SOLAR POWER SYSTEMS: a FEW RULES
to determine their CAPACITY.

The capacity of solar power systems is defined by the number of solar panels installed and the size of the battery bank.

With more capacity, you will be able to run more and bigger appliances at the same time for longer periods of time. The house batteries only act as a temporary energy storage place.

As a rule:

  • never discharge a battery bank more than 25 % of its capacity.
  • have enough charging power in the solar power kit to recharge the battery bank within the next day.

The solar energy collected in daytime by solar power systems will need to have the capability to recharge in full your deep cycle batteries (specifically designed to be charged and recharged) in this reasonable time.

This means that:

  • the charging power of the solar power systems have to be greater than the daily consumption.
  • the capacity of the battery bank has to be sufficient to store that solar energy until it is used.

Very often, people think of adding more batteries to solve their electricity power shortage but the problem is not really there.

They need to add more solar panels to their solar power system.

Everything has to be well balanced with the daily consumption you have. This is why you need to know what is your daily amp-hour consumption.

We did this exercise when we covered the sizing of the battery bank.


HOW MANY SOLAR PANELS IN YOUR SOLAR POWER KIT?

Knowing your daily energy consumption, it will help you determine how big a solar power kit you need.

Each solar panel is rated in watts which is derived from multiplying its peak voltage by its peak power amperage. (remember watts = volts X amps)

You can have a 100 watt panel which has a a rated peak power of 16.1 volts and 6.2 amps for example. The information is available in the spec sheet of the panel.

Notice that this 6.2 Ah is its maximum capacity of producing energy per hour in standard test conditions rarely found in reality. Also, this figure comes with a factor of tolerance in the production of the panels mentioned by the manufacturer. This factor is between 5% and 10 %.

A 5 % factor is preferable since it tells you that the real output should be within 5% of the rated peak power mentioned in the spec sheet. (5.89 to 6.2 with a 5% factor)

How many amps solar panels will produce in a day in the real world is unknown.

Obviously, it depends on the day weather and the season. A winter cloudy day with less daylight hours will produce less power than a blue sky mid- june day.

The latitude of your location will also play a role. (the more south, the better).

If for example, your daily consumption is around 55 Ah, you would ideally need a battery bank with a capacity 4 times larger then 220 Amp-hours.

Two 6-volt golf-cart batteries of 220 amp-hours in series would do. This respects the rule that you should not discharge your battery bank daily, more than 25 % of their total capacity.

There is another guiding rule that stipulates : use 1 watt of solar panel output for every 2 amps of battery bank capacity.

With the same 220 Ah battery capacity (and 55 Ah of daily consumption representing 25% discharge), using this 1:2 guiding rule, we should have one 110 watt solar panel or two solar panels of 55 watts as a minimum. (110 watts : 220 Ah)

But if you are RVing more often in the northern part of the continent with less hours of sunlight, just to be safe, a 1.5 watts output for 2 Ah capacity is better.

With is rule in mind, we are going to necessitate 165 watts of output or a solar panel kit of two 85 watt solar panels.

Finally, knowing that for every amp used, factor a need of 1.2 amp to return due to inefficiencies of the solar power system, this means that for our 55 Ah daily usage, we need to return 66 amps.

To be conservative, if we take into account only 5 hours of sunlight per day, divide this 66 Ah by 5 hours/ day and it equals 13 amps (with rounding) of energy we have to provide every hour with the solar power kit installed.

Following with this example, our two panels of 85-100 watts each with a minimal rated peak power of 6.5 amps each (66Ah/5 = 13Ah) should be sufficient for most of our needs.

Keep in mind that these are minimum requirements or minimal charging capabilities respecting different guiding rules.

With solar energy, it is always better if you can afford it, to have a solar power system with more capacity than less.


IN SUMMARY:

- with an estimated 55 Ah of daily usage, requiring a 220 Ah battery bank (25 % daily discharge rule), you need 66 amps of output (55Ah X 1.2 factor).

This is provided by a solar power system totalizing around 165 Watts (1.5 watts : 2.0 Ah battery bank capacity) in the northern hemisphere.

Two 85 to 100 watts panels with a minimal rated peak power of 6.5 amps each should be sufficient.

When you select your solar panels, the rating for the peak power voltage is also very important to keep in mind knowing that the voltage will decrease as the cell temperature increases.

You want to have access to a voltage as high as possible since it is the voltage that charges the battery.

Some RVers decide to attach in seriestwo 12-volt solar panels to build the equivalent of one 24-volt panel, the same we do with batteries.

This has for effect to increase substantially the voltage to charge the batteries.

If you decide to do that by simply buying off-grid 24-volt panels which exist already, always make sure that you have the space to install them all on the roof of the RV.

These panels are bigger and you need to select the proper controller . It will have to bring down the output from 24 to 12 volts since we have a 12-volt battery bank system (you remember). These controllers are available.

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