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November 2016

Your ultimate guide for solar panel and battery matching

For solar panels between 10W and 150W, here’s your ultimate guide on what battery to use for charging. During sunny days, we can expect an equivalent charging time of between 4-6 hours irregardless of solar panel size. Even though there are 12 hours of daytime, the equivalent energy harvest for solar in the Philippines is 4-6 hours.

Here’s how to use the table. If you need to charge a 10Ah battery, you would need at least a 20W solar panel to fully charge it. The “solar time” it will take for it to get fully charged is 4.4h. If you have a 50W solar panel, a 20Ah battery can be fully charged within 3.5h. Our recommendation is to use only the solar panels covered in the yellow area for each battery capacity between 10Ah to 75Ah.
The reverse is also true, if you have a 10Ah battery, don’t bother using a 10W solar panel to save on cost because the battery will never be fully charged, even on sunny days. There isn’t 8.8 hours of solar time to do that.
Note that we assumed lead acid batteries here where only 50% of its rated capacity is charged and discharged over its lifetime. Getting more than 50% will severely degrade and reduce the lifetime of a lead-acid battery. So the usable energy for a 20Ah battery is effectively only 10Ah. You can have 10h of run time for a 1A load. A 10W bulb for example can run the whole night on a 20Ah fully charged battery.
Connecting 2 solar panels in parallel will double the charging current and thus reduce the time it takes to fully charge a battery. When you connect 2 100W panels for example, you double the charging current to about 10A, reducing the time it takes to fully charge a 75Ah battery by half. So instead of 6.6 solar hours, it will only take 3.3 solar hours for the 75Ah battery to get fully charged.

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