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1. Enter your battery voltage. For instance, if you’re using a 12V battery, you’d enter the number 12.
2. Enter your battery capacity in amp hours. If you have a 50Ah battery, you’d enter the number 50. (If you only know your battery’s capacity in watt hours, first convert watt hours to amp hours.)
3. Select your battery type. Select “Lead acid” if you’re using a flooded or sealed (AGM or gel) lead acid battery. Select “Lithium (LiFePO4)” if you’re using a lithium iron phosphate battery.
4. Optional: Enter your battery depth of discharge as a percentage. If your battery is 80% discharged, you’d enter the number 80. (If you have a lead acid battery, keep in mind that they should usually only be discharged 50%.)
5. Enter the wattage of your solar panel or solar array. If you’re using a 100W solar panel, you’d enter the number 100. If you’re using a 400W solar array, you’d enter the number 400.
7. Click “Calculate” to get your results. Your estimated charge time is given in peak sun hours. You can use our peak sun hours map or calculator to find out how many peak sun hours your location gets. For example, let’s say your estimated charge time is 8 peak sun hours and your location gets on average 4 peak sun hours per day. In that case, you know it’ll take about 2 days for your solar panel(s) to charge your battery.
Besides using our calculator, here are 3 ways to estimate how long it’ll take to charge a battery with solar panels.
I’ll run through each method step by step, starting with the simplest and ending with the most complex.
Note: None of these methods is perfect. Each makes a number of assumptions that aren’t obvious to the untrained eye. I talk more about these assumptions at the end of this section.
This is one of the more common ways you’ll see people estimate charge time. It''s simple but inaccurate. For this one, your battery and solar panel need to have the same nominal voltage.
1. Divide solar panel wattage by solar panel voltage to estimate solar panel current in amps. For example, here’s what you’d do if you had a 100W 12V solar panel.
2. Divide battery capacity in amp hours by solar panel current to get your estimated charge time. Let’s say you’re using your 100W panel to charge a 12V 50Ah battery.
3. If using a lead acid battery, multiply charge time by 50% to factor in the recommended max depth of discharge of lead acid batteries.
This way takes into account two important factors that the first method doesn’t: battery depth of discharge (DoD) and solar charge controller efficiency. Incorporating DoD adds flexibility. You can estimate charge time regardless of what state of charge your battery is at.
1. Multiply battery voltage by battery amp hours to get battery capacity in watt hours. For example, let’s say you have a 12V 100Ah battery.
2. Multiply battery watt hours by battery depth of discharge to estimate how much of the battery’s capacity has been discharged. Let’s say your battery is discharged 80%.
3. Multiply solar panel wattage by rule-of-thumb charge controller efficiency (PWM: 75%; MPPT: 95%) to estimate solar output. Let’s say you’re using a 200W solar panel and an MPPT charge controller.
This last method builds on the previous one. It takes into account system losses to give you an even more accurate estimate.
1. Multiply battery voltage by battery amp hours to get battery capacity in watt hours. For example, let’s say you have a 12V 200Ah battery.
2. Multiply battery watt hours by battery depth of discharge to estimate how much of the battery’s capacity has been discharged. Let’s say your battery is discharged 50%.
3. Divide discharged battery capacity by the battery’s rule-of-thumb charge efficiency factor (lead acid: 85%; lithium: 99%) to get the amount of energy required to fully charge the battery after factoring in losses during charging. Let’s say you’re using a lead acid battery.
4. Multiply solar panel wattage by rule-of-thumb charge controller efficiency (PWM: 75%; MPPT: 95%) to estimate solar output. Let’s say you’re using a 400W solar array and an MPPT charge controller.
5. Multiply solar output by 100% minus a fixed percentage to take into account system losses. The National Renewable Energy Laboratory’s PVWatts Calculator uses 14.08% as its default value for system losses, so I’ll use that number here.
All these methods make assumptions. And they all leave out factors that affect solar charging time in the real world. Here are a handful of the main ones:
Our charge time calculator takes into account a couple of these variables for a more precise estimate. But, alas, it can’t predict the weather…yet.
Never connect a solar panel directly to a battery. Doing so can damage the battery.
Instead, connect the battery then solar panel to a solar charge controller. Charge controllers regulate the current and voltage coming from solar panels to safely charge the battery.
Tutorial: How to Connect a Battery to a Solar Panel
About Solar panel battery charge time calculator
As the photovoltaic (PV) industry continues to evolve, advancements in Solar panel battery charge time calculator have become critical to optimizing the utilization of renewable energy sources. From innovative battery technologies to intelligent energy management systems, these solutions are transforming the way we store and distribute solar-generated electricity.
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