1. What size?
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 (usually no longer than a second unless specified in the inverter’s specifications). Some appliances, particularly those with electric motors, need a much higher start up surge than they do when running. Pumps, compressors, air conditioners are the most common exampleanother common one is freezers and refrigerators (compressors). You want to select an inverter with a continuous rating that will handle the surge rating of your appliance so you don’t prematurely burn out the inverter. Don’t rely on the inverters surge to start your equipment because inverters don’t like to operate in their surge mode unless the manufacturer claims to have a longer surge time than normal.

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).
You can use the following formula to determine the size:
Volts * Amps = watts
or
Watts / Volts = amps
1250 Watt example:
1250 / 120 Vac = 10.41 amps ac (typical number found on equipment)
or
1250 / 12 Vdc = 104.1 amps dc (battery drain per hour)
Here is an example:
First, you need to determine what items you need to power during a power failure and for how long. Here is a brief example (watt requirements vary):
Lights  About 200 watts
Fridge  About 1000 watts
Radio  About 50 watts
Heater  About 1000 watts
Total wattage needed is 2250 watts. The fridge and heater have a start up power requirement so let's allow 2x the continuous wattage for start up requirements. 2250 * 2 = 4500 watts
Second, select an inverter. For this example, you will need a power inverter capable of handling 4500 watts. The continuous power requirement is actually 2250 but when sizing an inverter you have to plan for the start up so the inverter can handle it.
Third, you need to decide how long you want to run 2250 watts. Let's say you would like to power these items for an 8 hour period. Well this can be tricky because heaters and fridges run intermittently. Let's assume all of the appliances will run 40% of the 8 hr period which is 3.2 hours of actual run time. We need to convert the ac watts to dc amp hours because that's how batteries are rated.
To convert ac watts to dc amps per hour you divide the watts by the DC voltage (usually 12v or 24volts). Let's use 12volts since it is the most common.
2250 watts / 12 vdc = 187.50 dc amps per hour
187.50 is now your power requirement per hour
You have now determined that 187.50 is your power requirement per hour and now you need to multiply that by total hours of run time which is 3.2 in our example.
187.50 dc amps per hour 3.2 hours = 600 dc amps
Because you are using an inverter, you want to calculate the loss for converting the power which is usually around 5%.
(600 dc amps * 5%)+ 600 dc amps = 630 dc amps per hour (this is how much power you need in an 8 hour period running your appliances 40% of the time)
Fourth, now that you know your total power requirement is 630 dc amps we can select a battery source. Most typical deep cycle batteries are 6 volts or 12 volts. I will give you two examples using each voltage.
12 volt battery example:
If you select a 12 volt battery rated at 100 dc amps you will need 6 or 7 batteries in parallel (I will explain parallel vs. series later).
630 dc amps / 100 dc amp battery = 6.3 batteries
6 volt battery example:
If you select a 6 volt battery rated at 200 dc amps you will need 6 to 7 batteries in series.
3.15 * 2 = 6.3 batteries
No, I didn't make a mistake. When you use 6 volt batteries, you have to connect them in series to reach 12 volts.
What is series and parallel you ask?
When you connect batteries is parallel you are increasing amps. When you connect batteries in series you increase voltage. In the battery world, it is better to limit your parallel strings. It is better for your power system. In this example, I would recommend using 6 volt batteries because of the number of batteries this example requires.
How do we charge these batteries? You will need a charger to charge the batteries when you have access to city power. Most deep cycle batteries need a "smart" charger so the charger doesn't damage the batteries. In this example, you will need at least a 40 amp charger if not bigger. The bigger the charger, the faster the charge. Make sure your charger is for 12 volt batteries because the system we just identified is a 12 volt system.
You will also need cables. For this example, a 4 AWT (0000) cable is required to handle 4500 watts of start up power. That is huge cable. You may also want to consider an inline fuse. A 500 amp for this example is perfect. To figure out the size of fuse you divide your ac watts (start up) by dc voltage.
4500 watts / 12 vdc = 375 amps
You would need a 375 amp fuse or bigger. I recommend a 500 amp just incase you were to max out the 5000 watt inverter.
This is just a brief example. There are many different ways to set up your system. You can use solar panels, wind etc.
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2. Difference between modified, pure and digital pure sine wave inverters?
There are 4 major types of inverters  digital pure sine wave, pure sine wave (or "true" sine wave), modified sine wave (actually a modified square wave), and square wave.
Digital Pure Sine Wave
A digital pure sine wave is what you get from your local utility company and (usually) from a generator. The sine wave produced by a digital pure sine doesn't have the noise of an analog pure sine wave inverter.

major advantage of a digital pure sine wave inverter is that all of the equipment which is sold on the market is designed for a pure sine wave. This guarantees that the equipment will work to its full specifications.

Some appliances, such as motors and microwave ovens will only produce full output with digital pure sine wave power.

A few appliances, such as bread makers, light dimmers, and some battery chargers require a digital pure sine wave to work at all.

audio equipment, satellite systems, and video equipment, sound and look cleaner using digital pure sine wave inverters.

these are the most expensive of the inverter designs and outperform all other types of inverters, regardless of use.
Analog Pure Sine Wave
The sine wave produced by an analog pure sine wave inverter, is very similar to that of the digital pure sine wave inverter. The key difference is that the analog switching causes noise or static on the ac wave.

generally most appliances, motors, microwaves, chargers, and power tools will produce full power and not cause any buzzing or negative effects.

these types of pure sine inverters are not recommended for medical equipment unless manufacturer approved.

use this inverter for electric shavers and emergency flashlights, garage door openers, laser printers and large strobes used in photography
Modified Sine Wave (quasisine)
A modified sine wave inverter actually has a waveform more like a square wave, but with an extra step. A modified sine wave inverter will work fine with most equipment, although the efficiency or power of the equipment will be reduced with some.

Motors, such as refrigerator motor, pumps, fans etc will use more power from the inverter due to lower efficiency. Most motors will use about 20% more power. This is because a fair percentage of a modified sine wave is higher frequencies  that is, not 60 Hz  so the motors cannot use it.

Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises.

Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the peak of the line power  basically, the modified sine has a flat top rather than a peak  this may cause the occasional double trigger. Because the modified sine wave is noisier and rougher than a digital pure sine wave, clocks and timers may run faster or not work at all.

Items such as bread makers and light dimmers may not work at all  in many cases appliances that use electronic temperature controls will not control. The most common is on such things as variable speed drills will only have two speeds  on and off.

most equipment will operate without any noticeable difference, and because the lower cost, makes this the most common inverter sold and generally the only type found at your local retailer.
Square Wave
Very few but the very cheapest inverters any more are square wave. A square wave inverter will run simple things like tools with universal motors with no problem  but not much else. These are seldom seen any more except in the very cheap or very old ones. Of course we don't sell these. Too cheap!
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3. How do I hook up my inverter to my battery?
Series Wiring
Series wiring refers to connecting batteries to increase volts, but not amps. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the negative terminal of the other, then you have series wired the two together. In this case, you now have a 12 volt battery and the rated 350 amps does not change. If you were to series wire four L16's you'd have 24 volts at 350 amps, and so on.
Parallel Wiring
Parallel wiring refers to connecting batteries to increase amps, but not volts. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the positive terminal of the other, and the same with the negative terminal, then you have parallel wired the two together. In this case, you now have a 6 volt battery and the rated 350 amps increases to 700 amp hours. If you were to series wire four L16's you'd have 24 volts at 350 amps, and then parallel wire these four to the four other that are in series, then you'd have a 24 volt battery at 700 amps.
Five basic wiring types

