Off-grid Solar
Pure Sine Inverter

A pure sine inverter? Might sounds intimidating, a trigonometry term (sine wave) and electrical engineering terminology (inverter). However, it is an essential component in every off-grid power system.

The Important features of the Off Grid Inverter

What are the important features of a pure sine inverter?

• The purity of the pure sine wave, also known as true sine wave is measured by THD (Total Harmonics Distortion. It needs to be bellow than 5% (rings a bell? High Fidelity sound systems also have a distortion limit. With musical ear you can hear the higher harmonics of the 60Hz sound when the inverter operate)
• Versatile mounting options
• Comparatively light weight (don’t expect a weight your small child can lift though…)
• One (or more) communication ports (so a controller or a PC can “talk and listen" to the inverter)

The Main Specifications of the True Sine Wave Inverter

What are the main specifications for a pure sine wave inverter you want to look into?

• DC voltage rating and the tolerance; you want a nominal 12V (9-17VDC) inverter for 12V batteries, 24V for 24V batteries
• Rated maximum battery input current
• AC 120VAC, 60Hz (plus or minus 0.4%)for the US; most other countries, 230V 50Hz
• Power capability in Watts (AC side) to fit your home consumption
• Power Surge capability in Watts – to accommodate for source (batteries) voltage changes and load current changes (when heavy load such as a washing machines starts, a transition to a higher current consumption causes a transient that the inverter must cope with)
• Efficiency What percentage of DC power is actually converted to useful AC power? Look for numbers around 90% or exceeding 90%
• Over-current protection, over-temperature protection and other protections
• Heat removal method (usually a fan)
• AC and DC circuit breakers, remote control (dry contacts) is a desired feature

True Sine Wave Inverter Cost Considerations

Cost is expected to be $0.8-1 per Watt AC.
I recommend buying the pure sine inverter from a one stop reliable solar store with a good reputation together with the other parts of the solar kit. The shop will provide you with system analysis and a good installation tips. The store will provide you a reliable after sale support
(dont forget - it is a small scale power plant you are building)

Application for Off Grid Power systems

A pure sine wave inverter can be used with off grid solar, home wind turbine or hydro power systems not connected to electric grid. You may find a pure sine inverter with a built-in AC battery charger.

Technical Background

Some words on the pure sine inverter. When electrical power generation and distribution started by the end of the 19th century, two giants struggled about the right concept. Edison fought for Direct Current (DC), Tesla fought for Alternate Current (AC).Eventually, Tesla won this battle, and the utility grid is AC. The main reason for that is distribution efficiency. Power is Voltage (Volts) times Current (Ampers), to deliver the same Power, if the Voltage is x times higher the current is x time lower. The utilities High Voltage Power lines run thousands of miles on wires. Wires resist the flow of current, this “friction” ends up in warming the wires. This is waste of energy (the utility is in the business of selling energy to customers and not to heat up wires…). The heat is proportional to the Current squared therefore low current/high voltage is what is good for long distance electrical power distribution. AC can be readily transformed (using a transformer) from low voltage to high voltage or from high voltage to low voltage. DC can not be transformed that way.So, the utility generator generates AC electricity, having accurate sine wave shape, accurate frequency (60 Hz in the US) and pretty stable voltage. All residential appliances, electronic devices, lighting bulbs are built to work with the exact shape, frequency and voltage as specified and the utility is committed to supply compatible AC electricity.

Solar panels generate DC. A pure sine inverter is needed to convert the DC to AC compatible with the shape, frequency and voltage to the standards.
Here lies the difference between on grid tie inverter and offgrid solar inverter. The grid tie inverter “senses” the grid and the grid “discipline” it, so compatibility is “a built in feature”. The Off-grid solar inverter must have “a self discipline” to be compatible (the appliances, electronics, lighting bulbs “don’t know” they are connected to an off-grid solar inverter, for them “life is as usual")

Here comes the term pure sine wave. See bellow two pictures side by side, of “pure sine wave" (left side) and “modified sine wave” (right side)

The utility grid supplies "pure sine wave" - this is what the home appliances "like" and need to get. The main difference between the pure sine wave inverter and the modified sine wave inverter is in the wave shape.
To be honest, some appliances might work on modified sine wave supplied by what is called a modified sine wave inverter. The difference is mainly in the THD value. The price is a bit cheaper, however - it is an headache to keep two different inverters and to keep two separate sets of wall outlets. Keep it simple, install one good pure sine wave inverter. It is a bit more expensive than the modified sine wave inverter but it makes your life simpler and your home appliances will live longer.