Battery Charge Time Calculator

How long will it take to charge your battery from solar? Enter capacity, current level, panel size, and sun hours — get your answer.

Battery details

Battery capacity?

Current charge level?

Battery bank voltage?

Solar panel array size?

Charge controller efficiency?

Peak sun hours?

hrs/day

Time to full charge

1.1 days

Energy needed15,360 Wh

Daily charge added14.55 kWh/day

Solar power to battery2,910 W

Charge current60.6 A

% charged per sun hour15.2%/hr

Note: This assumes ideal conditions (full sun output during charging). Real-world charging takes 15–30% longer due to clouds, panel temperature, and charge controller absorption phase.

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How to Use This Calculator

Enter your battery capacity and current charge level

The battery capacity in Ah and the current charge level (state of charge, SoC) together determine how much energy needs to go in. A 400Ah 48V battery that's at 20% charge has 80% to refill — that's 15,360 Wh to replace. Your MPPT charge controller or BMS display shows the current SoC. If you have no display, a voltmeter reading can estimate SoC from the resting voltage.

Solar panel array size

Enter the total wattage of your solar panels — sum of all panels. Eight 375W panels = 3,000W. The array size (along with peak sun hours) determines how fast energy flows into the battery. More panels means faster recharge, but the charge controller limits the actual current delivered.

Charge controller efficiency

The charge controller converts solar panel output to the right voltage and current for your battery. MPPT (Maximum Power Point Tracking) controllers achieve 95–98% efficiency. PWM (Pulse Width Modulation) controllers run at 70–80% — they're much less efficient and waste significant solar energy. For any system above 200W, MPPT is the right choice. Popular MPPT brands: Victron SmartSolar, Renogy Rover, EPever Tracer.

Peak sun hours

Peak sun hours is the key variable that converts your panel's peak wattage into daily energy production. In Phoenix (6.5 PSH), a 3,000W array delivers about 18.6 kWh/day to the battery. In Seattle (3.5 PSH), the same array delivers only 10 kWh/day. The result shows both the minimum hours of uninterrupted full sun needed and the number of calendar days at your location's average sun hours.

The Formula

Battery total Wh = Ah × Voltage Wh needed = Battery Wh × (1 - Current SoC%) Effective solar power = Panel watts × Charge controller efficiency Charge current (A) = Effective solar power ÷ Battery voltage Hours of full sun to charge = Wh needed ÷ Effective solar watts Daily charge added (Wh) = Effective solar watts × Peak sun hours Days to full = Wh needed ÷ Daily charge added

Important: "Hours of full sun to charge" assumes continuous maximum solar output — useful for lab-bench sizing but not representative of real days. "Days to full" is the practical answer for real-world planning, accounting for your location's average daily solar production.

Example

Home battery at 20% — Denver, CO

A Denver homeowner has a 400Ah 48V LiFePO4 battery bank at 20% charge after a cloudy period. Their 3,000W solar array uses a Victron SmartSolar MPPT at 97% efficiency. Denver averages 5.5 peak sun hours.

Battery bank400Ah at 48V = 19,200 Wh

Current SoC20% (15,360 Wh needed)

Solar array3,000W

MPPT efficiency97%

Peak sun hours5.5 hrs/day

Result

Effective solar to battery2,910 W

Daily charge added16.0 kWh/day

Full sun hours to charge5 hr 17 min

Calendar days to full~1 day (at 5.5 sun hours)

Denver's strong sun gets this system recharged in just under one full solar day. In winter (3.5 PSH), the same recharge takes 1.6 days. This highlights why sizing for winter sun hours matters for off-grid and backup systems.

FAQ

What is an MPPT charge controller and why does it matter? ▼

An MPPT (Maximum Power Point Tracking) charge controller continuously finds the optimal voltage to extract maximum power from your solar panels, then converts it efficiently to the battery's charging voltage. Compared to the older PWM (Pulse Width Modulation) type, MPPT extracts 20–30% more energy from the same panels. For a 3,000W system, that's 600–900W more charging power — potentially 1–2 extra hours of charge time saved daily. MPPT is essential for any system where the panel voltage significantly exceeds battery voltage (e.g., 100–150V panel strings charging a 48V battery).

Why does charging slow down as the battery approaches full? ▼

Battery charging has three stages: Bulk (constant current, fastest charging — 0–80% SoC), Absorption (constant voltage, tapering current — 80–95% SoC), and Float (maintenance voltage, minimal current — 95–100%). The bulk stage is when solar power is most effective. Above 80% SoC, the charge controller reduces current to prevent overcharging, and charging appears to stall. This is normal and protective. This calculator models bulk-phase charging; actual full charge takes about 30% longer than the calculated time.

Can I charge a battery faster by adding more solar panels? ▼

Yes, up to the charge controller's maximum output current rating. If your MPPT controller is rated for 60A at 48V, it can deliver a maximum of 2,880W to the battery (60A × 48V). Adding more panels beyond what generates 2,880W won't speed charging unless you also upgrade the controller. For faster charging from a large battery bank, use a larger controller (or multiple controllers in parallel). Most home battery systems use 60A–100A MPPT controllers.

How do I find peak sun hours for my location? ▼

Use NREL's PVWatts calculator at pvwatts.nrel.gov — enter your address and it provides monthly and annual average solar irradiance data. General US averages: Phoenix, AZ: 6.5 PSH; Las Vegas, NV: 6.2; Denver, CO: 5.5; Dallas, TX: 5.3; Atlanta, GA: 5.0; New York, NY: 4.5; Chicago, IL: 4.0; Seattle, WA: 3.5; Anchorage, AK: 2.5. For battery charging, use your winter minimum PSH for worst-case planning — that's when batteries are most needed and solar output is lowest.

What if my solar panels produce more power than the charge controller can handle? ▼

This is called "clipping" — when solar production exceeds the controller's capacity, the excess is wasted. It's actually common and often intentional: slightly oversizing the solar array compensates for low-sun hours, panel degradation, and cloudy days. A rule of thumb: you can safely oversize the solar array up to 20–30% above the controller's rated wattage. Beyond that, you're wasting panels. The calculator shows the effective watts delivered to the battery (panel watts × controller efficiency), which is the controller-limited value.