What do you want your system to provide? back up power if grid fails.
solar Inverters for off-grid battery-based systems are designed for stand-alone power systems that can operate independently of the electric grid. Many of these inverters have AC inputs for grid power as well; so, if grid power is available to the site, it can be used for back-up power and emergency battery charging the way one would use a generator. In this way, an "off-grid" inverter can have the option of grid back-up. This is different than what is called grid-tie. Grid-tie systems can be of two types: batteryless and battery-based.
Batteryless grid-tie systems don't use batteries to store electricity, instead they use the electric grid for that purpose: electricity is drawn (bought) from the grid when energy use is higher than system production, and electricity is fed (sold) to the grid when more energy is produced than is used. The biggest downside to a batteryless grid-tie system is that when the grid has a blackout, so do you. Batteryless grid-tie inverters require a signal voltage from the grid; without it they shut down. The only way to gain true independence from the grid is with a battery-based system.
As utility companies become more accepting of grid-tied RE systems with buyback or net metering capabilities, they are becoming more popular. Having a batteryless system not only does away with the considerable expense of buying and periodically replacing batteries; also avoided are the hassles of maintenance, as well as the need to monitor the batteries to make sure that the energy demands placed on them balance the energy production, lest they become too drained. Since excess demand will simply be handled by the grid, the result is a very self-regulating system that needs little attention.
The biggest drawback to a batteryless grid-tie system is the fact that the system will shut down when the grid goes down and will not provide back-up power during a power outage. This is done in order to protect utility workers who will be trying to restore grid power.
Battery-based grid-tie systems combine the best of both worlds: surplus power can be sold to the utility company, and power can be bought from the utility's electric grid when needed. Plus, if grid power fails, the system does not shut down with it. Programming is still being developed to keep up with all the choices possible with this arrangement. When to sell, when to charge the battery, etc.
Price is obviously a big consideration. Buying too much inverter is a waste of money: getting an inverter with more power or more features than needed, or getting a pure-sine wave inverter when modified-sine wave power would run all your loads perfectly well. But not buying enough inverter can also be a waste: getting a cheapo that will be a door stop in a year, a quality one that is smaller than you need or might soon need, or a mod-sine to power sensitive electronics. The very first step in selecting an inverter is to look critically at the loads it will need to run. The cost of your inverter, and of your power system as a whole, is directly related to the amount of electricity you use.
When you size the solar inverter, you must first figure out and add together the wattages of all the continuous loads that will likely run simultaneously, in a highest electrical use scenario. (Relax, absolute precision isn't important; it's best to overestimate a bit). These include lights, home entertainment, computer, microwave, etc. (To help you with this, see the Solar Sizer for a more complete listing of household loads.) This total must be the same or less than the continuous wattage rating of the inverter you buy.
LIST OF COMMON SURGE LOADS:
Air Compressor (15A)-- 1100-9000 W
Circular Saw (not worm drive)-- 2000- 4000W
Refrigerator (Energy Star)-- 400 W
Table Saw (1.5hp)-- 2000 -7000w
Vacuum Cleaner-- 1200 W
Washing Machine--700- 3000w
Well Pump (1hp)--1000-3000w
(These are average surges, actual wattages may vary.)
Next, to determine the surge capacity of the inverter you need, add up the surge watts of all the loads that might start at the same time. Then add this total to the continuous wattage total you calculated above. Think of the water pump and the washing machine turning on at the same time as the refrigerator compressor (three surge loads) while you microwave dinner with the TV and half the lights in the house turned on (continuous loads).
If your solar inverter is undersized, a combination of loads such as this will trip the inverter's output breaker; this is known as nuisance tripping, and it truly is a nuisance.
A solar inverter sized by these minimum guidelines will dip its voltage during the starting surge. This is not harmful, but it will cause lights to dim. Fluorescents may blink off, and desktop computers may crash. To eliminate voltage dips, oversize the inverter by an additional 50% minimum plus the watts capacity required to handle other household loads at the same time.
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