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Inverters for Stand Alone Solar Powered Systems

When you decide that you want to power your entire home from solar power one of the decisions that installers make is the size of the inverter this is the one of the most important pieces in the puzzle and its pretty important to know how to size it.

The inverter is the instrument that converts Direct Current (DC) to Alternating Current (AC) the efficiency of this instrument effects the sizing of the battery pack, the solar array and even the amount of load (the amount of electricity required by an appliance) you are able to turn on at any given time.

There are several inverters out there in the market today and of several sizes. Inverters usually come with a 1/2 hour rating and a surge rating, both will be in VoltAmps(VA).

Typically what you will see is, that the 1/2 hour rating is the size of the expected load measured in VA as well which is the power that the appliance requires with a ‘power factor’ adjustment (remember that Watts and VA are similar but the latter takes into account the real and non-real part of power). There is usually a 10% safety factor on the load and a correction factor for efficiency of the inverter.

The surge factor is just the ability of the inverter to handle multiple appliances being turned on at the same time. Simply put surge is the initial amount of power that, for example, a washing machine needs to make the motor start spinning. These can initially, for a couple of seconds, multiply the amount of electricity consumed by a factor of 6! Usually the surge is approximately 2.5-3 times the 1/2 rating and this is to account for that reason mentioned previously.

The battery is the instrument that is usually sized on the watt-hours of demand that is needed and is also corrected for inverter efficiency because batteries store charge and discharge current in the form of DC and not AC. The inverter in some cases can have the ability to charge the battery as well then another efficiency factor needs to be taken into account.

With respect to solar panels they also produce power in the DC form and thus need to adhere to the efficiency of the inverter when sizing and chosing the correct inverter can effect weather you need one or two more panels!

Nonetheless, most of the design decisions are made in unison with all the components of a stand alone power system but the inverter plays an important role because of the nature of the electricity being generated (in DC) and the appliances that we use (in AC) today.

Written by Prateek Chourdia

MEngSc – Photovoltaics and Solar Energy, UNSW

Solar Energy Analyst

Solar Choice

© 2010 Solar Choice Pty Ltd


Comments

  1. Hi,
    I may be on the wrong page here but I have a few questions relating to solar power generated for home. If I install 12 panels which are 180 watt this equals 2.16 kw & I have a 2kw inverter, is that ok? Also how does the elcrticity the panels will produce during the day (sunlight hours) affect the power I use at the same time? Guessing that my usage is lower than my production at peak sunlight, does that mean, at that time, there will be a net feed in to the grid even though I am taking electricity from the grid, at that time? Sorry if my questions are simple or repetetive but I am a complete novice at this caper. If I am writing to the wrong area could you please forward this to the correct person for help please?
    Many thanks,
    Alan.

    1. Hi Alan,

      I assume your system will be grid-connected?

      Whether a 2.16kW solar array will work with a 2kW inverter would depend on the inverter you have, but in most cases that small disparity shouldn’t be a problem. (Read about inverter efficiency curves.)

      As for the timing of solar power production and use, with a grid-connected system, if you are using electricity when your panels ar producing it, it will go to your appliances/lighting first. Only the excess will be exported to the grid. This is what you get paid for if you are on a solar feed-in tariff scheme. If you are not on a feed-in tariff scheme (e.g. if you are in NSW and signing up after the closure of the Solar Bonus Scheme), you are best off to time your electricity usage to match when your panels are producing power.

      Hope you found this helpful.

  2. we are about to tidy up our website and get it more professional looking, would it be okay to include this article you have written on one of our pages. Its well written and informative.
    Thanks
    Darrell Wilson

    1. Hello Darrell,
      Thanks for your interest and comment. We would be happy to have you summarise our article and create a link to our page on your blog, but please do not copy the article wholesale and post it. Thanks!

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