How to size Solar Charge controller & Inverter?

Charge controllers, prevent excessive overcharge of the batteries within a battery based power system. Unlike other types of generators, solar panels can be short circuited or open circuited without causing damage to them. Controllers contain a relay that opens the charging circuit, terminating the charge at a pre-set high voltage and, once a pre-set low voltage is reached, closes the circuit, allowing charging to continue.

We will later go into more detail about different Solar Charge controllers. But for now let's state that if you will have a total Solar panel watt under 200 watt and are under a budget you can go for the cheaper PWN controller. If you want maximum efficiency and have more than 200 watt then go for a MPPT controller.

Now, to figure out how big your solar charge controller would need to be for your calculated parameters from . you might need to take your solar panel current or the Amperage specs into consideration, which may be simply gotten by dividing the panel's wattage rating with its voltage rating.

Example 8:

100 watt / 12 volt = 8.3 Amps.

We have so far applied a "plus tolerance" to all the previous parameters, so let's show some generosity to the Amp spec of the panel also, and instead of sticking to the 8.3 amps limit, you might be happy raising the level to around 10 Amps.

Example 9:

Take the number of panels x watts to get the total watts of the solar array. You then divide it by the voltage of your battery bank to get amps, add 25% to allow for cold temperatures and as always, round up. Example: 2 - 140 watt solar panels in series = 280 watts / 12 VDC battery bank + 25% = 29.18 amps. You would choose a 30 amp, 12 VDC charger in this example. Another example would be 4 - 250 watt solar panels = 1,000 watts / 24V battery bank = 41.7 amps + 25% = 52.09 rounded up = 60 amp controller. Note; Solar charge controllers are rated and sized by the solar panel array current and system voltage. These examples are simple calculations for smaller systems.

Example 10:

 Go to the Solar charge manufacturer website and check if they have a calculator.

For Victron Energy go to


Assessing Inverter Specifications

Finally we boil down to the inverter specifications, and determine the reasonably exact capacity that would keep the unit compatible with the discussed results, and keep the load running without issues, whenever required. When you reach an inverter for 2000 watt and current over 100 amps then you need to carefully size all components and cables so you do not burn anything up.

If you have made a list of the things you will power, then add everything up that you know you will use at the same time to get your peak watt figure. Here you might want to add some extra becuse you might in the future add more stuff.

Let's say from Example 6 refrigerator 200 watt when running, two lights at 75 watt and charging three phones at around 30 watts each.

200+150+90= 440 watt so you want to go for a minimum 500 watt inverter


Next: Look at DIY Solar 12v vs 24v vs 48v? or Plyg n play systems