When trying to figure out sizing of all your components and fitting the pieces of the puzzle together, it ban get extremely confusing. I found a great explanation at a website the markets quality products and information. This great source of quality products and information is

 Hurricane Wind Power

Their explanation for sizing inverters is one of the best I have ever read.

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 example-another 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.

Click this link for the complete explanation and a tool for

 calculating battery sizing