[IslandCrow]

On the slide to a new energy level after Peak Oil starts to make itself felt, I expect periodic disruptions to electricity supplies. I am slowly working towards the initial goal of being able to 'easily' survive periodic power cuts. The main problem area that I am now considering is that I work from home and need a computer for work.[/end of rational for posting in www.peakoil.com]

I am having problems calculating the required system size that I need. I think I understand the concept of VA (volt-amps) and how this might differ from Watts. The questions that I have relate mostly with the transformers.

For example. My computer has a 19Volt and 4,74 Amp rating which gives a VA rate of just over 90 VA, which happens to be close to the 90 Watt rating of the computer.
The transformer for the computer has the following information: 100V-240V / ~1.5A(1.5).
Since I am in Europe and run on a higher voltage than in the States is the VA of the transformer 240 x 1.5 = 360 VA?
If I was in the States would it only be 110 x 1.5 = 165 VA?

I know the transformer gets hot, but the step up from 90VA for the computer to 360 VA for the transformer for the computer seems to be too large. Can anybody comment on this?

Thanks for your help. *BAIK*

[MD]

$this->bbcode_second_pass_quote('IslandCrow', 'I') know the transformer gets hot, but the step up from 90VA for the computer to 360 VA for the transformer for the computer seems to be too large. Can anybody comment on this?

Thanks for your help. *BAIK*

[WatchfulEye]

$this->bbcode_second_pass_quote('IslandCrow', 'F')or example. My computer has a 19Volt and 4,74 Amp rating which gives a VA rate of just over 90 VA, which happens to be close to the 90 Watt rating of the computer.
The transformer for the computer has the following information: 100V-240V / ~1.5A(1.5).
Since I am in Europe and run on a higher voltage than in the States is the VA of the transformer 240 x 1.5 = 360 VA?
If I was in the States would it only be 110 x 1.5 = 165 VA?

I know the transformer gets hot, but the step up from 90VA for the computer to 360 VA for the transformer for the computer seems to be too large. Can anybody comment on this?

[IslandCrow]

Thanks MD.

You have made me feel a lot better. It seem I could get by with a smaller system than I was fearing, or I would get a longer working time with the bigger systems that I had been looking at.

I realize that this will not help in a major power-down situation, but it might help in the early stages of disruption to supplies, like last week we had two short (less than 5 minutes) power outages.

[IslandCrow]

Thanks also WatchfulEye.

Yes I should get one of those metres. It would be easy to work out my office needs, as I have all the equipment on one power strip (easy to switch off after work and so save the drain on standby).

[skyemoor]

I have a similar laptop and have a meter I've hooked up several times, as I have battery backup for the entire house (off-grid during outages) .Here's how you determine your storage requirements;

- The transformer, as WatchfulEye noted, is sized for the greatest possible power draw from the laptop, which is when it is fully operational, the hard disk is spinning up, and the fan is on full blast.

- The normal operating draw for the laptop is significantly less. My transformer states; 19.5v and 3.34A which would translate to ~65VA or roughly 70 W taking the inductive load into account. However, when measured over time, it averages about 18 W during a typical hour of use.

Now to help with the rest of the calculations, are you planning on setting up a standard UPS or are you going to go for having a battery (or two), a charger, and an inverter?

[IslandCrow]

$this->bbcode_second_pass_quote('skyemoor', 'N')ow to help with the rest of the calculations, are you planning on setting up a standard UPS or are you going to go for having a battery (or two), a charger, and an inverter?

[skyemoor]

If you are focused on the laptop, you can reduce DC/AC => AC/DC conversion losses with a DC/DC converter, such as this.

For planning how much solar to use, follow the steps in this guide I made for pandemic preparedness. For battery storage, determine how long you want to run the laptop on batteries. Deep cycle batteries are available in different voltages, most commonly 2v, 6v, and 12v. They are also rated for the number of amps they can provide in a given time frame, such as “amp-hour.” An important battery energy storage calculation is:

Volts x Amp-Hr = Watt-Hr

If you need roughly 20W for 2 hours per day, that's 40 Watt-Hrs per day. The same laptop run for 10 hours would use 200 Watt-Hrs. A 12v battery with 100A-Hr capacity has the equivalent of 1200 Watt-Hrs capacity.

Keep in mind that even deep-cycle batteries can be damaged if they are frequently discharged more than 50%. For this reason, provide a margin of error by never discharging a battery below 50% of its capacity. At all times then, you want to maintain the battery reserve charge above the 50% Depth of Discharge (DoD) level. So multiply your daily energy needs by 2, which would bring our example up to 2500 x 2 = 5000 watt-hr (or 5 kW-hr).

We can stop here and use a rule of thumb that there would be another 40% losses and that you could run your laptop for about 18 hours with the above-mentioned battery starting out with a full charge. There are finer calculations if you want to be more certain.

Battery charging losses are also a consideration during the charging phase, as the electrochemical processes within the battery are not 100% efficient, so subtract 25% for this loss. Most of this loss is due to heat and gases generated during charging.

Cloudy days decrease PV-panel energy production. To account for this, multiply the daily watt-hr requirement by the number of cloudy days you want to maintain operation.

As batteries age, they gradually lose their capacity as their interior structure deteriorates and eventually becomes too weak to effectively store energy. To ensure that your system will operate correctly throughout a battery pack’s useful life, add 20% to your energy storage calculation.

Battery packs must match the system voltage, so if you have two 12v batteries for a 12-volt solar system, these would be connected in parallel, and you would add each of the amp-hours of the batteries together. If you had the two 12-volt batteries in a 24-volt system, you would connect them in series. For example, consider the following energy storage need and system voltage level calculation.

Total battery storage = Energy Storage Need x Battery Reserve x Charging Losses x Number of Cloudy Days x Battery Aging Factor

Using an example of a 12-volt device needing 1200 watt-hours daily with 3 days of cloudy day storage, we can determine the minimum battery energy storage requirements and the number of batteries needed.

Total battery storage = 1200 x 2 x 1.25 x 3 x 1.2 = 10368 watt-hours

If we use 12-volt AGM batteries that are rated at 108 amp-hr each, they produce:

12 volts x 108 amp-hrs = 1296 watt-hrs

10368/1296 = 8

So we would need at least 8 of these batteries in parallel in our pack to meet our storage needs for 3 consecutive days without the sun.

[IslandCrow]

Skyemoor - Thank you for all the information. It is helpful, and I am learning things. The DC/DC adapter was a good tip that I had not thought of.

This thread has helped influence me towards getting a system (batteries / charger & invertor) that could later fit into a solar or wind system. However, I will need to spend more time on working on just what size set-up I would need for now.

At least there is a solar/wind system distribution firm on the next island community and I will approach them for more details and prices when I am a little clearer on how much power backup I need.

In the meantime I will have to learn how to cook using the wood-burning cook stove that was installed yesterday (I need to wait another day or two for the cement around the chimney to dry properly before I fire it up properly). I find my preps are an interesting mixture of hi-tech and low-tech solutions!