Do It Yourself

Designing an Off Grid Solar Electric System

First of all, you need to understand difference between power and energy. Power over a time is energy. For example, typical kettle (power is written on the label, let’s say it is 2kW), working constantly over one hour will consume 2kWh of energy. Typical family home with gas heating is using about 10kWh of electric energy per day.  

Heating with electric energy is waste as it is much more efficient to run heat pump compressor to heat (about 3 times more efficient than heating directly using electric heater or immersion element).  

Let’s assume you have gas for heating and cooking, you not wasting energy and all you need is 5kWh of energy per day which is 5kWh x 365days = 1825 kWh per year.  South facing 2kW solar system will deliver just about that amount over the year, but problem is that most of it during summer. There could be a day where it will generate 8kWh a day and there can be a day when you generate not even 1 unit over day. 

Next step is battery capacity given in Ah on data label. In simple words typical 12V car battery with capacity of 100Ah should deliver current flow of 100Amps over 1h, or 10Amps over 10hours, or 1Amp over 100hours. There is loses involved but lest assume it is perfect battery ☺.  

1Amp flowing from 12V battery can deliver 12Watts of power! As stated above 1Amp can flow 100hours so that battery should deliver 12Watts over 100hours which is 1200Watt hours or 1.2kWh of energy.  Now cut that half and you about to have real values for car battery which is not designed to store energy.  

Gel and AGM lead batteries are closer to those values but still they not designed to give 100% of it’s capacity where lithium batteries are closest to those values. 

So far we know that we need 5kWh of energy per day, 2kW solar system can deliver 0.2 to 8kWh of energy per day, and 12V 100Ah lithium battery can store 1kWh of energy. 

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