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How to select an Inverter / Solar Backup

Selecting an Inveter - Solar and Backup

How to select an inverter for a solar system. Covers sine wave, modified sine wave, grid tie, and backup power.

We carry many types, sizes, brands, and models of inverters. Various options are also available. Choosing which one is best from such a long list can be a chore. There is no "best" inverter for all purposes - what might be great for an ambulance would not be suited for an RV. Power output is usually the main factor, but there are many others.

There are many factors that go into selecting the best inverter (and options) for your application, especially when you get into the higher power ranges (800 watts or more). This page should give you the information you need to get your selection down to what will work best for you.


Some Basics First...

Watts

The poor watt is often misunderstood. Watts are basically just a measure of how much power a device uses when turned on, or can supply. A watt is a watt - there is no such thing as "watts per hour", or "watts per day". If a something uses 100 watts, that is simply the voltage times the amps. If it draws 10 amps at 12 volts, or 1 amp at 120 volts, it is still 120 watts. A watt is defined as one Joule per second, so saying watts per hour is like saying "miles per hour per day".

Watt-hours

A watt-hour (or kilowatt hour, kWh) is simply how many watts times how many hours that is used for. This is what most people mean when they say "watts per day". If a light uses 100 watts, and it is on for 9 hours, that is 900 watt-hours. If a microwave uses 1500 watts, and runs for 10 minutes, that is 1/6th of an hour x 1500, or 250 WH. When you buy power from your friendly utility (look at your last bill), they sell it to you at so much per kWh. A kWh is a "kilowatt hour", or 1000 watts for one hour (or 1 watt for 1000 hours).

Amps

An amp is a measure of electrical current at the moment. (Amps do not come in "amps per hour" or "amps per day" either). Amps are important because it determines what wire size you need, especially on the DC (low voltage) side of an inverter. All wire has resistance, and amps flowing through a wire makes heat. If your wire is too small for the amps, you get hot wires. You can also get voltage drops in the wire if it is too small. This is not usually a good thing. An amp is defined as 1 Coulomb per second.

A Coulomb is the charge of 6.24 x 1018 electrons. Therefore, 1 Amp is equal to the charge of 6.24 x 1018 electrons passing a point in a circuit in 1 second.

 

Amp-Hours

Amp-hours (usually abbreviated as AH) are what most people mean when they say "amps per hour" etc. Amps x time = AH. AH are very important, as it is the main measure of battery capacity. Since most inverters run from batteries, the AH capacity determines how long you can run. See our battery page for much more detailed information.


Watts - Or What Size do I Need?

Peak Power vs Typical or Average

An inverter needs to supply two needs - Peak, or surge power, and the typical or usual power.

  • Surge is the maximum power that the inverter can supply, usually for only a short time - a few seconds up to 15 minutes or so. Some appliances, particularly those with electric motors, need a much higher startup surge than they do when running. Pumps are the most common example - another common one is refrigerators (compressors).
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  • Typical is what the inverter has to supply on a steady basis. This is the continuous rating. This is usually much lower than the surge. For example, this would be what a refrigerator pulls after the first few seconds it takes for the motor to start up, or what it takes to run the microwave - or what all loads combined will total up to. (see our note about appliance power and/or name tag ratings at the end of this section).
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  • Average power would usually be much less than typical or surge, and is not usually a factor in choosing an inverter. If you run a pump for 20 minutes and a small TV for 20 minutes during a one hour period, the average might be only 300 watts, even though the pump requires 2000. Average power is only useful in estimating battery capacity needed. Inverters must be sized for the maximum peak load, and for the typical continuous load.

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