BECOMING YOUR OWN POWER COMPANY
Most renewable energy systems built in the last 5 years have been installed in homes where the bulk of the power used is 120 volt AC. In order to provide 120-volt AC power from PV panels and a bank of batteries you need a device called an inverter.
An inverter takes the energy that is stored in a battery as direct current (DC) and turns it into alternating current (AC). Modern inverters will take in the direct current at 12, 24 or 48 volts and put out alternating current at either 120 volts or 240 volts.
The advent of large, efficient inverters has turned renewable energy from a curiosity available to a few tinkerers to a viable option for homeowners.
Inverters are rated by the amount of watts of power they can provide continuously. For example, a 1500-watt inverter will provide 1500 watts of power all day long. However, it is also capable of providing as much as 3 times that much power over a short period. This is to allow for starting electric motors that typically draw a lot more current when they start up than when they are running.
Getting the right sized inverter for your installation is important. You need to calculate the maximum amount of power you will use at any given time and get an inverter that will provide at least 20% more than that. There are inverters as small as 100 watts output that plug into a car cigarette lighter and will power a laptop to ones as big as 4000 watts that will run a large home.
I always recommend purchasing the largest inverter you can afford for your system. Since the cost of the inverter will be from ¼ to 1/3 of your system cost you don't want to find yourself needing to get a larger inverter 3 or 4 years down the line when your needs increase.
Going through the process of calculating your loads will ensure that you don't get an inverter that is too small for your needs. But since inverters cannot be upgraded to a larger size easily or cheaply it makes sense to start with the biggest one you can afford.
Some inverters can be "stacked" or connected together to create either a higher wattage output or a higher voltage output. Trace inverters have had this function for a few years and it can come in handy in certain situations.
The earthship style home I wrote about in a previous column uses two small 120-volt Trace inverters connected together to produce 240-volts to operate a 240-volt deep well pump. Since the inverters only run the pump they do not need to have a large wattage output.
With nothing else in the house that needs 240-volt power it made sense to limit its application to the well pump and save having to buy two larger, more expensive inverters for the whole system in order to include power to the well. A single 4000-watt Trace inverter provides power to the house.
Inverters used to be considered a luxury in renewable energy homes. But modern inverters have efficiency ratings of over 90% and the cost per rated watt for even the best inverters has dropped from 3 dollars to around 70 cents, with some costing as little as 50 cents or less. This puts the price of a good quality 1500-watt inverter at around $750.
In addition to the decreased cost of the inverter itself it is much cheaper to wire a home with standard wiring than with the heavier gauge wire required for DC systems. Also fixtures and appliances for DC systems are limited and expensive. It is possible to save the cost of the inverter itself in the reduced cost of construction and the purchase of fixtures and appliances.
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