Solar Formulas

W = Watts (Wh)  |  V = Volts  |  A = Amperage {I = Current}  |  T = Time

Watts = Volts x Amps

WattHours = Volts x Amps x Time

Amps = Watts / Volts

Volts = Watts / Amps


How to calculate your solar power requirements:
There are three things to consider when choosing a solar panel or creating a solar system. You need to know what appliances you will be using and how much energy they require to work, how much energy your battery can store and which solar panel will replenish your stock of energy in the battery in line with your pattern of use.

How much energy will your appliance/s use over a period of time?

The power consumption of applicances is given in Watts (e.g. A 21" florescent light is 13W)

To calculate the energy you will use over time, just multiply the power consumption by the hours of use. The 13W light, on for 2 hours, will take 13 x 2 = 26W will be used for the 2 hour period. Repeat this for all the appliances you wish to use, then add the results to establish total consumption

How much energy can your battery store?

Battery capacity is measured in Amp Hours (e.g. 12v18ah). You need to convert this to WattHours by multiplying the ah figure by the battery voltage: 12v x 18ah = 216Wh (This is how much Watts is available in total. NB: NO STANDARD BATTERY SHOULD BE DISCHARGED FOR MORE THAN 50% - meaning the 18ah should use only 9ah or 108Wh in total of its capacity to maintain a healthy status)

The 13W light can use +-8 hours of the 108Wh on its own = 13W x 8 hours = 104Wh in total used, if 3 lights are used at 13W each = 3 x 13 = 39W / 108Wh = 2.7 hours available to be used if all 3 light are on at the same time

12v7ah = 84Wh | 12v18ah = 216Wh | 12v50ah = 600Wh | 12v100ah = 1200Wh (Battery storage in WattHours)

How much energy can a solar panel generate over a period of time?

The power generation rating of a solar panel is also given in Watts. To calculate the energy it can supply to the battery, multiply Watts by the hours exposed to sunshine, then multiply the result by 0.85 (this factor allows for natural system losses). For the solar 10W panel in 4 hours of direct sunshine, 10W x 4 hours x 0.85 loss = 34Wh (The solar panel is charging the battery to 34W (or 2.8ah = 34Wh / 12v) during the full 4 hours of direct sun)


Some examples with losses (more in depth specs)
Find out what the Wattage (Amperage) is of the appliances you wish to run. Most appliances have a rating at the back of the unit

1 x Small LED TV = 100W  |  5 x Energy saving lights (15W each) = 75W  | 1 x Small fridge = 200W

Once you have the correct rating you need to work out how long you will be using each appliance:
1 x Small LED TV (100W) for 5 hours = 500Wh
5 x Energy saving light (75W) for 8 hours = 600Wh
1 x Small fridge (200W) for 6 hours = 1 200Wh
Total Watts per day = 2 300W
Total Watts per hour = 375Wh

Working out losses more closely: (PS. this adds upwards)
20% for battery | 10% for inverter | 10% for distribution loss
2 300Wh per day / 0.8 (20%) = 2 875Wh (20% for the battery)
2 875Wh / 0.9 (10%) = 3 194Wh (10% for the inverter)
3 194Wh / 0.9 (10%) = 3 549Wh (10% for distribution loss)
Total usage = 3 549Wh

PV module (solar panel) calculation:
South Africa enjoys on average about 5 hours of sunlight. To work out the amount of panels required:-
3 549Wh / 5 hours = 710Wh (per hour)
Add 9% for temperature coefficiency of the PV panels = 710Wh / 0.91 = 780Wh
Devide your solar panel size needed into the daily Watthour required = (Say you use 200W 24v solar panels) 780Wh / 200W = 4 x (200W24v) solar panels will be required

Battery calculation:
For battery sizing you always need to cater for a 50% discharge to prolong the life of your batteries. Any extra discharging will dramatically reduce the life span of your batteries.
Battery size = Daily usage x 2 (for the 50% factor) / System voltage = 3 549Wh x 2 (50% factor) / 24v = 296aH at 24v.
The easiest way to get to this is 6 x 100aH (12v100ah) batteries = 600aH (remember the 50% factor)
(3 x connected in parallel and 2 x connected in series). This battery bank setup will give your around 1 day storage. If 2 days or more is required for backup purpose please just times with the days required

Inverter sizing:
When sizing your inverter you need to use your total Watt per hour. In the case of the example your total Watts per hour = 375Wh
In this case you would need around a 500W to 800W Pure Sine Wave Inverter.
NB: When bying inverters for computers, TVs etc (all electrical appliances) please always ask for a Pure Sine Wave Inverter)

Charge controller calculation:
Charge controller = Total PV Wp (solar power per hour) needed / System Voltage (24v)
(780Wh rounded) = 800Wh / 24v system used = (33.33A) The closest controller size would be 35A Charge controller to be used

Back to top