Mini Off-Grid Power System Calculator

Average daily load (DC) from Loads Sheet 1

(Wh)

Total Daily Load Inc Wiring Losses (DC)

(Wh)

Number of days required in batteries

Days

<<<Enter days of battery autonomy – usually between 4 and 14,

Wh of storage required

Wh

depends on how critical the system is and the availability of back-up sources.

kWh of storage required

kWh

Voltage of battery bank

VDC

<<<Select desired battery voltage – often 12V for small systems, 24V for medium and 48V for large.

Ah capacity of battery bank

Ah

For boats & vehicles, it often makes sense to match the battery bank voltage to the alternator voltage.

Number of sunlight hours (worst month)

hrs

<<<In UK, in mid-winter, this ranges from 1hr in the far south, to 0.5hrs in Scotland. Depends on latitude.

Sunlight Intensity

W/m2

In the summer, the average is about 4x the winter hours (in the UK).

Minimum wattage of solar pv array

Wp

In Spain, the mid-winter figure is more like 2hrs, with 5hrs in the summer.

Suggested MPPT Controller Current Rating

A

Meteo Deviations

Allowance for weather deviations

<<<This figure allows for deviations from the average meteo data, which can on occasion reach 40%.

The more critical the power system, the higher this figure needs to be.

Losses

Cable / Fuse / MCB Losses

<<< Standard losses – can be adjusted.

Battery losses

<<< Gel or AGM batteries are often 90% or higher efficiency. Wet / flooded batteries are lower, often 75-80%

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Cable Sizing Calculator

Distance (m)

Current (A)

Watts

Voltage

Losses (%)

Cable Size (mm2)

Rounded Up Cable Size Current Rated

Cable Size Max Current

Solar PV Array to Controller

mm^2

A

Controller to Battery Bank

mm^2

A

Battery Interconnects

mm^2

A

Inverter DC Cables

mm^2

A

Other Cable X (All Manual Entry)

mm^2

A

Other Cable Y (All Manual Entry)

mm^2

A

Other Cable Z (All Manual Entry)

mm^2

A

Inverter Wattage Rating

W

<<< Taken from load calculator sheet.

Inverter Current (Cont)

A

Mains Charger

A

<<<Enter current rating of mains charger (to add to overall charge current).

Total Charge Current (Mains + PV)

A

Peak DC Current (Charge or Discharge)

A

Selected Controller Current

A

Solar Controller Current (Suggested Min)

A

<<< Suggested minimum current rating of MPPT controller.

Solar Controller Voltage Rating (Min)

Voc

<<<Ensure Charge Controller has higher max pv input voltage than this.

Controller Current (Manual Input)

A

<<< Enter current rating of chosen charge controller.

Solar PV Array – Minimum Power

Wp

Solar PV Panel – Rated Wattage

Wp

<<<Enter Power in Watts of chosen solar pv panel.

Number of pv panels in parallel

<<< Enter number of pv panels to be wired in parallel up to controller current limit

Number of series strings of panels

<<< Enter number of pv panels to be wired in series up to controller voltage limit

Solar PV Panel Voltage (Vmp)

Vmp

<<<Enter Voltage Max Power (Vmp) of chosen solar pv panel.

Solar PV Panel Voltage

Voc

<<< Enter Voltage Open Circuit (Voc) of chosen solar pv panel.

Solar PV Array Voltage (Vmp)

Vmp

Chosen Solar PV Array Power (W)

Wp

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