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
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