How Many Solar Panels Do I Need For 50 Amps?

If you have been shopping for RV solar panel kits, you have probably seen a lot of 50 amp power kits online. They are often used in campers, but just how many solar panels is 50 amps? And is that enough for a typical RV or boondocking adventure?

The general rule is that a 100 watt solar panel is good for 30 amps a day, so two 100 watt panels is good for 50 to 60 amps. A 100ah lead acid battery in an RV can use 50 amps per day before recharging.

How to Calculate Amps, Watts and Volts

To explain how that 100 watt = 30 amp guideline works, you have to understand watts, volts and amps. To be specific, how to convert one into the other and how to interpret the results.

Solar panel output is measured in watts. Battery capacity is measured in amps. Most batteries and solar panels are 12 volts, but 24 volts and higher are available.

The formulas are simple:

• Watts / volts = amps
• Amps x volts = watts
• Watts / amps = volts

If your 50 amps is 12 volts:

50 x 12 = 600 watts
600 watts / 12 = 50 amps

These conversions assume you are using a 12V battery. If you have a 24V or 48V, replace the number and use the same steps. However the results will differ:

50 x 24 = 1200
50 x 48 = 2400

So the conversions are easy enough. But why did we say that as a rule, a 100 watt solar panel is good for 30 amps a day?

To answer that we have to take a look at how solar panels work, and why you need 2 x 100W panels to yield 50 amps.

Solar Panel Output and Rating

Solar panel ratings are based on maximum possible output. It does not necessarily mean the panel will produce the stated amount consistently.

In theory, a 100 watt solar panel can generate 8.3 amps an hour (100 / 12 = 8.3). With 6 hours of sunlight that is 49.8 amps, almost 50 amps a day. However, solar panels only produce peak output when the sun is at the optimum position. That is, high above the horizon and striking the solar cells. But the position of the sun changes, and this affects solar power output.

Realistically you can expect solar panels to produce peak output at 11:00 or noon. In some cases it might not even reach peak output, but only get close to it.

Instead of 8.3 amps, expect around 5 to 6 amps an hour with a 100W solar panel. With 5 hours of sunlight that is 30 amps total. And with two of those you have 60 amps more or less.

Depending on the weather the two PV modules might give you more than 60 amps. But even in less than perfect climate you should be able to get 50 amps. If you are looking for efficiency and reliability, we recommend the HQST 100W solar panels as they work in houses, RVs and other locations.

PV Module Efficiency Rating

Besides the weather, the efficiency rating is another factor. The greater the efficiency rating, the more energy from the sun gets converted into current.

Efficiency ratings range from 15% for the really low end modules up to 23% for high end solar panels. Those few percentage points make a huge difference especially during imperfect conditions.

In ideal weather any solar panel can produce solar power. But it is during dark overcast days you can separate the high rated modules from the inefficient ones.

Panels with higher efficiency rating may cost more, but you will be happy for it during cloudy days. They will still be able to convert energy into solar power, something inefficient PV modules cannot do.

How Long Will 50 Amps Last?

It would be great if a 50 amp battery lasts for 50 hours right? 1 amp = 1 hour seems logical. Unfortunately, battery capacity and charging does not work that way.

Battery rating is 5 amps per 20 hours, meaning if the load is consistently 5 amps an hour, it will last 20 hours. But that rarely happens because appliance amp draws vary, sometimes more than 5, sometimes less. In the real world, a 50 amp battery may last 10 to 15 hours.

The heavier the load, the faster the battery gets depleted. 5 amps per 20 hours does not necessarily mean drawing 10 amps = 10 hours. It depends on the battery type, age, discharge rate, capacity and operating temperature among other factors.

Battery Type

You cannot use the full capacity of lead acid batteries because it results in deterioration. The recharge point is 50%, so if you have a 50ah battery, only 25ah is available.

Lithium batteries can be drained to 0% and still be recharged. While manufacturers claim this is safe, it is not a good idea to do this regularly. Discharging at the 35% level is the lowest most lithium battery owners allow.

In other words, you will rarely ever have full use of the battery power. At most you can get 75% to 80% with lithium, but you pay a premium for the price. You just have to adjust your calculations according to the battery discharge rate.

Parasitic Draw

Some appliances and devices still draw power even when ‘turned off”. TVs in standby mode, the sleep / hibernate mode in laptops, Blu-ray players, booster antennas etc. All of these are parasitic draws because they consume power even though they are ostensibly shut down.

These are the most likely reasons your battery drains overnight, which is often mistakenly blamed on charge controllers. Check which appliances and devices have a standby mode and you will realize how much of a power draw they are.

Not Fully Charged

If the battery is not fully charged, you won’t get maximum capacity. A lithium battery with a 100% discharge will not last long if you topped it at 70%. This applies to other types of batteries as well.

So the solution seems to be to charge to 100% right? Unfortunately it is not that simple. Topping a battery to 100% is not healthy in the long term. The worst thing you can do to solar batteries is to discharge at 0% and recharge to 100%.

Partial discharges are the most ideal way to use solar batteries. It does not push the internal circuits to the limit and optimizes performance. The drawback is you cannot avail of the maximum capacity.

Age

The older a battery gets, the less efficient it runs. When a battery is new you can expect 90% to 95% efficiency. With age and wear and tear, the battery efficiency goes down and after a few years, may drop to 60% or lower.

Have you noticed how a battery loses power faster than it used to even if you run the same load? Or how much longer it takes to recharge now? Those are signs of wear and tear and that it is nearing the end of its life cycle.

Is 50 Amps Enough for an RV?

RV power consumption varies, but most are in the range of 30 to 50 amps. While 30 amps a day is still common, it makes more sense to switch to 50 amps.

50 amps can run a lot of appliances and devices. For an RV this is usually the amount consumed per day. With a 100ah lead acid battery and 50% discharge rate, you will get a lot of mileage out of it.

50 amps is equal to 600 watts, which is capable of running:

• Several LED bulbs
• 200 to 500W computer
• 3 cup coffee maker, usually 400 to 500W
• 50 inch TV, about 100W
• Portable fan, which ranges from 50 to 100W
• Slow cooker from 70 to 100W

And you can probably run a lot more than these depending on your RV setup.

You consume 50 amps a day, so you decide to get a 100ah lead acid battery. 100ah is the right size because you can only use half of the capacity.

To recharge the battery you use 2 x 100W solar panels. Presuming typical weather conditions, each module produces 30 amps a day for 60 amps in total.

You can run appliances and devices on the two solar panels during the day. At night you still have 50ah left because the battery has not been used. That is 100ah available every day.

When morning comes and the battery is 50% empty, you can recharge it with the solar panels. You can still power your devices because there is still 50% left in the battery. As long as there is power coming in from the solar panels, you can keep going.

For the purpose of this guide we are using 50 amps as example. But you can use other solar panel sizes too. In fact you can charge batteries on electricity for even longer power use.

Conclusion

One of the things newcomers to solar power learn is there are a lot of numbers involved. Lots of formulas and conversions to work through. Hopefully this guide was able to help you understand how many amps you need and if 50 amps is sufficient.