If you are reading this, I believe you heard lot of these claims as well. There are lot of people saying that you must turn off electrical appliances before the power comes back from a blackout. They said that leaving them at on state will damage them. Many people believe that when the power comes back from a blackout and appliances are on, it will add huge amount to the energy bill… Let’s go, we will uncover what is true and what is myth.
For the sake of picking a specific electrical appliance, let us consider air conditioning unit throughout this discussion.
Myth #1: The air conditioning system will get damaged when it is turned on together with the power resumption after a blackout.
This is a myth because most people will believe and think of an instant damage. It is not the case. Air conditioning systems will not be damaged easily. Air conditioners were tested at abnormal conditions during the development stage. Powering on the them at low voltage is a common abnormal test condition, not only for air conditioning but to all appliances that need power from the wall outlet. Before the product is launched to production, there is a certification process that needs to be done. This certification process includes the abnormal voltage test.
However, there is a possibility that the immunity of the air conditioning unit components will weaken over time if a blackout is going to happen very frequent. But blackouts happening very frequent is not a usual scenario.
Myth #2: Air conditioning system will get damaged by low voltage alone
This is not true because it is the current that will cause overheating to the motor winding. However, the high amount of current is caused by the low voltage. To maintain the same power, the current will increase to compensate for the voltage drop. For instance, the air condition setting is at the maximum coldness during a blackout. When the power comes back, the air conditioning unit will target the set coldness which equals a certain amount of power. To maintain this power, the current will go higher since the voltage is low.
Truth: The air conditioning system will draw huge current when the power is back
This is true. Even on a normal start-up (not coming from a blackout), there is a huge current draw, and this is called an inrush current. During start-up, especially at cold start (off for longer time), the motor will draw a very high current because the rotor is stationary. The motor winding will simply act as a short circuit for a very short duration. Air conditioning unit inrush current may go as high as 4-20 times the steady state current. A steady state current is the amount of current that the motor will draw at steady level of the input voltage and a particular output power.
Let’s assume here a 2-hp air conditioning unit and consider a wall outlet voltage of 230 volt. The 2-hp will give a total watt of 1,492 watts (1 hp = 746 watts). This will result in a steady state current of:
Steady state current = watts / volts = 1,492 watts / 230 volts = 6.5 amps
Assuming the worst-case inrush current, that is times 20, the maximum current at start-up is:
Inrush current (startup) = 20 x 6.5 amps = 130 amps
This current is only happening at very short duration. This inrush current is unavoidable as it is part of the electrical property of a motor of the air conditioning unit. The behavior of the current at start-up and going to steady state voltage would be same as below.
When the power just came back from a blackout, the input voltage during this time is very low and unstable. The reason behind this is that all the loads (within the area where power is interrupted) will be reconnected at once, causing a huge current draw that will cause the voltage to drop and will need some time to stabilize.
When the air conditioning system is not turned off when the power is back, the equivalent current will be higher than the steady state current. This will not last long but significantly has longer time than the inrush current. In some places in the world, the line voltage may reduce by as low as 15% from the usual level. For a 230-volt wall outlet, the resulting voltage is 195.5 volts. Supposing this is the case, the current draw of the air condition is:
Current draw at low voltage = 1,492 watts / 195.5 volts = 7.63 amps
The behavior of the current at start-up, with a low voltage and then steady state voltage would be same as below.
As illustrated in the figure above, the current draw of the air conditioning unit is higher when the voltage is low.
Myth #3: There is a big increase in the energy bill when an air conditioning system is turned on together with the power resumption after a blackout.
This is not true. The inrush current as explained above will only happen in a very short duration. The effect of this is negligible though the current spike is high. Supposing the wall outlet voltage will not go back to its normal level right away that causes a high current draw, still the effect is negligible since the total power could still be the same (considering an ideal power factor). Furthermore, staying at low voltage scenario for long period is not going to happen often.
Numerical Analysis for Myth #3
For the sake of this numerical analysis, let us consider a 2-hp air conditioning unit in the earlier portion of this discussion. The steady state current of this air conditioning unit is 6.5A, while the inrush current is 130 amps (this is 20 times the steady state current). The inrush current is going to happen only in a very short period. It could be in microseconds to milliseconds. But for the sake of discussion, we will consider 1 second.
The monthly energy consumption under steady state (normal use) at 12 hours daily usage is:
Energy consumption per month at steady state voltage = (1,492 watts / 1000) x 12 hours x 30 days = 537.12 kW-hour.
Assuming an energy cost of 0.3$ per kW-hour, the monthly energy bill is:
Energy bill per month at steady state voltage = 537.12 kW-hour x 0.3$ per kW-hour = 161.136$
Energy Cost Adder Due to Start-up Inrush Current
The above number is not considering yet the impact of the start-up inrush current. For an inrush current of 130 amps at 1 second duration, the energy can be approximated as:
Energy due to inrush current = 230 volts x 130 amps x [1 second x (1 minute / 60 second) x (1 hour / 60 minute)] = 8.31 watt-hour
Considering the air conditioning unit is turned on twice a day, the monthly energy due to inrush current is:
Energy due to inrush current per month = 8.31 watt-hour x 60 = 498.6 watt-hour per month
The corresponding energy bill due to the inrush current in a monthly basis is:
Energy bill addition per month due to inrush current = (498.6 /1000) x 0.3$ = 0.15$
Is this worth saving? I leave that to you!
Energy Cost Adder Due to Low Voltage Right After the Blackout
When the air conditioning unit operates at low voltage, the current will increase to maintain the power. This means that power consumption is the same. The impact of the higher current will be on the rms loss in the wire from the meter going to the air conditioning unit. However, the RMS loss is a function of the wire resistance which is very small since household wiring is relatively short, this will result in insignificant cost adder also.
A typical 14 AWG wire has a resistance of 2.5 ohms per 1000 feet. The difference in the low voltage and normal voltage wire power loss is:
Power loss difference = (7.63 amps x 7.63 amps x 2.5 ohms) – (6.5 amps x 6.5 amps x 2.5 ohms) = 145.54 watts – 105.625 watts = 39.92 watts
Supposing the low voltage duration will reach an hour and will happen four times a month (which is very unlikely), the monthly energy cost adder is:
Energy bill addition per month due to low voltage = (39.92 / 1000) x 1 hour x 4 x 0.3$ = 0.05$
To summarize the above analysis, the total amount added to the monthly energy bill is 0.2$ (0.15$ + 0.05$) if you are going to let your air conditioning unit on when the power is just coming back from a blackout. You will be the judge if this number worth saving!