Off Grid Home Forums Technical Discussion Solar car battery chargers – what do I need? Re: Solar car battery chargers – what do I need?


You are going to hate me but the answer is almost never.

If you just take the numbers you get

13w = almost 1 amp

75 amp batt full flat

so 75 hours of sunlight.


you should not take the bat to more than 20% discharged for long bat life or to 80% discharged for a short bat life.

Also charging a bat is very ineficient so for every amp you use you need to input 1.5 amps. Add in the self discharge rate of the bat & you need even more. A 13 watt panel will keep you full battery full whilst NOT in use. Count on 5-8 hours of power in summer & 1-2 in winter. So in summer you might get 5 – 8 amps from the panel resulting in 3 to 5 amps into the batt per DAY. A better bet is to work out how much power you actualy use daily / weekly whilst in the van. Then calc the bat size needed & the charging source & size needed. To give you some idea a standard kettle would need 230 HOURS of the 13watt panel charging a batt to run for ONE hour.

Will you be able to charge from site power, the tow car or a genny?

We are looking at going of grid soon & will need a 2kw (2000 watts) solar panel to provide the needed charge rate to supply less than 1kw of daily power year round. Just for the short summer we can get away with a 400watt pannel as there is more sunlight hours.