How to calculate and choose your Victron MPPT controller: a practical guide

The solar charge controller is the component I most frequently find to be incorrectly sized in installations by customers who have bought parts separately. Choosing the wrong MPPT controller means losing solar output, damaging the battery or spending more than necessary. In this guide, I explain exactly how to work out which solar charge controller you need.

The difference between a PWM and an MPPT controller: when does it matter?

A conventional PWM regulator connects the panels directly to the battery whilst it is charging, which means that the panel voltage must be the same as the battery voltage. An MPPT charge controller constantly calculates the panel’s maximum power point and converts the excess voltage into additional current.

In practice, MPPT extracts between 15 and 30% more energy from the same panels, particularly on cold, cloudy days and in the early hours of the morning. For installations with more than 200W of panels, the difference in actual runtime is significant. For systems under 100W and very basic set-ups, PWM may be sufficient if budget is the main priority.

The three parameters that determine your MPPT charge controller

1. Maximum input voltage (Voc at no load)

Each solar panel’s technical specifications state its open-circuit voltage (Voc). If you connect panels in series, the voltages add up. The maximum voltage that can reach the solar regulator occurs on the coldest days of winter, when the Voc rises above the nominal value.

Accurate calculation: Voc × number of panels in series × 1.25 (temperature factor).

Example: 2 × 400W panels with Voc = 37V in series → 37 × 2 × 1.25 = Estimated maximum 92.5V.

2. Maximum charging current

The current supplied to the battery by the MPPT controller is calculated by dividing the total power of the panels by the battery voltage, using an efficiency factor of 90–95%.

Example: 800W of solar panels, 24V battery → 800W / 24V × 0.95 = 31.6A charging current.

3. Compatible battery voltage

Victron SmartSolar photovoltaic charge controllers automatically detect the battery voltage (12V, 24V or 48V, depending on the model). Some models only work with 12V and 24V; the TR models support up to 48V. Check the product details before purchasing if your battery bank is 48V.

Comparison table: when to use each Victron SmartSolar model

The 30A solar regulator (MPPT 100/30) is the best-selling model for medium-sized campervan and boat installations. The 20A solar regulator (75/15, pre-set) is suitable for low-power installations. For off-grid domestic installations or those with 48V batteries, it is common to upgrade to the 150/60-Tr or higher models.

The most common mistake: underestimating the voltage

Most selection errors arise from focusing solely on the current (more amps = better) without checking the maximum input voltage. With modern 400W panels that have a Voc of 37–40V, connecting three in series results in 111–120V at rest, which rules out 100V models and necessitates a 150V system.

The rule: First, select the maximum voltage, then adjust the current.

How to calculate your solar regulator in 2 minutes

If you enter the Voc of your panels, the series/parallel configuration and the voltage of your battery, the Victron MPPT calculator from FV Componentes It tells you straight away which SmartSolar model is the best fit. It’s the quickest way to verify the calculation or spot any constraints you hadn’t considered.

Do you have any questions about your specific installation?

If you’ve decided on your panels and chosen your battery but aren’t sure which MPPT charge controller is suitable, please email us with the details (panel make and model, number of panels, series/parallel configuration and battery voltage) and we’ll confirm the model for you on the same day.