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- What size charge controller do I need for 400 watt solar panel?
- How much power does a 400 watt solar panel produce?
- How does solar irradiance affect solar panels?
- How many amps does 400 watts produce?
- How do I choose the right size solar charge controller?
- Choosing the right size solar charge controller
- How long will it take a 400 watt solar panel to charge my battery?
- What can you run off a 400 watt solar system?
- How many batteries do I need for a 400 watt solar system?
What size charge controller do I need for 400 watt solar panel?
As a general rule, the average 400 watt solar panel has a current output of 10 amps and open circuit voltage of 50 volts. A charge controller size of 60 volts input and current rating of at least 20 amps will be needed. MPPT charge controllers are preferred over PWM.
A 400 watt solar panel measures about 80 inches x 40 inches (2032mm x 1016mm), or an area of 22 square feet (a little over 2 square meters!)
It’s the biggest size of solar panels – it weighs about 50 pounds (22kg) and tends to be used for complete installations mounted on roof-tops. It’s more cost-effective to install the larger wattage panels.
However, if you did want to use just one or two, for battery charging or building a backup solar generator for home or camping, then you’ll need to know the electrical specifications.
Video – 400 Watt Solar System Install With MPPT Charge Controller
How much power does a 400 watt solar panel produce?
The Electrical Properties shown in the image below are taken from an LG solar panel specification sheet.
It shows the voltage and currents associated with the LG 400 watt panel, also efficiency.
Notice that all of the values depend on the Standard Test Conditions (STC). Basically, these are ideal laboratory conditions and are very rarely found in real-life situations.
In general, you can expect about 75% of the power output described in the manufacturer’s spec sheet, for a 400 watt panel, read 300 watt to be realistic.
For sizing a solar charge controller the two numbers that interest us are the Open Circuit Volts (Voc) 49.3 volts, and MPP Current (Impp) 9.86 amps.
MPP stands for Maximum Power Point. (read more about MPP here.) This is when the values of voltage (Vmppp) and (Impp) give maximum power output in watts:
Vmpp 40.6 x Immp 9.86 = 400.3 watts (STC)
Furthermore, the MPP value happens when the internal resistance of the connected load is equal to the internal resistance of the solar panel, called the Characteristic Resistance.
This becomes important when choosing the type of solar charge controller to use (more later on.)
Not all 400 watt solar panels have the same specifications.
The table below compares some popular solar panels:
Table – 400 watt solar panel specifications compared
# of cells
Vmp (volts max power)
Price guide ($)
Power at MPP
kWh/day at 4 peak-sun-hours
kWh/yr 4 peak-sun-hours
2,067mm (81.4in) x 1,046mm (41.2in) x 54mm (2.1in)
1,644mm (64.7in) x 1,204mm (47.4in) x 40mm (1.6in)
2,008mm (79.1in) x 1,002mm (39.4in) x 40mm (1.6in)
2000 X 992 X 35 mm
22.5 kg (49.6 lbs
How does solar irradiance affect solar panels?
There are many factors affecting solar panel output power and a complete solar power system suffers from many losses – see infographic below:
Read more about Solar System Losses.
Infographic: Solar PV System Losses
By far the biggest factor affecting output is irradiance. This is a measure of the sun’s energy reaching the solar panels and varies by location.
It’s measured in kilowatt-hours per square meter per day or year (kWh/m2/day) or peak-sun-hours and is used to determine how much energy a solar panel will deliver where you live.
Peak-sun-hours is found for your location by using historical data from a site like Globalsolaratlas.
Simply enter you city and read off the value in peak-sun-hours.
Multiply the value by the solar panel rated wattage to find how much energy in watt-hours the panel will deliver per year.
The example below is for Las Vegas:
2734 peak-sun-hours x 400 watt = 1093.6 kWh/year
How many solar panels would you need to power your home?
How many amps does 400 watts produce?
A typical value of current for a 400 watt solar panel is just under 10 amps. Note that this is not much less that the panel’s Short Circuit Current (Isc) which is 10.47 amps.
If you ever have a solar panel with no label, this is an easy way to approximate the maximum current possible under working conditions.
It just so happens that Impp is about 95% of Isc, but how do you find Isc?
Simply set a multimeter to its current setting and use it to short the solar panel leads together. ISC will be shown on the multimeter scale.
Multiply the value by 0.95 and you have the panel’s maximum working current.
Note: Shorting solar panel leads together through a multimeter can’t damage the panel. Because the panel lead resistance is so low, the voltage is effectively zero.
This means power (watts) is also zero:
0V x 10.47 amps = 0 watts
How do I choose the right size solar charge controller?
Before choosing the size of charge controller, we need to talk about the type of controller to use.
There are two basic types – MPPT or PWM.
What is the difference between MPPT and PWM solar charge controllers?
The cheapest and most basic type of solar charge controller is called Pulse Width Modulation (PWM).
Basically, it chops up DC current into square-shaped packets and adjusts to voltage level by changing the width of the pulses.
An MPPT (Maximum Power Point Tracking) controller is more expensive and operates in an entirely different way.
I previous said that the Maximum Power Point of any solar panel is when the load’s internal resistance matches that of the panel – this is how the MPPT controller works.
The controller searches for and matches the impedance that will give the maximum power output for the prevailing sola rpanel conditions.
It always delivers the maximum charge current that the solar panel can produce.
My own tests have shown that an MPPT charge controller can deliver up to 40% more power than PWM – I recommend using them in general.
Renogy Solar Charge Controllers
Choosing the right size solar charge controller
The two important values to consider are input volts from the solar panel and the amount of current that will pass through the solar charge controller.
Check the specs for any basic 12 volt output solar charge controller, but a basic value for input voltage is 0 to 60 volts, with currents ranging from 10A, 20A, 30A, 40A or 60A.
Voc (Open Circuit Voltage for a 400 watt solar panel is just under 50 volts, so a 60 volts controller would be OK, with current rating of 20A (400 watt panel output current is about 10 amps).
I tend to use a controller that’s twice the current rating I need, just in case I want to expand solar panel power later.
What is the difference between series and parallel solar panels?
If you want to use two 4oo watt solar panels to double your power output, be careful how you connect the two together.
In parallel you’ll double the output current to 19.72 amps, so you might want to upgrade to a solar charge controller with a higher current rating.
In series the current stays the same, but the voltage doubles to 98.6V, so you’ll have to upgrade to a solar charger model that will accept a higher voltage.
How long will it take a 400 watt solar panel to charge my battery?
If using an average value of 4 peak-sun-hours for irradiance, then a 400 watt solar panel will generate 1600 watt-hours of energy during the day.
What size battery would this amount of energy charge?
1600 watt-hours is equal to 133Ah. Assuming a deep-cycle lead-acid battery with 50% Depth of Discharge, then the battery size that could be recharge in a day would be 266Ah.
This doesn’t seem to make sense, as a 400 watt panel can generate just 10 amps of current, and 133Ah divided by 10 amps = 13.3 hours, minimum.
However, remember that the maximum power voltage is going into the solar charge controller is 40.6V – over 3 times more than the battery charging voltage.
The beauty of MPPT charge controllers is that they convert the maximum power into a higher charging current i.e. the higher voltage gets converted to approximately 14 volts and the extra voltage is converted into extra current. (A very simplified explanation!)
What can you run off a 400 watt solar system?
A 400 watt solar panel will run a variety of home appliances, but an inverter will be required to convert the DC volts from the solar panel to the AC volts required by most appliances.
It isn’t a good idea to rely on the sun continuously, due to shading issues. Also, if appliances need to be run at night then batteries will be required to ensure a secure supply of power.
The table below gives some idea of the type and power ratings of common appliances that a 400 watt panel could power:
Electric Can Opener
Home Sound System
Electric Fence (25 miles)
Electric Hedge Trimmer
How many batteries do I need for a 400 watt solar system?
A 400 watt solar panel can deliver on average 16oo watt-hours of energy per day. This is calculated by:
400 watts x 4 peak-sun-hours = 1600 watt-hours
If you wanted to store this energy in a battery for night-time use, then it would seem that a battery of 1600 watt-hours would be OK. The equivalent Ah for a 12 volt battery would be:
1600 watts/12 volts = 133Ah
Unfortunately, it doesn’t work like that in the real world!
In the case of a deep-cycle lead-acid battery, a 50% depth of discharge is recommended to maximize battery life, so you only have access to 50% of the rated capacity.
This means that the 400 watt solar panel system would need a 266Ah capacity battery.
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