wired in before the transformer, so the transformer will not be a phantom load.)

As an alternative, you may consider using a DC powered pump. It will be completely independent from the inverter. Efficient DC pumps have been developed especially for renewable energy systems. They can pump water using 1/3 to 1/2 the energy of an AC pump. DC pumps are specialized and therefore more expensive than AC pumps, but an extra $1000 spent on a DC pump can save $2000 in total system cost.

Quality: You get what you pay for

A good inverter is reliable and able to handle a wide variety of loads without wasting lots of energy. It is well protected from surges from nearby lightning and static, and from surges that bounce back from motors under overload conditions. A good inverter is an industrial quality device that is proven and certified for safety, and can last for decades. A cheap inverter may soon end up in the junk pile, and can even be a fire hazard. Consider an inverter to be a foundation component. Buy a good one that allows for future expansion of your needs.

Seek professional help

Safe and effective system design is critical. Where multiple voltages and power sources are used, the electrical codes (National Electrical Code in USA) can be quite complex. It is best to seek professional help in the design stage. We hope this article has been useful in getting you started.

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