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: |
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: |
Working out losses more closely: (PS. this adds upwards) |
PV module (solar panel) calculation: |
Battery calculation: |
Inverter sizing: |
Charge controller calculation: |