PRACTICAL CONSIDERATIONS & APPLICATION NOTES ON SOLAR SYSTEM ASPECTS AND SIZING

 

SOLAR PANEL ENERGY PRODUCTION
As a rule-of-thumb each so-called peak-Watt ( Wp) of solar panel power can deliver around 3.5 watt-hours of energy per day. This can be more in summer and in certain areas ( e.g. Kalahari desert), but less at say the coast. ( and during bad weather spells!)

Therefore a 40Watt solar panel would supply about 40 x 3.5 = 140Watt-hours per day. As a further example, an array of 10 x 50W solar panels can provide 1750 Watt-hours per day.

ENERGY CONSUMPTION OF APPLIANCES / LOADS
The energy requirements are determined by the power that each appliance ( light, TV set, radio, refrigerator, etc.) uses and the number of hours per day each appliance is on. If more energy is used than is produced in a given time, the energy store ( battery or batteries) will run out. It is like a water storage tank or bucket: if more is taken out than is replenished, the tank will eventually run dry!

NOTE: The appliance power of interest for the purpose designing the solar system is the electrical INPUT power it needs.
For example: A "800Watt" microwave oven actually requires much more input power than 800W; the 800W in this instance commonly refers to the power the appliance produces at the "output" i.e. microwave power it produces to cook food.

 

EXAMPLE:
If 4 lights each consuming 9W are to be used according to the following scheme:

1^st light on for 4hours per night
2^nd light 3 hours
3^rd light 1 hour
4^th light 1 hour

the total energy requirement would be 81Watt-hours. To this must be added system losses and inefficiencies which can between 5% and 30%-plus. The chief contributor could be the inverter (where applicable), with 8%-30% loss factor and more!, depending on the quality /design technology etc. of inverter. For this and other reasons it is important to consider carefully the inverter choice in solar systems.

Assuming that the 4 lights in this example are 12V dc lights, and no inverter is employed, the total energy requirement including losses can be estimated to be:

81Wh plus 8% losses = 87.5Whours

To cover this daily energy requirement, we need 87.5 / 3.5 = approximately 25W of solar power.

To this one may want to or have to add extra capacity for re-charging after a bad weather spell. If this extra capacity is not added, lighting use would need to be reduced temporarily after a bad weather spell, to allow the solar panel(s) to recharge batteries fully, since they would have gone into deep discharge during the cloudy period. Some loads may or cannot be reduced ( e.g. critical telecommunication equipment ), and therefore extra solar power capacity is required to recharge batteries fully and reasonably quickly after bad weather, while at the same time still providing for the normal load.

 

BATTERY CONSIDERATIONS
Other than perhaps solar water pumps and grid-connected systems, solar systems usually use batteries to store the energy produced by the solar panels. The appliances in turn draw energy from the batteries, either directly ( for 12Vdc, 24Vdc, 36Vdc and higher DC voltage appliances / loads) or through inverters, which convert the battery voltage to the required AC voltage of the appliance ( 220V or 110V etc)

A number of factors determine battery choice and size, but for purpose of this discussion, a determining consideration is that the battery should be sized so that the normal daily "draw-down" on it's capacity should be less than 20%. E.g. A nominal 100Amp-hour battery should not be discharged by more than 20% on a daily/nightly basis. That means 20Amp-hour capacity withdrawal per day/night, which translates to 240Watt-hours of energy withdrawal for a 12V battery ( amp-hours x battery voltage = Watt-hours)

Of course, deeper discharges may and do occur ( e.g. bad weather spells, intermittent high loading), but these should be occasional events, after which it is very important to permit the battery to recover to full charge as soon as possible.

In a solar application, battery life is highly dependent on the number and depth of discharge cycles the battery is subjected to. In this regard, it should be noted that different types of batteries have different "cycling" capabilities: a car-type battery can probably withstand 60-100 deep ( 80%) discharges, 12V bloc GEL-type batteries are designed for 400-500 deep ( 80% ) discharges and so on.

INTERMITTENT REQUIREMENTS
Up to now the discussion has assumed that the energy needs would be needed more or less daily for extended periods. However, applications such as weekend/holiday use, appliances are used for short periods of time with relatively long periods on non-use. In such cases one would one would increase the energy storage capacity ( i.e. batteries) and reduce the solar power generators, since after the "weekend", the solar panel(s) can recharge the batteries over an extended period of time.

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