Documentation sections

Solar Panel Sizing Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug solar-panel-sizing. Calculate solar panel wattage, battery capacity, and charge controller current for off-grid photovoltaic systems based on load and sun hours

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of solar-panel-sizing
Name	Unit	Range	Default	Description
loadPowernumber	W	at least 0.1	100	Continuous power consumption of the load
loadHoursnumber	h	0.1 … 24	24	Hours a day the load runs. 24 = always on.
peakSunHoursnumber	hr	at least 0.1	5	Average peak sun hours per day for your location
systemVoltagenumber	V	at least 1	12	System Voltage
batteryCapacitynumber	Ah	at least 1	100	The battery you have or plan to fit: compared with the required capacity as the days it lasts at 50% depth of discharge
daysAutonomynumber	none	1 … 30	3	Number of cloudy days the system must operate without sun
numLoadsnumber	none	0 … 9	0	Loads beside the base load, each with its own power and hours a day (0–9)
load1Powernumber	W	0 … 10000	50	Power additional load 1 draws while it runs
load1Hoursnumber	h	0 … 24	4	Hours a day additional load 1 runs
load2Powernumber	W	0 … 10000	50	Power additional load 2 draws while it runs
load2Hoursnumber	h	0 … 24	4	Hours a day additional load 2 runs
load3Powernumber	W	0 … 10000	50	Power additional load 3 draws while it runs
load3Hoursnumber	h	0 … 24	4	Hours a day additional load 3 runs
load4Powernumber	W	0 … 10000	50	Power additional load 4 draws while it runs
load4Hoursnumber	h	0 … 24	4	Hours a day additional load 4 runs
load5Powernumber	W	0 … 10000	50	Power additional load 5 draws while it runs
load5Hoursnumber	h	0 … 24	4	Hours a day additional load 5 runs
load6Powernumber	W	0 … 10000	50	Power additional load 6 draws while it runs
load6Hoursnumber	h	0 … 24	4	Hours a day additional load 6 runs
load7Powernumber	W	0 … 10000	50	Power additional load 7 draws while it runs
load7Hoursnumber	h	0 … 24	4	Hours a day additional load 7 runs
load8Powernumber	W	0 … 10000	50	Power additional load 8 draws while it runs
load8Hoursnumber	h	0 … 24	4	Hours a day additional load 8 runs
load9Powernumber	W	0 … 10000	50	Power additional load 9 draws while it runs
load9Hoursnumber	h	0 … 24	4	Hours a day additional load 9 runs

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of solar-panel-sizing
Field	Unit	Description
energyPerDay	Wh	Energy Per Day
panelWattageheadline	W	Required Panel Wattage
panelCurrent	A	Panel Current
batteryAh	Ah	Required Battery Capacity
chargeControllerAmps	A	Charge Controller Current
batteryAutonomyDays	days	Days the entered battery carries the load at 50% depth of discharge; reaching the days of autonomy needs the required capacity

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "solar-panel-sizing",
    "inputs": {
      "loadPower": 100,
      "loadHours": 24,
      "peakSunHours": 5,
      "systemVoltage": 12,
      "batteryCapacity": 100,
      "daysAutonomy": 3,
      "numLoads": 0,
      "load1Power": 50,
      "load1Hours": 4,
      "load2Power": 50,
      "load2Hours": 4,
      "load3Power": 50,
      "load3Hours": 4,
      "load4Power": 50,
      "load4Hours": 4,
      "load5Power": 50,
      "load5Hours": 4,
      "load6Power": 50,
      "load6Hours": 4,
      "load7Power": 50,
      "load7Hours": 4,
      "load8Power": 50,
      "load8Hours": 4,
      "load9Power": 50,
      "load9Hours": 4
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("solar-panel-sizing", {
    "loadPower": 100,
    "loadHours": 24,
    "peakSunHours": 5,
    "systemVoltage": 12,
    "batteryCapacity": 100,
    "daysAutonomy": 3,
    "numLoads": 0,
    "load1Power": 50,
    "load1Hours": 4,
    "load2Power": 50,
    "load2Hours": 4,
    "load3Power": 50,
    "load3Hours": 4,
    "load4Power": 50,
    "load4Hours": 4,
    "load5Power": 50,
    "load5Hours": 4,
    "load6Power": 50,
    "load6Hours": 4,
    "load7Power": 50,
    "load7Hours": 4,
    "load8Power": 50,
    "load8Hours": 4,
    "load9Power": 50,
    "load9Hours": 4,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "solar-panel-sizing",
    "inputs": {
      "loadPower": 100,
      "loadHours": 24,
      "peakSunHours": 5,
      "systemVoltage": 12,
      "batteryCapacity": 100,
      "daysAutonomy": 3,
      "numLoads": 0,
      "load1Power": 50,
      "load1Hours": 4,
      "load2Power": 50,
      "load2Hours": 4,
      "load3Power": 50,
      "load3Hours": 4,
      "load4Power": 50,
      "load4Hours": 4,
      "load5Power": 50,
      "load5Hours": 4,
      "load6Power": 50,
      "load6Hours": 4,
      "load7Power": 50,
      "load7Hours": 4,
      "load8Power": 50,
      "load8Hours": 4,
      "load9Power": 50,
      "load9Hours": 4
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "solar-panel-sizing",
	  "inputs": {
	    "loadPower": 100,
	    "loadHours": 24,
	    "peakSunHours": 5,
	    "systemVoltage": 12,
	    "batteryCapacity": 100,
	    "daysAutonomy": 3,
	    "numLoads": 0,
	    "load1Power": 50,
	    "load1Hours": 4,
	    "load2Power": 50,
	    "load2Hours": 4,
	    "load3Power": 50,
	    "load3Hours": 4,
	    "load4Power": 50,
	    "load4Hours": 4,
	    "load5Power": 50,
	    "load5Hours": 4,
	    "load6Power": 50,
	    "load6Hours": 4,
	    "load7Power": 50,
	    "load7Hours": 4,
	    "load8Power": 50,
	    "load8Hours": 4,
	    "load9Power": 50,
	    "load9Hours": 4
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "solar-panel-sizing",
  "values": {
    "energyPerDay": 2400,
    "panelWattage": 600,
    "panelCurrent": 50,
    "batteryAh": 1200,
    "chargeControllerAmps": 62.5,
    "batteryAutonomyDays": 0.25
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:solar-panel-sizing",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "loadPower": {
          "min": 0.1,
          "max": null,
          "unit": "W"
        },
        "loadHours": {
          "min": 0.1,
          "max": 24,
          "unit": "h"
        },
        "peakSunHours": {
          "min": 0.1,
          "max": null,
          "unit": "hr"
        },
        "systemVoltage": {
          "min": 1,
          "max": null,
          "unit": "V"
        },
        "batteryCapacity": {
          "min": 1,
          "max": null,
          "unit": "Ah"
        },
        "daysAutonomy": {
          "min": 1,
          "max": 30,
          "unit": ""
        },
        "numLoads": {
          "min": 0,
          "max": 9,
          "unit": ""
        },
        "load1Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load1Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load2Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load2Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load3Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load3Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load4Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load4Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load5Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load5Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load6Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load6Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load7Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load7Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load8Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load8Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        },
        "load9Power": {
          "min": 0,
          "max": 10000,
          "unit": "W"
        },
        "load9Hours": {
          "min": 0,
          "max": 24,
          "unit": "h"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "loadPower": 100,
      "loadHours": 24,
      "peakSunHours": 5,
      "systemVoltage": 12,
      "batteryCapacity": 100,
      "daysAutonomy": 3,
      "numLoads": 0,
      "load1Power": 50,
      "load1Hours": 4,
      "load2Power": 50,
      "load2Hours": 4,
      "load3Power": 50,
      "load3Hours": 4,
      "load4Power": 50,
      "load4Hours": 4,
      "load5Power": 50,
      "load5Hours": 4,
      "load6Power": 50,
      "load6Hours": 4,
      "load7Power": 50,
      "load7Hours": 4,
      "load8Power": 50,
      "load8Hours": 4,
      "load9Power": 50,
      "load9Hours": 4
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "loadPower"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageSolar Panel Sizing Calculator, at these values