Electric Solar System – Calculating the Power

If you are interested in applying the electric solar system for your residence you might want to know how to roughly calculate or design the system. There are three main solar kits involved in this calculation: Your home solar panels, solar batteries and solar inverter. Let’s have a look on them one by one:

Home Solar Panels

The electric solar panels output is rated in watt (W) which equals to the voltage (Volt) multiplied by the current (Ampere). The most common output is 12VDC which is the nominal voltage of the solar batteries. To charge a 12VDC battery you will need voltage higher than 12VDC so by default most of the solar panels are designed to produce an effective output up to 17VDC in a hot sunny day.

If you have a 12VDC 100 watt solar panels, the maximum current per hour produced in direct sunlight is: Current = Power / Voltage = 100W/17VDC = 5.9A.
Note that the actual voltage generated by the solar panels is 17V so we have to put this number into calculation instead of using the 12V rated voltage.
The next factor to put into calculation is the average hours of peak sun per day in your area. If you have 5 hours of peak sun the power generated by your electric solar system per day will be 100W X 5hr = 500 watt-hours.

Solar Batteries

Solar batteries are rated in Amp-hours and designed for deep cycle discharging. Ideally you should give 20% extra capacity to anticipate heavy discharging during cloudy or no sun condition.

What does it mean if you have a 12V 350Ah battery? Multiply the voltage by the amp and you will get 4200 Wh or 4.2 kWh power kept in the battery.
If you prefer a 24V system you can put two 12V batteries in series, similar to stacking up 2 X 1.5V batteries for your flash light. The current will remain the same but the voltage is doubled.

Connecting 2 X 12V batteries in parallel will double the current but the voltage will remain 12V.

Solar Inverter

Solar inverter is needed to convert DC voltage into AC. To power your 6 Amp 120VAC inverter with 5 hours duty cycle using a 12VDC battery you will need a 30 amp hour load (6 A X 5hrs). This is not the real drain value of the electric solar system. You should divide the load voltage by the battery voltage, in our example here is 120/12 = 10 and then multiply it by the inverter’s amp hours (30 Ah). Total current needed from the battery to power your inverter here is 300 Ah. If you add a 20% margin you will need to buy a 360 Ah 12VDC battery.

The SolarCost Admin

Related posts:

1. Solar Kits
2. Home Solar System – Useful Hints
3. Portable Solar Panels
4. Solar Charge Controller
5. Solar Inverter