#solarpanels #solarpower #solarbattery

Portable solar panels come in all shapes and sizes. The 80 watt solar panel is one of the more popular, offering a nice balance between power and size. You can use it for any number of applications, off grid or grid tied. But can an 80 watt solar panel charge a 12 volt battery?

The answer: it will take 3 days for an 80 watt solar panel to charge a 12 volt 100 amp hour battery provided there are 5 hours of sunlight available. The less sun hours available, the longer the charging time will be.

So How Much Power Does an 80 watt Solar Panel Produce?

An 80 watt solar panel can produce 400 watts in 5 hours, assuming ideal conditions and peak sunlight. The solar panel however, cannot produce 80 watts an hour the whole day. In fact, most panels produce about 70%-90% of the maximum capacity. If you want to charge a 75 amp hour 12 volt battery, it could take a couple of days.

It would be nice if these solar panels always produced 80 watts, but there are numerous factors that affect energy production. These include available sunlight, panel design, location etc.

Let's say you have an 80 watt solar panel and want to charge a 30 amp hour 12 volt battery. Suppose there are 6 hours of sunlight available. To calculate:

Multiply 30 amp hours by 12 volts, that is 30 amp hours times 12 volts.

Then divide watts by sunlight hours available, and you get the solar panel watts you will need.

In this scenario, multiply 30 by 12 and you get 360. Divide 360 by 6 and you get 60 watts. So an 80 watt solar panel is sufficient to charge a 30 amp hour 12 volt battery in 6 hours. If you have a larger solar panel then the charge time will be faster.

Can this same solar panel charge a 40 amp hour battery? Let's do the calculations.

40 amp hours times 12 volts = 480.
480 divided by 6 hours equals 80.

You get 80 watts, which is right at the limit. Does this mean an 80 watt solar panel can charge a 40 amp hour 12 volt battery? No, because some energy is lost during the procedure. We can only get the theoretical maximum of 80 watts if the solar panel is perfectly oriented towards the sun and there will be no clouds during the 6 hours of charge time.

That's not likely to happen so you should get a larger solar panel to compensate for it. If the solar panel is 80 watts, assume that it can produce up to 60 to 75 watts maximum. This applies to other solar panel sizes as well. Note that this calculation assumes there are 6 hours available. You have to adjust the hours as necessary, but the steps are the same.

So What about Output Panel Voltage & Charge Controllers?

The voltage panel on solar panels affects charge time as well. All open circuit panels have a voltage difference, and this lowers the voltage that can be drawn from it. For 80 watt solar panels the maximum point is usually 20 volts 4 amps.

What does this mean? If you use a PWM charge controller, any power or voltage at or above 14.4 volts is discounted. PWM charge controllers only use 14.4 volts as needed for loading, The rest is discarded. If you have an MPPT charge controller, you will be able to use more power and voltage.

So What is the Safe Battery Discharge Level?

The discharge level refers to how low the power has dropped. With lead acid batteries it should not be allowed to drop below 50%. If you have a 70 amp hour lead acid battery, recharge it when the level drops to 35 amp hours. This is necessary to prolong the lifespan of the battery. You'll have to replace it eventually, but you put off that time by years.

Lithium ion batteries are a different matter. Some manufacturers say their battery can be discharged to 0%, while others say recharge at 35%. Check the instructions that came with your battery for the best results. So how does this affect the performance of an 80 watt solar panel?

Discharging a lead acid battery at 50% does two things. First, you won't need as much power from the solar panel. In our calculations we used the whole battery capacity, under the assumption it was 100% drained. But what if you recharge it at 50%?

It would look like this:

20 amp hour times 12 volts = 240
240 divided by 6 = 40

Recharging a lead acid battery at 50% only requires 40 watts, something your 80 watt solar panel can handle. Even if there are only 5 hours of sunlight available, the amount required is still within reach.

20 amp hours times 12 volts = 240
240 divided by 5 = 48

Rounding off you need 50 watts to charge the battery, which your solar panel can produce. This calculation assumes the solar panel produces 20 amps per hour.