Thursday, December 8, 2022

How to calculate the electrical consumption to buy a generator for a house?

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Electric generators or generator sets are very useful devices, which come in different models and with different capacities. That is why when selecting the most suitable for a house, several criteria must be taken into account. The main criterion is the electricity consumption generated in the home, which must be covered by the generator to be purchased.

In this article we will describe in detail each of the aspects to consider when buying a Electric generator, in a way calculate electricity consumption of appliances, tools and other equipment that work in a home.

Physical units and their conversions

We must know very well the physical units related to the electricity consumption of the devices, to be able to carry out the calculations correctly.
Let’s first go over the terms of:

  • Amperage (A): is the electrical current that a device requires to operate.
  • Voltage (V): is the electrical voltage supplied by power plants to companies and homes. This can vary by country, and ranges from 100V to 440V.
  • Watt (W): is the unit of electrical power.

To calculate electrical power (W), that is, the electrical consumption of an appliance, multiply the electrical current (A) by the voltage (V): A * V = W
Generally the power of the devices is expressed in watts, but when the power is high, multiples such as the kilowatt (kW) or the megawatt (MW) are often used.
1 kW = 1,000 W
1 MW = 1,000,000 W

  • Kilovoltiampere (kVA): it is a multiple of the volt-ampere (VA), which is the unit of the apparent power and the complex power of an electrical appliance.

The kVA is the unit used to indicate the power of a Electric generator.

How to calculate the electrical power of a device?

For calculate electricity consumption of our entire house, we must begin by knowing how the electrical consumption of a single device or electrical appliance is calculated.
Taking into account the definitions and relationship of the physical units already mentioned, we will go on to present an example:

Suppose we have a laptop with 1A at 220V, applying the formula to calculate the electrical power, we will obtain the consumption of this device: 1A x 220V = 220W

Usually, electrical equipment has a label or sheet where its electrical power is indicated, so we would not have to calculate it. However, there are also equipment that only indicates its amperage (A) and its voltage (V), in which case we must apply the indicated formula to know the electrical power of said equipment.

In the event that one or more of the devices that we have in our home, has a high electrical power, we will find that this is expressed in kW or MG. In either case, we would only have to convert these values ​​to W, using the equivalence values.
For example, suppose we have a powerful tool, whose label indicates that its power is 350kW. To convert this value to watts, we just have to multiply it by the equivalence: 350kW * 1000 = 350,000W

The following table shows some devices, with their respective electrical power:

Appliance, equipment or tool Electrical power electrical consumption (W)
Fluorescent lamp 35
100 watt bulb 100
Video device 100
Refrigerator / Fridge * 800
Freezer / freezer * 700
Microwave oven 1500
Coffee maker 1200
Hair dryer * 1500
Radio 200
TV 250
Computer 400
Printer 350
Bench grinder * 400
Hedge trimmer * 500
Chainsaw, blade length 25 to 40 cm * 1500
Hand lamp 500
30 cm diameter emery 2500
Drill * 700
12.7 cm diameter circular saw * 1000
Manual sander * 1020
20 cm diameter floor sander * 2000
Pressure Washer / Hydrojet, 1 Hp 3500
20 Hp compressor * 2200

Inductive power equipment

Some electrical appliances have rotary motors for the moment of starting, and at that moment they require inductive power, which is much higher than the electrical power they require to function, once they have started. An example of this equipment is washing machines, refrigerators or refrigerators, pumps, among others (see the devices with * in the table above). For some of these devices, the inductive power they require is 3 or 6 times higher than the standard operating power. That is why the calculation of electrical consumption for these equipments must be different.

Depending on the type of equipment start-up, its consumption must be calculated:

  • Direct Start (DOL): the standard electrical power is multiplied by 6.
  • Start Star Triangle (ET): the standard electrical power is multiplied by 3.
  • Soft Starter or Variable Frequency Drive (VF): the standard electrical power is multiplied by 2.5.

Let’s take the following examples as a guide:
1 Fridge = 375W x 3 = 1125W
1 Water pump (1HP) = 735w x 3 = 2205W

To facilitate this calculation, we will show in the following table the values ​​corresponding to the inductive power, according to the horsepower (HP) of some devices.

Motor Power (HP) Consumption in continuous use (W) Starting power (W)
1/8 275 850
1/6 275 850
1/4 400 1050
1/3 450 1350
1/2 600 1800
3/4 850 2600
one 1100 3300
two 2200 660

How to calculate the electrical consumption in our house?

Once the ways of calculate electricity consumption of the devices, we can proceed to calculate the electricity consumption in our home. For this we only need to make a list with each and every one of the devices, equipment or appliances that work with electricity, and take note of the electrical power of each of them.
Once the list is obtained, we must verify that all the electrical power values ​​are expressed in the same unit (watts W, for example). Otherwise, we must make the relevant conversions, in order to continue.
By having all the values ​​in the same power unit, we must add them to obtain the total value. In the following table, we will show an example:

Quantity Appliance, equipment or tool Electrical power – electrical consumption (W) Inductive power factor Total (W)
10 100 watt bulb 100 NA 1000
one Refrigerator / Fridge 800 3 2400
one Microwave oven 1500 NA 1500
one Coffee maker 1200 NA 1200
one Hair dryer 1500 3 4500
two TV 250 NA 500
one Computer 400 NA 400
one 30 cm diameter emery 2500 NA 2500
one Drill 700 3 2100
one Planer 700 3 2100
Total electricity consumption of the house 18200

The sum of the power required by the appliances is 18200 W
This value is converted to kW, dividing it by 1000: 18200/1000 = 18.2 kW.

Then, to obtain the required KVA, divide the value expressed in kW by the cosine of Fi: 18.2 / 0.8 (Cosine Fi (F)) = 22.75 KVA

When obtaining the final value, it is recommended to increase it by 20% to have a safety margin: 22.75 + 20% = 27.3 KVA

In conclusion, you need a Electric generator covering 27.3 KVA.


  • It is recommended not to start all the devices at the same time, once they are connected to the Electric generator.
  • Try to stay connected to generator, only those devices that are in use.
  • Perform proper maintenance for your Electric generator.

Electric generator – Maintenance

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