Estimate how many hours of peak sunlight you need to fully charge your battery bank.
Estimated Charge Time:
How to Calculate Solar Battery Charge Time
Calculating the time it takes to charge a battery using solar energy is essential for sizing an off-grid power system. Whether you are powering a camper, a remote cabin, or a backup home system, understanding the relationship between battery capacity and solar output ensures you never run out of power.
The Solar Charge Formula
The basic formula used by our calculator is:
Time (Hours) = (Battery Capacity in Wh × Depth of Discharge) / (Panel Wattage × System Efficiency)
Key Variables Explained
Battery Capacity (Ah): The total energy storage of your battery measured in Amp-hours.
Battery Voltage (V): Usually 12V, 24V, or 48V. Multiplying Ah by Voltage gives you Watt-hours (Wh).
Solar Panel Wattage (W): The rated power of your solar panels under Standard Test Conditions (STC).
System Efficiency: No system is 100% efficient. Losses occur in the charge controller, wiring, and battery chemistry (typically 70% to 85%).
Depth of Discharge (DoD): How much of the battery you are recharging. If your battery is half full, you only need to charge 50%.
Realistic Solar Charging Example
Imagine you have a 100Ah 12V Lead Acid battery that is completely empty (100% DoD) and a 200W solar panel.
Component
Value
Total Energy Needed
1200Wh (100Ah x 12V)
Effective Panel Output (80% Efficiency)
160 Watts
Calculation
1200Wh / 160W
Estimated Time
7.5 Hours
Factors That Slow Down Charging
While the calculator provides a mathematical estimate, real-world conditions often vary:
Sun Angle: Panels produce the most power when the sun is directly perpendicular to the surface.
Cloud Cover: Overcast skies can reduce solar output by 50% to 90%.
Temperature: Solar panels are actually less efficient in extreme heat.
Controller Type: MPPT controllers are roughly 15-30% more efficient than PWM controllers.
function calculateSolarTime() {
var ah = parseFloat(document.getElementById('battCapacity').value);
var v = parseFloat(document.getElementById('battVoltage').value);
var w = parseFloat(document.getElementById('panelWatts').value);
var eff = parseFloat(document.getElementById('systemEfficiency').value);
var dod = parseFloat(document.getElementById('dischargeDepth').value);
var resultBox = document.getElementById('solar-result-box');
var resultValue = document.getElementById('solar-result-value');
var resultNote = document.getElementById('solar-result-note');
if (isNaN(ah) || isNaN(v) || isNaN(w) || isNaN(eff) || isNaN(dod) || w <= 0 || eff <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Calculation Logic
// 1. Total Watt-hours of the battery
var totalWh = ah * v;
// 2. Watt-hours required based on Depth of Discharge
var whToCharge = totalWh * (dod / 100);
// 3. Real-world output of the panels
var effectiveWatts = w * (eff / 100);
// 4. Time in hours
var hours = whToCharge / effectiveWatts;
// Formatting the output
var displayHours = hours.toFixed(2);
resultValue.innerHTML = displayHours + " Peak Sun Hours";
var dailySun = 4.5; // Average peak sun hours per day
var days = (hours / dailySun).toFixed(1);
resultNote.innerHTML = "Based on your inputs, you need approximately " + displayHours + " hours of full direct sunlight. In a typical region with 4.5 peak sun hours per day, this would take about " + days + " days to complete.";
resultBox.style.display = "block";
}