Power

I don’t think a School Server can achieve it’s potential if a teacher cannot count on the School Server’s function, even when the power is not available. In many developing countries, adequate power all the time, is an objective that will be reached sometime in the future.  Until then rolling brownouts are a fact of life.

This makes battery backup of the school server, AP’s, and external internet modem, quite important. The only standard input voltage for this group of devices is 120Vac or 240Vac, depending on the country. So a UPS (Uninteruptable Power Supply) is the device that is needed.  But the storage capacity of a typical UPS is only 9AHr (108 watt hours). A typical computer uses 40-90 watts, and will completely drain the UPS battery in 30 minutes or so.

The large 100 AHr battery shown on the left, compared with the 9 AHr UPS can keep the laptop running for 30 hours; the numbers:

  • 100 AHr battery times 12 Volts equals 1200 watt hours)
  • 1200 watt Hr times .5 = 600 watt Hr (batteries survive longer if only discharged 50%)
  • 600 watt hours divided by 20 watts = 30 hours.

As I’ve thought  about living with rolling load shedding, and expensive power, I realized that I needed to rethink my assumptions.  What really matters is that the school server, and the internet be available during the school day — say from 7am to 5pm.  So the battery needs to be big enough to last 10 hours per day.  Each area needs to decide whether more than one day of charge is advisable, given the probability of an outage lasting more than 24 hours.

Since kernel 2.24, Linux has had the feature that the system can be waked at a given time from a suspended state by the bios. If we use this feature, we’ll be able to use a smaller backup battery and save expense: initial investment in battery and ongoing power expense.

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