Estimate how long it takes to charge your battery bank with solar power.
Total Amp-hours of your battery bank.
12 Volts
24 Volts
48 Volts
How empty is the battery? (e.g. 50% needs recharge).
Accounts for controller, wiring, and heat losses (Typ: 75-85%).
Energy Required to Recharge:0 Wh
Real-World Solar Output:0 Watts
Effective Charging Current:0 Amps
Estimated Charge Time:0 Hours
Understanding Solar Charge Rates
When planning an off-grid solar system for an RV, boat, or cabin, one of the most critical calculations is determining how long it will take to recharge your battery bank. This Solar Panel Charge Rate Calculator helps you estimate the charging duration based on your specific battery capacity and solar array setup.
How the Calculation Works
Calculating solar charging time involves physics and real-world efficiency factors. It is not as simple as dividing battery size by panel wattage. Here is the logic used in this tool:
Total Energy Capacity (Wh): We first determine the watt-hours of energy your battery holds by multiplying the Amp-hours (Ah) by the Voltage (V).
Energy Deficit: Batteries are rarely drained to 0%. The calculator considers the "Discharge Depth" to calculate exactly how much energy needs to be put back into the battery.
Real-World Efficiency: A 100W solar panel rarely outputs 100W. Factors like heat, angle of the sun, wiring resistance, and charge controller efficiency (PWM vs MPPT) reduce output. We use a default efficiency factor of 75% to provide a realistic estimate rather than a theoretical maximum.
Key Terms Explained
Amp-Hours (Ah): The measure of electric charge a battery can hold. A 100Ah battery can theoretically provide 1 Amp for 100 hours or 100 Amps for 1 hour.
System Efficiency:
PWM Controllers: Usually 70-80% efficient.
MPPT Controllers: Usually 90-95% efficient, though total system losses (wiring/heat) still result in a total efficiency around 80-85%.
Peak Sun Hours: This calculator gives the duration of charging time assuming consistent sunlight. In reality, you only get 4-6 "Peak Sun Hours" per day in many locations. If the calculator says "10 Hours" to charge, it may take 2 days of real-world sunlight to fully top off the battery.
function calculateSolarCharge() {
// 1. Get Inputs
var batAh = parseFloat(document.getElementById('batteryCapacity').value);
var batVolt = parseFloat(document.getElementById('batteryVoltage').value);
var depth = parseFloat(document.getElementById('dischargeDepth').value);
var panelW = parseFloat(document.getElementById('panelWattage').value);
var count = parseFloat(document.getElementById('panelCount').value);
var eff = parseFloat(document.getElementById('efficiency').value);
// 2. Validation
if (isNaN(batAh) || isNaN(panelW) || isNaN(count) || isNaN(depth) || isNaN(eff) || batAh <= 0 || panelW Amps = Watts / Volts
var chargingAmps = realSolarWatts / batVolt;
// Time to charge = Energy Needed / Power Input
var chargeTimeHours = 0;
if (realSolarWatts > 0) {
chargeTimeHours = energyNeededWh / realSolarWatts;
}
// 4. Update UI
document.getElementById('resultsArea').style.display = 'block';
document.getElementById('resEnergyNeeded').innerText = energyNeededWh.toFixed(1) + " Wh (" + (batAh * (depth/100)).toFixed(1) + " Ah)";
document.getElementById('resRealWatts').innerText = realSolarWatts.toFixed(1) + " Watts";
document.getElementById('resAmps').innerText = chargingAmps.toFixed(2) + " Amps";
// Format time nicely
if (chargeTimeHours > 0) {
document.getElementById('resTime').innerText = chargeTimeHours.toFixed(2) + " Hours";
} else {
document.getElementById('resTime').innerText = "Infinite";
}
}