It is common for those with new solar configurations to worry about whether or not they are working. The worry is understandable because:

1. solar is $$$; and
2. solar can seem magical; and
3. both poorly running systems and mostly/fully charged systems show low levels of output for different reasons.

The last point may be counterintuitive. A fully charged system is observed to deliver very little power because the bank is already charged; the system is loafing, waiting for a load. It only cranks up power when you add a big load or start charging a depleted battery. With lead chemistry batteries this also applies in Absorption stage, when the battery reduces the amount of current it will accept.

A poorly running system is observed to deliver very little power because it isn't set up right (or is shaded, etc) and can't do any better. Battery voltage is too low³⁾ and doesn't meet needs.

Also counterintuitively, systems with lots of panel start generating usable power so early after sunrise that they can finish bulk charging well before solar noon. This means the user would not see the system at full power during normal circumstances. This effect is particularly strong when the system is overpaneled.

This page is to help solar beginners tell what their system is doing without special equipment.

Icons and blinking lights are often misleading or oversimplied. Tracer/Renogy controllers are infamous for confusing battery graphics. Others like the Victron 75/15 accurately show which charging stage is running.

Measurements, on the other hand, are useful when we learn how to read them.

To tell at a glance what your system is doing you need to find a couple pieces of info about your system. You will only have to look them up once; it might be worthwhile to write them down on a sticky note near your solar gear.

• Vabs - the charge controller's Absorption setpoint, hopefully tweaked to your battery manufacturer's recommendation
• Vfloat the charge controller's Float setpoint

If you have an MPPT controller also look up these pieces of info:

• Vmp - the voltage at which your panel makes maximum power. This will be stated in the specs and on the label on the back of your panel. For so-called 12v panels this is usually around 17-19v.
• Voc - the voltage of the panel at open circuit (“unconnected”). This will be higher than Vmp, probably in the low 20s. Found in the same place as Vmp.

These checks are admittedly crude but will help see if your system is getting it done. No expensive or specialized equipment is required.⁴⁾ MPPT controllers in particular will reveal a great deal of information by how they interact with the panels.

For the examples below we will assume the system is set up this way:

Vabs 14.6v
Vfloat 13.8v
Vmp 17v
Voc 20v

In addition,

Vpanel - measured voltage of panel output
Vbatt - measured voltage of battery bank

During bulk charging with MPPT

• Vpanel == Vmp
• Vbatt climbing toward Vabs
• Current delivered to battery holds steady
• Example: Panel voltage at Vmp == 17v, battery voltage 12.9v and rising, controller output steady at 4A.

During bulk charging with PWM

• Vpanel == Vbatt; both climbing toward Vabs⁵⁾
• Controller output increasing as Vpanel & Vbatt increases⁶⁾
• Example: Panel & battery 12.9v and rising, battery charging amps 2.5A and rising.

During Absorption charging with MPPT the battery will need less and less current.

• Vpanel starts to creep up from Vmp toward Voc
• Vbatt == Vabs until the controller's criteria for ending absorption is met
• Controller output decreasing as current demand drops.
• Example: Panel voltage 17v and rising, battery voltage held at Vabs == 14.6v, controller output amps 2A and dropping

During Absorption charging with PWM

• Vpanel & Vbattery == Vabs until the controller's criteria for ending absorption is met
• Controller output decreasing as current demand drops.
• the controller may get warmer as current demand drops⁷⁾
• Example: Panel and battery voltage held at Vabs == 14.6v, controller output 2A and dropping

During Float with MPPT

• Vpanel close to Voc
• Vbattery == Vfloat
• controller output minimal⁸⁾
• Example: Panel voltage held steady at 19.8v (near Voc), battery voltage holding at Vfloat == 13.8v, controller output < 1A

During Float with PWM

• Vpanel & Vbatt == Vfloat
• controller output minimal⁹⁾
• Example: Panel and battery voltage holding at Vfloat == 13.8v, controller output < 1A.

Adding a load can help reveal how much untapped power your system can access, and also prove that your system is working.

When adding loads to MPPT during absorption (or float)

• Vpanel drops down closer to Vmp again to increase power to meet the load
• Vbatt held at Vabs (or Vfloat) setpoint if possible
• If max power is required Vpanel will be Vmp.¹⁰⁾
• If demand outstrips supply Vbatt will drop while Vpanel stays pinned at Vmp.
• Example: Upon adding load, the controller starts hunting (tracking) a panel voltage that will meet the load; this will be closer to Vmp. Current output spikes to meet load demand.¹¹⁾ Output is limited by what the panel can provide under present conditions.

When adding loads to PWM during absorption (or float)

• Vpanel & Vbatt held at Vabs (or Vfloat) if possible¹²⁾
• If enough power is available no change will be observed but the controller may run cooler
• If demand outstrips supply the Vbattery & Vpanel will drop¹³⁾
• Example: Upon adding load, the controller releases power it had been dissipating in the PWM circuit. Current output spikes to meet load demand.¹⁴⁾ Output is limited to whatever power the panel puts out at Vabs & Vfloat.¹⁵⁾

These are general targets to ensure the system is working at the bare minimum to meet your needs. They give a big picture but no information about what is happening at a particular moment.

Are your batteries showing at least 12.1v (approx. 50% state of charge) in the morning? If so, continue to Afternoon.

If not, address this issue soon because discharging too deeply will damage the batteries. Consider these changes:

• increase your solar harvest
• decrease your power use
• increase your Vfloat and Vabs setpoints. Increasing Vabs to the manufacturer's max will help get the battery fully charged. If running PWM this will also allow the system to generate more power.
• run your loads through a low voltage disconnect so they turn off before hurting your bank
• conserve power!
• consider reducing your nighttime loads; charge devices like phones and tablets when the system has excess power.
• [And stop running things off an inverter! – secessus]

Are your batteries starting Absorption by noon-ish and completing Absorption by late afternoon? If so, continue to the next step.

Note: You can tell your batteries are in Absorption because the bank will be at Vabs, the controller's absorption setpoint.

If not, delay running heavier loads until Absorption is underway.

Are you hitting sundown with at least 12.7v¹⁶⁾ in your bank? If so, you can relax; your system is working well enough.

If not, either we are not harvesting enough power or we are using too much. Stop running heavier loads when the controller can no longer hold Vfloat.