This is a complex question that requires you to do some reading, as it confuses even electricians. The applicable reference is: NEC 220.40 Feeder and Service Calculations.
Typically if you are asking this question you should call an electrician to help you. However if you read the applicable NEC article you will be able to calculate your own service.
This can be done by using a clampmeter, this is a instrument that clamps over the main incoming cable ( hot wire ) where it enters the distribution board. You set the meter to read amps and this gives you the total amps being used at that time.
I always ask the customer to find their electrical useage for the last twelve months. Use the highest month in KWHr, divide that by days in month, then hours in day, then supply voltage such as 240-volts. This will show the highest average ampere demand for which your service conductors are supplying. Don't forget to convert to watts at the end.
Surprisingly, most homes average about 10 amps continuous or less. Minimum service rating is 100 amps.
If you are paying $.1 per KWHr and the bill is $150 over 30 days, then you used 1500 KWHr, or 8.68 amps per hour continuously on average.
You could simply install socket power readers on all the outlets that have electrical appliances attached to them, and that would give you an indication of your power usage. If you wanted to know amps, then that's something else all together.
In residential installations, this is usually not a necessary factor to calculate. The utilities companies usually only offer 100 amp, 200 amp or 400 amp service. The typical new home in today's market will use the standard 200 amp service.
If you have concerns like having a garage that you will do work in using electrical equipment, then you would want an electrician to calculate the additional load. 200 amp service is quite adequate for most purposes unless you have the aforementioned scenario. My son has a welding shop and business and operates the entire shop with 200amp residential service including a 3 phase generator.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized
IF YOU ARE NOT REALLY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
The equation used to find amperage when amps and voltage are given is I = W/E. Amps = Watts/Volts.
The total number of amps used in a given house will vary at any given time, depending on what lights are on, what equipment you are using, etc. To measure the amp draw at any given time, you would use an ampmeter and use it to measure each of the two wires feeding your panel.
add the amps from each breaker reguardless of voltage. If the total exceeds 80 amps, it means you cannot use all breakers at full amp. For example: If the total equals 90, then you will have to make sure ten amps are not being used at that time. Yes, you do have 100 amps, however it is safest to run eighty percent of your total amps., at one time. The number of breakers has noting to do with the amps. You can use one breaker rated at 80 amps, or you can use four twenty amp breakers.
Amps are the amount of power you're using. If you only have one light on then you're maybe drawing 1 or 2 amps. If you have everything in the house on you may be drawing 50 or 60 or more amps.
The SUM function is the main function used to calculate a total. It can also be done with SUBTOTAL, and it is possible with SUMIF, SUMIFS and DSUM.
Wire sizes and ampacity are regulated by code. # 14 wire 15 amps, #12 wire 20 amps, #10 wire 30 amps, #8 wire 40 amps, #6 wire 60 amps. These are the most common, a slight variation in amperage depending on the insulation factor.
You can power any amount of devices that total to a sum of 3000 watts. If you can not find the wattage on the devices nameplates then use the amperage of the devices. I = W/E Amps = Watts/Volts. 3000/120 = 25 amps. A total of 25 amps can be used from a 3000 watt generator.
You can power any amount of devices that total to a sum of 800 watts. If you can not find the wattage on the devices nameplates then use the amperage of the devices. I = W/E, Amps = Watts/Volts. 800/120 = 6.6 amps. A total of 6.6 amps can be used from a 800 watt generator.
You can power any amount of devices that total to a sum of 3250 watts. If you can not find the wattage on the devices nameplates then use the amperage of the devices. I = W/E Amps = Watts/Volts. 3250/120 = 27 amps. A total of 27 amps can be used from a 3250 watt generator.