How long will laptop run from UPS?

Question

The laptop has input: 18.5V, 2.7A
The UPS output is: 220V, 6.82A and also something about 1500AP
I have no idea how to work out how long I would be able to run the laptop off the UPS when the UPS is disconnected from the mains.

It all depends on unknown factors: 1) How good is the battery in the UPS? 2) How good is the battery in the laptop? 3) What peripherals do you have connected to the laptop? 4) What are you doing on the laptop? All these variables have a big effect on the longevity of the UPS power, and we cannot tell you what they are. One of the biggest factors is what you are doing on the laptop. Different things consume different amounts of power. If it is idling it will last longer. If you are playing games it will not last as long. — Majenko, Dec 23 '11 at 11:55
It would be useful if a few others of similar stature to Leon (how tall are you Leon? :-) ) weighed in in cases such as this and provided some perspective. Doing this in the technical forum raher than in Meta should be of direct value to all. Need not be long in each case. — Russell McMahon, Dec 23 '11 at 11:56
So: I see this as design as you need to do electrical calculatios which may well lead to related hardware actions. — Russell McMahon, Dec 23 '11 at 11:58
So what you are saying is I should use trail & error while at home to see how long it lasts with my normal usage? — maisc, Dec 23 '11 at 11:59
That's the only way really. There are so many variables it is impossible to give an accurate figure. Some UPS systems will provide an "estimated run time" in their software which you can look at. — Majenko, Dec 23 '11 at 12:19
Superuser.com is probably a good place for this... see http://superuser.com/search?q=ups — Kellenjb, Dec 23 '11 at 16:19
The real answer is: About 2 minutes shorter than you need it to run! — , Dec 23 '11 at 19:05
@RussellMcMahon, on topic is discussed on meta. Second, how is this not a clear case of something that is someone just wanting some consumer electronics help with the bonus of a site like SuperUser that will help. — Kortuk, Dec 24 '11 at 11:44

score 3 · Accepted Answer · answered Dec 23 '11 at 12:36

Re

The laptop has input: 18.5V, 2.7A
The UPS output is: 220V, 6.82A
and also something about 1500AP

These are informative but not directly related.

Input is to laptop by charger.
This IMPLIES that the cells are < 2.7 Ah each - say probably 2.2 Ah LiIon 18650 cells
BUT this is uncertain.

6.82A is notional UPS output max current.
They Are REALLY sayin that it is a 1500 VA or Watt inverter and the 6.82 was got by dividing 1500 by 220 . It will be approximate.

Question has many factors affecting it.

This is the general formula that applies and may be approached in several ways.

Time = Available_energy / rate of using energy
     = (UPS_battery_capacity_in_Wh x UPS_efficiency) / Laptop_Power_use

You can get a good measure of (UPS_battery_capacity_in_Wh x UPS_efficiency) by operating a known load on a full charge and seeing how long it ran.

BUT you may guesstimate that

 UPS  energy capacity ~~= Battery amp-hour rating x battery voltage x 0.8

or If battery Watt hour capacity is known.

 UPS  energy capacity ~~= Battery Watt-hour rating x 0.8

In both cases the 0.8 is an assumed efficiency of 80% from battery output to end use device.

__

The 18.5V, 2.7A is the laptop charger raing.
More useful is the battery amp hour (Ah) or Watt-hour(Wh) capacity. ither or both are almost always shown on the battery.

Determine N from N x 3.6V ~+ battery voltage.

Then laptop battery energy capacity = either

energy capacity = Watt.hours shown on battery

or

energy capacity = N x 3.6 x Battery amp hours capacity

Run laptop from fully charged battery using conditions similar to those to be used when UPS is in use. If you do not know thse use conditions then you cannot answer the question.

Laptop Run Hours = LRH.

Then

Laptop_power = energy usage rate = Battery_energy_capadity (from above) / LRH

Then

**UPS runtime ~~= UPS_battery_capacity_in_Wh x UPS_efficiency / Laptop_power**

EXAMPLE:

Made up data.

UPS Battery = 12V, 30 Ah.

UPS available energy = 12 x 30 x 0.8 = 278 Watt.hour

Laptop battery = 14.6V.

So N = 4 as 3.6 x 4 ~~ 14.6

Laptop battery capacity = 4Ah (on battery).

Laptop battery energy capacity = 3.6 x 4 x 4ah = 47.6 Watt.hour - say 48

Laptop is run from this battery and achieves 4.5 hours operation.

Laptop power use = Laptop battery energy / hours operaing   
 = 48/4.5 = 10.666 atts = say 11 Watts

UPS runtime - UPS energy/Laptop power

 = 278 Wh/11 W ~= 25 hours

This is longer than a UPS will usually opera a computer system.
This is due here to goodish battery in UP =30 Ah and rasonably low thirst laptop.

NB - note that I have taken UPS and laptop batery capacities as rated. If they are not new this will not be true.

score 3 · Answer 2 · answered Dec 23 '11 at 14:42

As others have said, you left out much important information, foremost of this is the battery capacity in the UPS. The question is basically how long the USP battery can power the laptop. It should be obvious that we need to know something about that battery.

However, I'll do the calculations by adding some assumptions. This won't provide the right answer necessarily, but it will show you how to calculate the time once you have the relevant values.

The laptop requires 18.5V at 2.7A worst case. For many operations it will draw less than that, but let's go with that and get a worst case number. 18.5V x 2.7A = 50W, which is what we will assume the laptop requires. There are at least two power conversion stages between the UPS battery and the laptop input, so let's say that only 50% of the battery energy makes it to the laptop. The rest is lost in heat due to conversion losses. That means we will assume the UPS battery is supplying 100W.

Now we have to pick battery characteristics. These vary widely, so I'm just going to pick something. This could be rather off for your particular UPS. Let's say the UPS uses a 12V lead-acid battery rated for 50 A-h. At best that means it can source 50A at 12V for 1 hour, or 600W for one hour. Since the drain is 100W, you have 6 hours of run time.

There are details I left out, but this is the basic logic. Note that the exact volts and amps out of the UPS and into the laptop adapter are irrelevant since we are only looking at power transfer. This same logic would hold for a 110V UPS with the same battery inside.

The 18.5V at 2.7A rating is probably the mains power supply rating. IF they allow full LiIon charge plus laptop operation at the same time you can guesstimate that it has 18650 cells with a nominal capacity of 2 t o 2.5 Ah.
If you assume 2 A for the battery at about 1C, that's about 700 mA for the PC putting laptop battery life at about 2000/700 or about 3 hours.| UPS battery life is then UPS_Ah / 0.7 x whatever efficiencies you wish to assume. 50% overall seems somewhat harsh but not super vastly so. — Russell McMahon, Dec 23 '11 at 19:46
@Russell: The OP asked how long he can run the laptop off the UPS. I therefore deliberately ignored any energy stored in the laptop itself. 50% is probably a bit harsh, but consider there are probably 3 switching power stages between the battery and the laptop DC input. First a boost to make some high voltage, then something like a class D to make 50 Hz sine at 220V, then a rectifier and switcher in the charger. Three stages at 80% each is 50% overall. Two stages would have to be unrealistically bad. So I agree with "somewhat harsh but not super vastly so". — Olin Lathrop, Dec 23 '11 at 20:54
Nice answer! I wonder how to find out the battery capacity of this UPS http://www.apcguard.com/BE550G.asp?gclid=CLfBpor_pLMCFQqe4Aod1zsAGw? I didn't find something like xxx A-h. — Tim, Oct 29 '12 at 00:45

score 1 · Answer 3 · edited Jun 11 '20 at 15:10

Even though this isn't really on topic here I will try to give an answer. First some clarifications:

The data you gave of your laptop input means, that the laptop has a maximum (!) power consumption of 18.5V * 2.7A = 50W. The Laptop will need much less power if it is not under full CPU load or if the battery is full (and doesn't get charged).
You need to know the battery capacity of your UPS. If you know the battery capacity in Ah you can give a rough estimate by multiplying it with it's voltage. Example: A lead-acid battery has a capacity of 7Ah and a voltage of 12V. This will mean it stores about 84Wh of energy. Let's assume a drawn power of 25W: The battery could supply that for about 84Wh / 25W = 3.4h
You need to take efficiency into account. If your UPS needs to supply 1W to it's output, it will draw maybe 1.25W from the battery. I don't know how efficient UPSses are usually, but 80% sounds reasonable to me. The same thing goes for the battery: It has an internal resistance, which means that when heavy loads are applied, a lot of energy will be wasted in the battery itself.
As already mentioned, battery age and condition is a big issue: An old and abused battery will only hold a fraction of it's nominal charge. This is why one usually changes UPS batteries after a period of time.

In conclusion I would go for an experiment to verify my etimate: Figure a way out to measure the state of charge of the UPS battery. Disconnect the UPS from mains and note the SOC of the battery every n minutes until the SOC reaches about 50% (batteries don't like being discharged all the way). Repeat this experiment with varying loads from the laptop.

score 0 · Answer 4 · answered Dec 24 '11 at 09:36

We have to guess because there are four things we don't know:

1) We don't know the UPSes rated wattage.

2) We don't know the UPSes rated run time at some specified load fraction.

3) We don't know the laptop's actual draw from its DC supply.

4) We don't know the efficiency of the laptop's power supply.

However, a typical 1500VA UPS has a rated wattage of around 1200W. A typical UPS has a 20 minute run time at half load. A typical laptop under normal use will draw about 80% of the rated maximum. Lastly, a typical laptop power supply is about 70% efficient.

Plugging all these guesses together, we get a 20 minute run time at 600 watts from the UPS or 200 watt hours. We also get about a 42 watt draw from the laptop. That would give 4.7 hours.

The only problem is that a typical UPS becomes less efficient as its load goes down. This is because a certain amount of power is required just to keep the UPS itself operating. The draw from the laptop is so low that this will be significant. So I put this all together into a guesstimate of 3.5 hours.

How long will laptop run from UPS?

4 Answers4