Power requirement:  8 x 30 watt lamps powered daily from 7 pm to 7 am. Peak load = 8 x 30 = 240 watts. Energy consumption/ day = 240 x 12 = 2880 watt-hours = 240 Amp-hours, assuming a 12 volt battery backup system

Discussion:  With the battery backup systems, mains power will be used when available. However this is unpredictable and could vary from 24 hours full availability to 24 hours zero availability. A battery backup system must be designed to provide a certain level of power availability under worst-case conditions; for instance, the design criterion might be to provide an overall level of power availability of 99% (ie approximately 3 days total power outage in the calendar year); or it might be to provide backup power under conditions of 3 days continuous grid blackout.

Battery backup power necessitates a method of charging the backup batteries. The two main methods are by solar power with energy from the sun or by power from the grid through a battery charger, when grid power is available. In general, charging via a battery charger powered from the grid results in lower initial capital costs  battery chargers are less expensive than solar panels and can potentially provide power day and night.

Solar power has the advantage of providing power when there are prolonged, even indefinite grid blackouts. Battery charger power becomes more attractive when grid power is reasonably reliable and regular, without prolonged blackouts - when grid power availability is of the order of 50% or better.

Design Criteria: The specified power requirement 8 x 30 watts x 12 hours is quite high for a stand-alone solar backup system and would require 8 to 10 pcs 140 watt solar panels for continuous stand-alone (ie without grid) operation, depending on the overall power availability specified. If a lower number of hours of operation can be tolerated, say 6 hours daily from 7pm to 1am, then the size of the solar capacity can be reduced. The solar capacity can also be reduced if one makes the assumption of significant grid availability. Many different system configurations are possible with a similarly wide range of variation in cost and system performance. A system which provides one day of reserve capacity or 6 hours of daily operation would be lower in cost than the systems we have specified here.


We propose the following systems for your consideration:

Partial Solar System/ National Grid (battery bank is standby)

4 x 140 watt solar panels, 8 x 88 Ah batteries, 1 x 1KW inverter, automatic changeover switch (inverter/mains), timer switch (7pm to 7 am), secure, weatherproof enclosure for ground components, installation of solar panels, ground components and cabling&. USD 4,300 (four thousand three hundred United States dollars).

System specifications: Reserve capacity 3 days operation, 12 hrs/day. After this, with no sunshine (height of rainy season) power availability is grid power availability. Note: System switches to grid when grid is available. Thus there will be grid charges

Battery Charger/ National Grid (Battery bank is standby)

1 x 30 Amp battery charger, 8 x 88 Ah batteries, 1 x 1KW inverter, automatic changeover switch (inverter/mains),  timer switch (7pm to 7am), secure, weatherproof enclosure for components, installation of components and cabling&.     USD 3,000  (three thousand United States dollars
)       
System specifications: Reserve capacity 3 days operation, 12 hrs/day.  After this, power availability depends on grid availability. Anywhere from 0% to 100%.


Full Solar System (Battery bank is main)

10 x 140 watt solar panels, 20 x 88 Ah batteries, 1 x 1KW inverter, automatic changeover switch (inverter/mains), timer switch (7pm to 7am), secure, weatherproof enclosure for ground components, installation of panels, components and cabling&.USD 7,000 (seven thousand United States dollars)

System specifications: Reserve capacity 7 days operation, 12 hrs/day. After this power will fail until sufficient sunlight to recharge batteries.


One year warranty on all systems (parts and workmanship)




Comparisons: (Note: Battery charger and partial solar  systems have 3 days reserve capacity. With no source of power (national grid or effective sunlight) input power will continue to be available from the system for 3 days. Full solar system has 7 days reserve capacity. Table below is AFTER reserve capacity has been exhausted.

	0% grid	50% grid	100% grid
Battery charger	0%	100%	100%
Partial Solar, dry season	67%	100%	100%
Partial Solar, mid-rainy season or heavy harmattan	0%	50%	100%
Full Solar, dry season	100%	100%	100%
Full Solar, mid- rainy season or heavy harmattan	0%	0%	0%

Note that table above applies only after the exceptional circumstance of zero grid power for 3 consecutive days and nights (battery charger) or zero grid power for 3 consecutive nights PLUS 3 cloudy days (partial solar system) or 7 consecutive cloudy days (full solar).

 

	0% grid	50% grid	100% grid
Battery charger	Never	30hrs	12 hrs
Partial Solar, dry season	Never	384 hrs	30 hrs
Partial Solar, mid-rainy season or heavy harmattan	Never	Never	very long but lights will work off grid
Full Solar, dry season	253 hrs	253 hrs	253 hrs
Full Solar, mid- rainy season or heavy harmattan	Never	Never	Never

 

Daily (running) Cost of power:

Daily cost of power with battery charger: Le3,000, assuming electricity @ Le1,000 / KWh
Daily cost of power with partial solar system:  up to Le3,000/ day assuming electricity @ Le1,000/ KWh. Note: System is grid preferred, battery standby
Daily cost of power with full solar: zero