The charge controller Amp (A) rating should be 10 to 20% of the battery Amp/hour (Ah) rating. For example, a 100Ah lead-acid battery will need a 10A to 20A solar charge controller, and a single 150W to 200W solar panel to generate the 10A* charge current needed for the battery to reach the battery adsorption voltage.

Always refer to the battery manufacturers specifications.

Before sizing a charge controller and purchasing panels or batteries you should understand the basics of sizing an off-grid solar power system. The general steps are as follows:

• Estimate your loads – how much energy you use per day in Ah or Wh
• Determine the battery size needed in Ah or Wh
• Determine how many solar panel/s you need to charge the battery (W)
• Choose the Solar Charge Controller/s to suit the system (A)

The first step is to determine what loads or appliances you will be running and for how long? This is calculated by – the power rating of the appliance (W) multiplied by the average runtime (hr). Alternatively, use the average current draw (A) multiplied by average runtime (hr).

• Energy required – Watt hours (Wh) = Power (W) x Time (hrs)
• Energy required – Amp hours (Ah) = Amps (A) x Time (hrs)

Once this is calculated for each appliance or device then the total energy requirement per day can be determined as shown in the attached load table.

The total Ah or Wh load is used to size the battery. Lead-acid batteries are sized in Ah while lithium batteries are sized in either Wh or Ah. The allowable daily depth of discharge (DOD) is very different for lead-acid and lithium,

On average, Lead-acid batteries should not be discharged below 70% SoC (State of Charge) on a daily basis, while Lithium (LFP) can be discharged down to 20% SoC on a daily basis.

Lead-acid (AGM or GEL) batteries can be deeply discharged but this will severely reduce the life of the battery if done regularly.

For example: If you have a 30Ah daily load you will need a minimum 100Ah lead-acid battery or a 40Ah lithium battery. However, taking into account poor weather, you will generally require at least 2 days autonomy – so this equates to a 200Ah lead-acid battery or an 80Ah lithium. Depending on your application, location, and time of year, you may even require 3 or 4 days autonomy.

The solar size (W) should be large enough to fully charge the battery on a typical sunny day in your location. This is not simple as there are many variables to consider including panel orientation, time of year & shading issues. This is actually quite complex, but to simplify things we can roughly work out how many watts are required to produce 20% of the battery capacity in Amps. Oversizing the solar array is also allowed by some manufacturers to help overcome some of the losses – see more details below.

Solar sizing Example: Based on the 20% rule, A 12V, 200Ah battery will need up to 40Amps of charge. If we are using a common 250W solar panel, then we can do a basic voltage and current conversion – 250W / 12V battery = 20.8A. So we would need at least 2 x 250W panels to get close to 40Amps charge.

Remember there are several loss factors to take into account so slightly oversizing the solar is a common practice.

The MPPT charge controller size should be roughly matched to the solar size. A simple way to work this out is using the power formula:

Power (W) = Voltage x Current or (P = V*I)

If we know the total solar power in watts (W) and the battery voltage (V), to work out the maximum current in Amps we re-arrange this to work out the current (I) – so we use the rearranged formula:

Current (A) = Power (W) / Voltage or (I = P/V)

For example: if we have 2 x 200W solar panels and a 12V battery, then the maximum current = 400W/12V = 33Amps. In this example, we could use either a 30A or 35A MPPT solar charge controller.

Due to the various losses in a solar system, it is common practice to oversize the solar array to enable the system to generate more power during bad weather and under different conditions such as high temperatures where power derating can occur. The main loss factors include – poor weather (low irradiation), dust and dirt, shading, poor orientation, and cell temperature de-rating (refer to the power temperature co-efficient on the solar panel spec sheet for more details

The various loss factors can add up as high as 20%. For example, a 300W solar panel will generally produce 240W to 270W in summer due to temperature power de-rating, and in winter or due to the low irradiance, depending on your location. For these reasons, oversizing the solar array beyond the manufacturers ‘recommended or nominal values’ will help to generate more power.

Oversizing by 150% or more is even possible on some higher-spec MPPT solar charge controllers. However, not all solar charge controllers are designed to handle the excess power when the solar is operating at full capacity and this can damage some controllers. Therefore, it is important to always check the manufacturer allows oversizing.

Also, you must NEVER exceed the maximum input voltage (Voc) or maximum input current rating of the solar charge controller!

IMPORTANT – Oversizing solar beyond the manufacturer’s recommendations is allowed on some higher-end MPPT solar charge controllers such as those from Victron and Morningstar. Oversizing can void your warranty and could result in damage or serious injury to persons or property – always ensure the manufacture allows oversizing and never exceed the maximum input voltage or current limits.

As mentioned, all solar charge controllers are limited by the maximum input voltage (V – Volts) and maximum charge current (A – Amps). The maximum voltage determines how many panels can be attached (in series), and the current rating will determine the maximum charge current and in turn what size battery can be charged.

As described in the guide above, the solar array should be able to generate close to the charge current of the controller, which should be sized correctly to match the battery. Another example: a 200Ah 12V battery would require a 20A solar charge controller, and a 250W solar panel to generate close to 20A. (Using the formula P/V = I, then we have 250W / 12V = 20A).

As shown above, a 20A PRISM 60V/20 MPPT solar charge controller together with a 12V battery can be charged with a 250w ‘nominal’ solar panel. Due to the losses described previously, it could also be used with a larger ‘oversized’ 300W to 330W panel. The same 20A Prism charge controller used with a 48V battery can be installed with a much larger solar array with a nominal size of 1160W.

Below is a simple guide to selecting a solar array to match various size batteries using the Rover series MPPT charge controllers. 

20A Solar Charge Controller – 50Ah to 150Ah battery

• 20A/100V MPPT – 12V battery = 250W Solar (1 x 250W panels)*
• 20A/100V MPPT – 24V battery = 520W Solar (2 x 250W panels)*

40A Solar Charge Controller – 150Ah to 300Ah battery

• 40A/100V MPPT – 12V battery = 500W Solar (2 x 250W panels)*
• 40A/100V MPPT – 24V battery = 1000W Solar (4 x 250W panels)*

In cold climates, the open circuit voltage (Voc) of the solar panel can increase significantly, up to 5V or even higher, which may result in the Voc of the solar array going above the maximum voltage limit of the solar charge controller and damaging the unit. See example below.

Example: A Prism 100V/50A solar charge controller will have a maximum solar ‘open circuit voltage’ or Voc of 100V, and a maximum charging current of 50 Amps. If you use 2 x 300W solar panels in series with 46Voc, then you end up with 92V. This seems ok, as it is below the 100V maximum.

However, in extremely cold conditions the panel voltage can go much higher than the Voc. This can be calculated using the ‘voltage temperature co-efficient’ of the solar panel (this is typically 0.3% for every degree below STC – 25°C cell temperature), but to simplify things, you can generally add 5V to the panel Voc.

In this case, we would end up with a Voc of 102V. This is now greater than the max 100V input voltage limit and could damage the unit or void your warranty.

Solution: There are two ways to get around this issue.

1. Use an MPPT solar charge controller with a higher input voltage rating such as the PRISM MPPT with 150v/50amps
2. Or a much easier option 2 – Connect the panels in parallel instead of series, so the maximum voltage will now be around 46V + 5V = 51 Voc, no worries if you are using a 12V or 24V battery system. Remember in this case the input current will double so the solar cable should be rated accordingly.

Note: Assuming you are using a 12V battery and 2 x 250W panels, the MPPT charger controller output current will be roughly – 500W / 12V = 42A max. So you should use a 50A MPPT solar charge controller.