Mid-Range MPPT Solar Charge Controller Request for Proposal (RFP) Template

Choosing the right charge controller is one of the most important decisions associated with designing and building an off-grid system.  This sample template was created to help installers, designers, and engineering, procurement & construction personnel to create an RFP that will attract responses from qualified parties that can meet key mid-range MPPT charge controller specifications to help ensure deployment of a reliable, high quality off-grid system.  A charge controller is considered to be the brain of a Photovoltaic (PV) system and an ineffective controller can cause premature battery failure and failure in the whole system.


The controller design life shall be 15 years or greater.

The manufacturer’s warranty period shall be 5 years.

The controller shall be 100% solid state.  Mechanical relays and cooling fans are not acceptable.

The maximum allowable open circuit voltage on the solar input shall be 115V or greater.


Peak efficiency must be >= 98%.

Efficiency at full power >= 94% with Vmp = 60V with 12 volt systems.

Efficiency at full power >= 95% with Vmp = 90V with 24 volt systems.

At a minimum there should be 7 battery charge setting options for various battery types.

The battery settings should be fully programmable allowing the adjustment of charge voltage, float voltage, equalization and temperature compensation. and custom programmability for optional voltage setting adjustments to accommodate a variety of battery types including sealed, flooded, and gel batteries.

Auto-equalization shall take place every 28 days or there must be an option for manual equalization for sealed or flooded batteries.

Voltage accuracy must be < 80 mV.  Controllers which do not list accuracy information will not be accepted.  Battery sense must be included to eliminate voltage drop inaccuracies (Battery voltage sense terminals must directly measure battery voltage at all times.  Inaccurate measurements of voltage drop can cause the controller to enter into absorption stage too early and disconnect loads during low voltage disconnect (LVD) prematurely).

The thresholds for all the battery charging settings (absorption, float, and equalize), must be temperature compensated and adjustable.  Default temperature compensation = -30mV/C @12V.  Custom settings must be available.  The controller shall include a built in temperature sensor and a remote temperature sensor (RTS) for more accuracy. 

MPPT Tracking Algorithm should be capable of sensing multiple maximum power points utilizing full sweeping at all times and continuously operate with the highest power.

Switch Options

The charge controller must have:

1. DC Load Control (Low Voltage Disconnect) to protect batteries from over-discharge.

2. High voltage disconnect protection for Solar charging and Loads to prevent overcharging or high voltage conditions to load from battery.

3. A means to establish dry contact alarm notifications (this can be established by connecting to a compatible relay switch accessory.)

4. Generator start option with dry contact relay switch(es)

5. Multiple switches for prioritized loads

Custom Settings

The following settings must be able to be customized and adjusted:

1. Charge voltage regulation setpoints (Absorption, Float and Equalize).

2. Absorption time with responsiveness to depth of discharge (DOD).

3. Float timeout

4. Equalize time with Equalize timeout setting (max. time > Absorption Voltage ) to prevent partial equalizations for extended periods of time.

5. Manual Equalize or Automatic Equalize settings (Auto includes frequency (# of days) between Equalize).

6. High voltage disconnect for Solar charging and Loads

7. A maximum regulation voltage setpoint limit to protect voltage sensitive loads.

Environmental Requirements

1. The controller’s circuit board shall be protected with a conformal coating.

2. The terminals shall be marine rated. All fasteners shall be stainless steel.

3. The controller shall be rated for operating temperatures from -40 °C to +60 °C.

4. The controller can operate to the rated maximum battery current at 45°C without derating.

5. The controller shall be rated for 100% humidity non-condensing conditions.

6. Case studies must be available to demonstrate reliable and long-term performance of the controller in warm and tropical environments.


The controller must be equipped with the following protections:

1. Solar Input: overload, short-circuit, high voltage warning, reverse polarity, high temperature, nighttime reverse current

2. Load Output: overload, short-circuit, high temperature, reverse polarity

3. Battery: reverse polarity

4. The controller shall continuously perform “Self-Diagnostics and report any faults or alarm conditions as they happen.

5. Automatic recovery without fuses

6. The controller shall be transient over voltage protected with nanosecond response Transient Voltage Suppressors (TVS’s) rated at least 4500 watts. Both the solar input and the load output shall be protected.

7. The controller will not be damaged with high input power levels. The controller is also able to continue operating continuously without exceeding the rated output current of the controller.

Meter Display Interface

1. The digital display should be included with the controller

2. The controller’s meter should display the following information

a. Battery Voltage, Net Battery Current, Load Current, Target Voltage

b. Charge State (Bulk, Absorption, Float)

c. Load State (LVD, ON/OFF)

d. Alarms, Faults

e. Solar Current, Array Voltage, Array Vmp, Array Voc

f.  Today’s Min/Max Voltages, Charge Ah, Load Ah, Minutes in Absorption/Float/Equalize

g. Digital and Graphical Historic Daily Logged Min/Max Voltages, Charge Ah, Load Ah, Minutes in Absorption/Float/Equalize

3.    The meter interface must be interactive.  Users must be able to program and adjust controller settings via the meter interface.

4.    Active Matrix Graphical Meter Specs

a.    Resolution > 128 x 64 or greater

b.    Viewing Area > 70mm x 40mm,

c.    Backlight LED Brightness, Contrast adjustable

d.    Auto Scroll Enable/Disable

e.    Backlight Timeout

f.     Auto Return to Main Screen

g.    Operating Temperature -20ºC to +60ºC

h.    Storage Temperature -30ºC to +80ºC


The controller must accommodate:

1.    Maximum wire size is 35mm. (#2 AWG).

2.    1/2″,3/4″, 1″ conduit or M20, M25, M32 glands to be offered in controller wiring box.

3.    Wire terminals that are accessible on the front of the controller. Access to terminals must be possible while the controller remains mounted.


Controller must support the following communications capabilities:

1.    Remote monitoring options for all system parameters including but not limited to operating Array Volts, sweep Vmp/Voc/Pmax, Charge State (Bulk MPPT), Absorption, Float, Equalize, Night, Fault, Disabled), Battery Voltage, Target Regulation Voltage, Charge Current, Battery temperature, Heat Sink Temperature, Charge Ah, Load Ah.

2.    Ability to download daily logged values for last 100 days or more.

3.    Remote data logging in real-time.

4.    The following daily logged values must be stored internally by the controller:

a.    Min./Max. Battery voltage

b.    Ah charge/load

c.    Max./min. Array voltage

d.    Alarms

e.    Faults

f.     The time that the controller has regulated battery charging in absorption, float, and equalize charging stages.

(Daily logged values must be stored internally and available for download into a spreadsheet or viewable with an optional graphical meter.  Additional communications adapter accessories may be required to view this data).

5.    The ability to program and control (System Reset/ Disable Charging and/or Load) the system remotely.


1.    The controller must be compliant with the requirements of EIA-62109, UL 1741, UL 62109, IEC 62109-1, US FCC Part-15 Class B.

2.    The controller shall be manufactured in an ISO9001 certified facility.