In a few days, there will be a power outage that will probably last for 8 hours. We had this issue before, but then only our part of de building was affected. It was temporarily resolved by using 2 industrial extension cords. But in this case, the whole street won't have power. So that's not an option.

Because we've done it before with extension cords, we where wondering if we can do it again using a power generator. I've counted about 15 computers. Assuming they use an estimated 250 watt average. But I'm aware this is a big guess.

What are the things to keep in mind when choosing a temporary backup generator?

The equipment we want to supply:

  • Mail server, file server, firewall and all other primary network devices use an APC
  • Other devices (8 computers, 8 27" screens) will be powered off. They are both on 2 power loops (hope I'm using the correct terminology). Once these are connected to the temporary power supply, the devices can be switched on again.
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    $\begingroup$ You need to clearly state what equipment you want to supply, with power draw. Then ask how to do it. Issues will be change over from grid to generator and power spikes, hopefully an EE can help you there. $\endgroup$
    – mart
    Jan 23 '17 at 14:06
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    $\begingroup$ You'll also want a UPS to bridge the gap between the power outage and the generator connecting. $\endgroup$ Jan 23 '17 at 15:37
  • $\begingroup$ We do have a UPS for our main computers. The other devices will be powered off, switched manually, and powered on again. I'll edit the question with some more detailed information. $\endgroup$
    – Diederik
    Jan 23 '17 at 15:39
  • $\begingroup$ What is the supply voltage in your parts of the region $\endgroup$ Jan 24 '17 at 12:43

Electronics can be quite sensitive to the quality of the power supply, particularly in terms of providing a stable voltage and the right sort of AC waveform.

There are two obvious solutions for backup power. The first is to use a generator powered by an IC engine. The complication here is that the engine output needs to be matched to the load by regulating the throttle which means that if the load is inconsistent the voltage and frequency can vary. So a simple generator consisting of an engine driving an alternator may need additional regulation of the electrical output to provide a supply suitable for electronics.

You can now get inverter generators which convert the AC generated by the alternator to DC and then use and inverter to convert it back to a more stable AC supply. A decent one will produce a stable voltage with a waveform very close to a pure sine wave.

The other alternative is to use inverters powered by batteries which can be charged by the normal mains supply when not required. This method has the advantage of providing a 'clean' power supply (with the right inverter) and can be a bit more modular. Deep cycle lead acid batteries are generally used for this sort of application as they are reasonably economical and can tolerate quite a lot of discharge without being damaged. Note that conventional car type batteries don't like being fully discharged and prefer to be have their charge regularly topped up.

Batteries also tend to deal better with fluctuating demand as they can deliver fairly high currents eg to cope with surged on startup of equipment.

You also need to account for the fact that whatever supply you use will really need to be able to cope with the peak demand as well as the sustained average.


The most practical and simplest is to review the breaker box. Below is an example breaker box for a typical US single family house. Note: US base houses are power using 120 VAC.

Breaker box

Add up total current for circuits that are of interest, which essentially will help the estimate the total current. Then calculate the total wattage for the generator.

Also remember the startup current for most electrical equipment is higher than steady state current. So if the startup sequence is staged appropriately a smaller generator might work. This requires knowledge of the startup current and steady state current for the electrical devices. Below are some reference that might help further investigate the solution. Note this is for a US single family house, the theory should be same for a larger business.

Note: It is best to consult a professional before making the final decision.


  • $\begingroup$ You also need to ensure that your UPS is connected correctly. If the planned power outage is to do some work on the wiring network, the supply company are not going to be happy if they find that you are applying power to the cables they isolated! Remember that transformers work both ways - so your 120V or 240V UPS may be applying 5,000V or more to the cables in the street outside your house, if it is incorrectly installed! (Not to mention that the UPS won't like trying to power up every house in the street as well as your own, in that situation....) $\endgroup$
    – alephzero
    Jan 25 '17 at 22:14

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