How to calculate UPS runtime calculation

Having a UPS between a home’s electrical installation and our computer is very important in case of any outages. Choosing the power of a UPS is not easy, it requires a series of calculations to know what capacity the UPS should have. Additionally, we will explain how to calculate how many hours the UPS can last.

What is UPS?

It is an energy storage system that allows an electronic device connected to it, such as a computer, to continue working for a while. Uninterrupted Storage Systems or UPS are essential in computing, above all to protect against power outages and thus prevent data loss or corruption. The time you can run a computing device depends on the capacity of the UPS and the consumption of the computing equipment.

Internally, we can say that the structure of a UPS is quite simple, based on one or more batteries and a set of electronic circuits. We have a circuit that converts the alternating current (AC) of our home setup to direct current (DC) to charge the batteries. In addition, there is an additional circuit that converts direct current to alternating current when a power outage is detected.

It provides more or less benefits depending on the quality of the UPS. Normally, the cheapest ones act as backup for when there is no power supply. The most advanced UPSs have the ability to protect against small transient voltage spikes and even suppress voltage noise. Some more advanced UPSs offer additional USB ports for charging devices such as smartphones or tablets. Some even have Ethernet connectors to eliminate electrical noise that may be in the wiring. Note that in this case there will be a decrease in internet speed.

Types of electrical power we need to know

The first thing to keep in mind is that the power UPSs usually mark is different from the power consumed by a computer. To do this, we must first know the following concepts about electrical power:

  • Electric power: It’s all that energy that does useful work. For example, what a light bulb consumes to illuminate. Its formula is: P=V*I*cos(φ); Here, «P» is the watt (W), «V» is the electrical voltage (V), «I» is the electrical power with electrical current (A), and «cosφ» is the power factor.
  • Reactive power: We’re talking about energy that doesn’t really do useful work. This type of energy occurs in installations with a large number of coils (normally motors). Its formula is: Q=V*I*sin(φ); Here, «Q» is reactive power, whose unit is reactive volt-amperes (VAr), «V» is electrical voltage (V), «I» is electrical current (A), and «sin(φ)» is the opposite power factor and has no unit.
  • Apparent power: Vector sum of active and reactive powers. Formula: S=V*I; where “S” is the total electrical power in volt-amperes (VA), “V” is electrical voltage (V) and “I” is electrical current (A)
  • Power factor: It is the ratio between the energy supplied (apparent power) and the energy actually consumed (active power). It is calculated as follows: FP=P/S; where “PF” is the ratio between active power and apparent power and has no unit, “P” is active power (W) and “S” is apparent power (VA). This value is the “φ” parameter.

How do I know the power of the UPS I need?

Well, now that we have a basic knowledge of the different electrical powers, it’s time to choose a UPS. First of all, we must know how much our computer consumes in order to be able to choose a UPS that supports our equipment.

The easiest way is to buy a wattmeter that plugs in and connects our computer. These devices allow us to know the consumption of our computer with sufficient precision. If we are going to play games or do heavy work, the ideal is to write the value in this case.

If we do not want to buy this device, we can roughly calculate the consumption of our computer according to the selected hardware. To see the consumption of the processor and video card, we can look at the manufacturer’s page. Next, we add about 50W to the highest value (this is the maximum consumption of the motherboard, RAM and SSD). It will more or less give us the maximum consumption of the computer, which is something very difficult to achieve.

What UPS power do I need?

Let’s say we measured the consumption to be 300W when we were playing games. Ideally, the UPS power is 400 W so that there is a margin in case there is more load or we have more connections. However, UPSs are expressed as apparent power (VA). So how do I know which UPS to buy?

With the formulas we explained above, we can easily know the UPS power we need. It is as simple as dividing the active power by the power factor. The formula would be: S=P/FP.

Ok, we don’t know what the power factor is and it’s not really necessary. Most UPSs on the market operate at a power factor (PF) between 0.7 and 0.6. Ideally, to be on the safe side, opt for a conservative power factor, choosing 0.6 as ideal. If we choose a UPS from a well-known and reliable brand, we can choose a power factor of 0.7. Be careful because if we choose a very low priced Chinese UPS we will only get a PF of 0.5 which is ideal.

For a 400W consumption, taking a power factor of 0.6, this gives us that the UPS should be at least 666.66VA. There is no UPS of this power, so we will choose one of 700VA.

How long will UPS last?

Now that we are clear on how to calculate the power of the UPS, it’s time to know how long it will last us. For this, we have a nice formula as follows: T = [(N*V*Ah*Ef)/S]*60

Ok, now we’ll explain what each parameter is:

  • T is the total uptime that UPS will have
  • N: UPS battery number, normally the manufacturer specifies this parameter
  • V: The voltage delivered by the batteries.
  • Ah: is the ampere/hour of the battery (Ah)
  • Ef: The efficiency of the batteries. Normally it is between 98-90%, but normally 95% is taken, which is an average term for calculating autonomy
  • S: is the apparent power of the UPS
  • 60: Represents an hour in minutes and is used to convert the result into an easily manageable unit of measure

Calculation example

Let’s say we have a 700VA UPS, two batteries, a battery voltage of 9V and 5Ah. We also assume an efficiency of 95% (interestingly, when we apply this in the formula, it’s already converted to 0.95). Now we simply replace the data in the formula so that:

T= [(2*9*5*0.95)/700]*60 —> The result of this is: T= 7.32 minutes

Thus, we will have more than 7 minutes to shut down the system safely with the selected UPS. We must bear in mind that the power the UPS is designed for is greater than the actual power consumption for there to be a margin, so the duration will be much greater. Regardless, the duration is more than enough to safely shut down the system.