Power Consumption Question

[Feline]

Think I know the answer to this - but would like a little reassurance!

Owing to a new local phenomenon known as "Recurrent Load Shedding" (like - no mains supply) I need to set up alternative power supplies for computers in Chez Feline (aka the Cathouse).

First thought was to buy a generator - but have now decided that I don't need no noisy gas-guzzling machine outside the backdoor, so have now embarked on the route of DC-AC inverters powered by deep discharge lead acid batteries.

When trying to do a power budget (in watts), I am faced with the fact that most of the equipment that will be hooked to the inverter have a label to the effect that the power requirements are: " 100v - 240v AC - 1.2A"

So on the face of it - this equipment could take between 120 watts and 288 watts. Hmmm!

Would I be right to assume that the label indicates that the equipment will take a maximum of 1.2 amps (ie. 120 watts), and this would (presumably) be at the lower input voltage (100v), so at 240 volts I could expect it to still take only 120 watts (which would mean only 0.5 amps current drain)?

Also - if anyone has any knowledge of average power consumption of a computer system as opposed to quoted panel sticker consumption - and would be prepared to share this information, I would be most grateful (what I'm getting at here is that a computer where the CPU is thrashing away at 100% is going to take more power that a CPU running at, say, 30% utilisation - so how much power does a lightly used computer actually consume?).

I also have to observe that this whole exercise has made me painfully aware of just how fragile our personal ecosystem becomes when you can't expect reliable house current from the wall socket - think computers, printers, and the myriad of rechargeable battery powered devices we rely on, all of which depend on house current! A lot of local businesses just have to close down without power (no backup power supplies on their telephone systems) and there are all sorts of downstream consequences (like traffic congestion owing to no traffic lights, and even the cellular network is taking strain because the duration of power outages exceed that planned for when installing power backup in local cells). :(

[Hyph]

Power figures can go all over the place, depending on what components are in the machine and how much time it spends powered on but idle.

Our data centre guys take the rated wattage and multiply by 0.6. They use that number for power consumption calcs. At least, that's what they tell me.

They use this as a metric for providing power, UPS and cooling - they seem like they know what they're doing.

[hellsbrink]

Sheesh, that is a difficult one!

It's impossible to say, tbh, as every system is different. Your idea of a "light" load could be different from mine!

Your best bet would be to get someone to test the power consumption at FULL load, and work from that figure as obviously there will be times when that load will be used and it will chew the battery power of anything you design to be used at a lower power.

Oh, do remember to include your monitor in your power calculations. A PC ain't much use if you have nothing to look at!

[green granite]

My Asus manual suggests that the power consumption for an "average" computer configuration is about 400 watts for a desktop, ( the variables are things like the no of hdds, cd/dvd readers etc) allowing for some loss in the psu that equates to roughly 2 amps @ 240V. A laptop would be less, and of course you could power it straight from the 12V batteries.

[Keef]

It's hard to judge how hard you're hitting your power supply. On "idle", it's pretty low. At other times, it'll get close to 100%. To be sure your batteries will hold out, I'd take 75% of the rated power of the power supply in the PC, plus 100% of the power consumption of the monitor, plus any other peripherals (printer, scanner...).

If you're using an inverter to convert the 12v to 230v, reckon on about 90% efficiency in that.

My guess would be that you're talking somewhere between 300 and 500 watts at 230 volts, so about 25 to 40-ish amps from the batteries. If your power outages are several hours at a time, that's a lot of amp-hours, so some very large batteries. Then, there's the question of how fast they will recharge when the power comes back on.

I'd be more inclined to go for a llittle generator. There are some very nifty little 1 kilowatt ones available for very reasonable prices. You then have the advantage of being able to have a lamp running too, so you can see what you are doing.

[Feline]

Thank you for all the replies!

I think I am going to have to acquire me an AC amp meter and take some actual measurements.

I must say that this was an area which I never thought about too much - for example, I vaguely thought LCD screens took a lot less than a CRT screen, but they take more than I thought. Likewise, even a modest laser printer takes about 500 - 600 watts - and that's a lot more than an ink jet printer at about 40 watts.

I have also discovered that there is more than one type of 12 volt lead acid battery - car batteries are designed to dispense very high currents on a low duty cycle: for a power backup application one needs so-called "deep discharge" batteries which are optimised to provide a lower current (tens of amps rather than hundreds of amps) over a sustained period.

My original thinking was to simply buy a generator but decided against this because if they're not used all the time they can be extremely difficult to start, particularly the cheap ones where you mix two-stroke oil in with the petrol - over a period of time the petrol evaporates leaving the carburettor full of two-stroke oil. Also, some of them have ridiculously low running time between services - one I looked at needed servicing every 24 running hours! Last but not least, they generate a lot of carbon monoxide - not nice close to a house!

So far I have acquired two 102 ampere hour deep discharge batteries. One of these will power my main desktop system (and the house's internet gateway) by means of a combined 1000 watt inverter (2700 watt surge rating!) with a built in three stage 10 amp battery charger (which tapers the charge rate so the battery doesn't get cooked). I just hope that I don't get anywhere near 1000 watts because that will mean some significant drainage on the battery!

The other battery will power an 800 watt inverter with a separate 10 amp battery charger for Mrs Feline's Apple Mac.

I also have a 150 watt inverter for lap top with 3G card and ink jet printer. With a bit of forethought I can quickly transfer work in progress to a memory stick and continue working (and communicating) on the laptop.

I also have about half a dozen fluorescent lamps with rechargeable batteries, so will be able to see what I'm doing after dark ...

At some stage I will likely also invest in a solar panel to recharge the batteries (we have plenty of sunlight!) - so I think I've more or less taken care of work continuity (important because I work from home - and no work = no income!)

Cooking (sort of) taken care of with bottled gas ...

It also occurred to me that a real neat solution would be to have a couple of rollers coupled to a generator: if you're at home then it's likely that your car will also be at home. Drive car onto rollers and put into gear and one should be able to produce a good few kilowatts!

[hellsbrink]

With the extra wear you would put on the car for your "generator", you'd probably end up being cheaper buying a proper gen set....

You can get all sorts of generators nowadays, they ain't all the big, stinky, smoky, noisy diesel ones of yore. Some that could easily run your PC are small and quiet enough to use in the actual house (but the fumes ain't so good, although you could duct that outside). With battery power alone you also have to think of how long the power could be out for, you would then need someting to charge the batteries up. I would get a small UPS to run things for a few hours, and then a small, portable, petrol gen set for when an outage lasts a bit longer. That should cover you until you get a decent solar setup, and having that gen set would no doubt come in useful as you could just put it into the back of the wagon and use it for other purposes as well if the mood takes you. Nothing wrong with doing something that gives you more options!

[green granite]

have a look at this for a generator 3Kw 17Hrs running on one refuel, and a noise level of 71 DBA

http://www.justgenerators.co.uk/pages/pramac_P4500es.htm

[Bushfiva]

Since you used the plural "computers", so you're likely to be networked, I offer this bit of advice from personal experience: make sure the hub/switch/routers, monitors and any external hard drives are on the UPS, too.

And since you're talking about load shedding on a budget, don't bother with the laser printers, etc. You want a system that maintains data integrity, not productivity.

The problem with many UPSes is that they can send a "power gone" signal to only one device. You should consider a hardware doohicky that can tell all your computers that power has gone, and that so many minutes remain. That way, you can have computers start shutting themselves down as the UPS runs down, or when the generator is down to the last 2 liters of fuel.

[Saab Dastard]

Bushfiva is spot on with his last post!

I have actually seen it in a corporate environment, where all the servers were on UPS, but none of the networking equipment (switches and routers). Power went down for 10 secs or so, servers and data were fine, but the comms were knocked out for nearly a day even after the power came back.

Hardly anyone knew there had been a power cut, as almost everybody had a laptop. Everyone thought the lights had just been switched off, until the complaints started.

Fortunately I was just an observer of the event, and responsible neither for the situation nor the solution. But I did learn a lesson that day!

As an aside, a laptop with its built-in 3-hour (ish!!) battery would be a very appropriate desktop replacement in your situation, Feline!

SD

[Keef]

If you are concerned with stability, and if you are determined not to use a generator (I'd go for a 4-stroke petrol generator - quiet, and a KW is easy)...

I'd consider running all the equipment off the inverter all the time, with an adequate charger to "float" the batteries while the mains supply is up. The power needed is almost the same, and when the mains drop, or hiccup, or fail, the equipment carries on running as if nothing had happened.

If you have occasional heavy power users - eg the laser printer - then feed them direct from the mains and don't use them during a power failure. You can always plug them into your battery supply if needed for a job.

Your two 102AH batteries will run your desktop, plus monitor, plus router and hub for about 2¼ to (max) 3 hours before they'll be exhausted - assuming your equipment needs about 400-500 watts total. That's around 2 amps AC at 230 volts.

A 4-stroke generator outside the house will run for many hours on one tankful - but of course the equipment will stop while you fire it up.

The ultimate, to me, would be the battery plus inverter "always on" so you have completely uninterrupted power, charged from the mains while the mains are "up", then a 4-stroke generator to supply the "recharge" power to the batteries when the mains are down. That way, you have the best of all worlds.

[The Flying Pram]

You can buy a power monitor in the form of a 13A plug adaptor which will give you a direct readout of power and current being used, and some of them will also provide a cumulative reading. Maplin do them for about £15. Virtually all computer PSU's are "switchmode" and tend not to like "Quasi Sinewave" inverters. Make sure you use a "Pure Sinewave" one.

[seacue]

I have a couple of 3 W screwin fluorescent lamps. They are fairly bright - especially if you can get the fake flame-shape cover off them. I have one powered by my UPS here by my laptop .

[Crashing Software]

Feline,

For everything you ever wanted to know about coping without mains power, may I suggest a search of the forums here http://www.canalworld.net/

Gems of information such as frequently using more than 50% of the available amp hours very quickly ruins batteries - and there's something called the Peukert effect that also impacts charge/discharge cycles.

[Feline]

Had to knuckle down to a spot of sustained work, so haven't made a lot of progress - but have modified some of my work habits so everything can be done on the laptop if necessary - and I reckon (with the kit I've got so far) I've got about twelve - fourteen hours endurance on the laptop.
Still trying hard to avoid going the generator route - may take a look at solar when I've got a little more time.

Thanks again everyone - will post again when I've gained a bit more experience.