Solar Panel Battery Charge Time Calculator (LFP vs Lead-Acid, MPPT vs PWM, 2026)

Name: Solar Panel Battery Charge Time Calculator (LFP vs Lead-Acid, MPPT vs PWM, 2026) Calculator
Author: Marko Visic

Solar battery charge time depends on five things: battery capacity, depth of discharge, panel wattage, charge controller type (PWM vs MPPT), and battery chemistry (lead-acid vs LiFePO4). The honest formula is hours of peak sun = (battery Wh × DoD) / (panel W × controller efficiency × battery efficiency). For a typical 12V 100Ah LiFePO4 battery with a 200W panel and MPPT controller, that comes out to about 5 hours of peak sun, or one full sunny day. For lead-acid, the bulk stage finishes in similar time but the absorption stage adds 3–5 more hours of float-charging that solar can't always provide. This guide does the math properly, accounts for chemistry, and gives a realistic charge-time table.

I built a 6 kW grid-tie array on my own house in 2024, but my first solar project years ago was a 100W panel charging a single 12V deep-cycle battery on a camper van. That is exactly the use case this calculator is built for. The math is simple in principle and full of traps in practice — most of which come from ignoring battery chemistry and MPPT vs PWM differences.

The Right Formula

Hours of peak sun needed = (Battery Wh × DoD) / (Panel W × η_controller × η_battery)

Where:

Battery Wh = battery capacity in watt-hours = Ah × nominal voltage (e.g., 100 Ah × 12 V = 1,200 Wh)
DoD = depth of discharge as a fraction (0.5 for lead-acid, 0.8–0.95 for LiFePO4)
Panel W = nameplate wattage of the solar panel
η_controller = charge controller efficiency (~0.78 for PWM, ~0.95 for MPPT)
η_battery = battery charge efficiency (~0.85 for lead-acid, ~0.98 for LiFePO4)

Then:

Days to charge = Hours of peak sun needed / Daily peak sun hours at your location

This is the formula every honest solar charge time calculator uses. Most older articles skip the controller efficiency and the battery efficiency, which is why their answers are 25–40 % too optimistic.

Worked Example — A 100W Panel Charging A 12V 100Ah LiFePO4 Battery

This is the most common DIY off-grid setup. Real numbers:

Parameter	Value
Battery	12 V × 100 Ah = 1,200 Wh
Depth of discharge	80 % (LiFePO4 safe DoD)
Energy needed	1,200 × 0.80 = 960 Wh
Panel	100 W (Renogy 100W 12V mono)
Charge controller	Victron MPPT 100/15
Controller efficiency	95 %
LFP charge efficiency	98 %
Effective charge rate	100 × 0.95 × 0.98 = 93.1 W
Hours of peak sun needed	960 / 93.1 = 10.3 hours
Daily peak sun hours	5 (U.S. average)
Days to charge	10.3 / 5 = 2.06 days

So about 2 days of solar to fully recharge a 100 Ah LiFePO4 from 20 % SOC.

With a PWM controller instead of MPPT:

Effective charge rate = 100 × 0.78 × 0.98 = 76.4 W
Hours needed = 960 / 76.4 = 12.6 hours
Days = 12.6 / 5 = 2.51 days

PWM is 22 % slower for the same panel and battery — about half a day longer to recharge.

Battery Chemistry Changes Everything

The biggest oversimplification in older charging articles is treating all batteries as equal. They are not. The two relevant chemistries for solar in 2026:

LiFePO4 (LFP) — Modern Standard

LFP batteries (Battle Born, Renogy LFP, Lion Energy, Eco-Worthy LFP) charge almost linearly from 0 % to ~95 % SOC, then taper for the last 5 %. They accept high C-rates (typically 0.5C, meaning a 100 Ah battery can accept 50 A of charge current). For solar charging, the battery is essentially never the bottleneck — your panel current is the limit.

LFP charging characteristic	Value
Usable depth of discharge	80–95 % (vs 50 % for lead-acid)
Charge efficiency	~98 %
Bulk stage SOC range	0–95 %
Absorption stage	Brief, ~5 % of cycle
Float stage voltage	~13.6 V
Cycle life	3,000–6,000
Self-discharge	~3 %/month

LFP is the right choice for any new off-grid solar system in 2026. The price premium over lead-acid (~2× upfront) pays back in cycle life (~5× longer) and usable capacity (~2× more per nameplate Ah).

Lead-Acid (Flooded, AGM, Gel) — Legacy

Lead-acid batteries (Trojan T-105, Renogy AGM, Lifeline GPL) have a three-stage charging cycle that severely limits practical solar charge time:

Bulk stage (0–80 % SOC): Constant current, voltage rises freely. Fast.
Absorption stage (80–100 % SOC): Constant voltage at ~14.4–14.8 V, current tapers. Slow — limited by battery chemistry, not by your solar.
Float stage (100 % maintenance): Constant voltage at ~13.5 V to offset self-discharge.

Lead-acid charging characteristic	Value
Usable depth of discharge	50 % (going below damages the battery)
Charge efficiency	~85 %
Bulk stage SOC range	0–80 %
Absorption stage	80–100 %, can take 4–6 hours alone
Float stage voltage	~13.5 V
Cycle life	500–1,200 (deep cycle)
Self-discharge	~5 %/month

The absorption stage is the killer for solar charging of lead-acid. Even if you have unlimited panel current, the battery will only accept a slowly-tapering charge during the top 20 % of its capacity. Solar systems often never get lead-acid batteries to 100 % during the day, because the sun goes down before the absorption stage finishes. This is why lead-acid solar systems suffer from chronic undercharging and short battery life.

LFP vs Lead-Acid Charge Time Comparison

For the same 100 Ah nameplate battery, charging from 50 % to 100 % SOC with a 200W panel and MPPT controller:

Battery type	Usable energy at 50→100 %	Effective charge rate	Bulk time	Total time (incl. absorption)
LFP 12V 100Ah (95 % DoD)	~600 Wh	186 W	3.2 hr	3.5 hr
AGM 12V 100Ah (50 % DoD)	~600 Wh	162 W	3.7 hr	6.5 hr (bulk + 3 hr absorption)
Flooded 12V 100Ah (50 % DoD)	~600 Wh	162 W	3.7 hr	8 hr (bulk + 4–5 hr absorption)

LFP is roughly 2× faster to a true 100 % charge than lead-acid for the same nameplate Ah and the same solar input. That is on top of the 2× usable capacity advantage.

MPPT vs PWM Charge Controllers

MPPT (Maximum Power Point Tracking) and PWM (Pulse Width Modulation) controllers have very different efficiency. For a full comparison including when each type makes sense, see MPPT vs PWM charge controllers. From Victron Energy's white paper:

Controller	How it works	Efficiency at 12V battery / 18V panel	Cost (2026)
PWM	High-speed switch; pulls panel down to battery voltage	~78 %	$15–30 (10 A)
MPPT	DC-DC converter; runs panel at Vmp	~95 %	$50–120 (10 A)

The 17-percentage-point efficiency difference is real and well-documented. On a 100W panel, MPPT delivers ~95W to the battery vs ~78W for PWM. The MPPT premium pays itself back in 6–12 months on any panel above 50W. There is no good reason to install PWM in 2026 except for sub-50W trickle-charge applications (gate openers, deer feeders, RV maintenance chargers).

For the rest of this article, the calculator assumes MPPT. If you have PWM, multiply all charge times by ~1.22.

Realistic Solar Charge Time Table (LFP, MPPT, 5 PSH)

Hours of charge required to fill the listed battery from 20 % to 100 % SOC, using an MPPT controller and 5 peak sun hours per day. Days are sun-hours / 5.

Panel	12V 50Ah LFP	12V 100Ah LFP	12V 200Ah LFP	12V 300Ah LFP	12V 400Ah LFP
100 W	5.2 hr (1.0 d)	10.3 hr (2.1 d)	20.6 hr (4.1 d)	30.9 hr (6.2 d)	41.2 hr (8.2 d)
200 W	2.6 hr (0.5 d)	5.2 hr (1.0 d)	10.3 hr (2.1 d)	15.5 hr (3.1 d)	20.6 hr (4.1 d)
300 W	1.7 hr (0.3 d)	3.4 hr (0.7 d)	6.9 hr (1.4 d)	10.3 hr (2.1 d)	13.7 hr (2.7 d)
400 W	1.3 hr (0.3 d)	2.6 hr (0.5 d)	5.2 hr (1.0 d)	7.7 hr (1.5 d)	10.3 hr (2.1 d)
500 W	1.0 hr (0.2 d)	2.1 hr (0.4 d)	4.1 hr (0.8 d)	6.2 hr (1.2 d)	8.2 hr (1.6 d)
800 W	0.6 hr (0.1 d)	1.3 hr (0.3 d)	2.6 hr (0.5 d)	3.9 hr (0.8 d)	5.2 hr (1.0 d)
1,000 W	0.5 hr (0.1 d)	1.0 hr (0.2 d)	2.1 hr (0.4 d)	3.1 hr (0.6 d)	4.1 hr (0.8 d)

For lead-acid batteries, multiply these times by 1.6× for AGM or 2× for flooded to account for absorption stage and lower charge efficiency.

Common Misreadings

"A 100W panel produces 100W per hour." No — 100 W is instantaneous power, not energy per hour. Over a day with 5 peak sun hours, a 100W panel produces 100 × 5 × 0.85 ≈ 425 Wh of usable energy, not "2,400 Wh per day."
"MPPT and PWM are basically the same." No — MPPT delivers ~25 % more energy from the same panel because it operates the panel at Vmp instead of forcing it to battery voltage. See the 100W panel amps article for the physics.
"My 100Ah battery holds 100 Ah of usable energy." Lead-acid: only 50 Ah usable (50 % DoD limit). LiFePO4: about 80–95 Ah usable. The "nameplate" Ah is not the usable Ah.
"More solar = always faster charging." Up to a point. Once the battery enters absorption (lead-acid) or its acceptance limit, adding more panels doesn't help — the bottleneck is the battery's own chemistry, not your solar input.
"My solar will fully charge a lead-acid battery every day." Often not. The absorption stage is slow and the sun sets before it finishes. This is why lead-acid solar systems chronically undercharge and die early. LFP doesn't have this problem.
"I can use a 12V car battery for solar." No — car (cranking) batteries are designed for short high-current bursts, not deep cycling. They will die in weeks under solar use. Use deep-cycle lead-acid (Trojan T-105, Renogy Deep Cycle AGM) or LFP (Battle Born, Renogy LFP). For the wiring side, see how to connect solar panels to a battery.

Bottom Line

For realistic solar battery charging in 2026, use LiFePO4 chemistry + MPPT controller. If you need help sizing the battery bank itself, try the solar battery sizing calculator. The combination charges roughly 2× faster than lead-acid + PWM for the same panel and battery, and the LFP has roughly 5× the cycle life.

A typical 100 W panel charges a 12V 100Ah LFP battery from 20 % to 100 % in about 2 days of average sun. A 200 W panel does the same job in 1 day. A 400 W panel does it in half a day. Use the calculator below for any specific panel + battery combination.

Battery voltage

Battery capacity

Battery chemistry

Target charge time

hrs

Required solar panel size

To charge a 100Ah 12V Lithium (LiFePO4) battery in 5 hours

Energy to charge

1.26kWh

If you use 100W panels

panels needed

If you use 200W panels

panels needed

171 kg

CO₂ avoided per year

0.04

equivalent US homes powered

trees planted equivalent

$74

estimated annual savings

Chemistry	Efficiency	Cycle Life	Panel Watts
Lithium (LiFePO4)	95%	3,000–5,000	252 W
Deep Cycle AGM	85%	500–1,000	283 W
Lead-Acid Flooded	80%	300–500	300 W

Tap to see sensitivity analysisSensitivity analysis

Sensitivity range
Scenario	Value
Low (-20%)	202 W
Expected	252 W
High (+20%)	302 W

Battery chemistry has the biggest effect \u2014 switching from lead-acid to lithium reduces required panel watts by ~20%.

What size battery do I need?See production for your state

Keep Reading

If you found this useful, these guides go deeper into related topics:

Frequently Asked Questions

How long does it take a 100W solar panel to charge a 12V battery?

For a 12V 100Ah LiFePO4 battery from 20 % to 100 % SOC, with a 100W panel and an MPPT controller at 5 peak sun hours per day, about 2.4 days. With a PWM controller, about 3 days. For a lead-acid battery the answer is similar in pure energy terms, but lead-acid has slow absorption and float stages that double the actual time to reach 100 %. The honest answer is: 2–4 days for a meaningful charge, 4–6 days for a complete 100 % top-up on lead-acid.

Can a 100W solar panel charge a 12V 100Ah battery in one day?

No. A 100W panel produces about 400–500 Wh per day in 5 peak sun hours after MPPT conversion. A 12V 100Ah battery holds 1,200 Wh of energy total. So one day of full sun delivers about 33–42 % of the battery's full capacity — not a full charge. To charge from empty to full in one day you need about 250–300 W of solar with MPPT, or 350–400 W with PWM.

How does MPPT vs PWM affect charge time?

Significantly. A 100W panel through a PWM controller delivers about 70–78 W of usable power to a 12V battery (because the panel is forced to operate at battery voltage instead of its 18V Vmp). Through an MPPT controller it delivers about 95–100 W. So MPPT charges 25–30 % faster than PWM for the same panel — a 4-day PWM charge becomes a 3-day MPPT charge. See [How Many Amps Does A 100 Watt Solar Panel Produce](/how-many-amps-does-a-100-watt-solar-panel-produce/) for the physics.

Does battery chemistry affect charge time?

Yes, dramatically. LiFePO4 batteries can accept charge at very high C-rates (typically 0.5C, sometimes 1C) and they spend almost all the charge cycle in bulk stage — fast charging from 10 % to 95 % SOC. Lead-acid batteries have a slow absorption stage at the top: bulk fills 0–80 % SOC quickly, but absorption (80–100 %) takes hours of low-current charging at constant voltage. A 12V 100Ah lead-acid takes ~3× longer to reach a true 100 % than the same Ah of LiFePO4 from solar.

What is the three-stage charging cycle for lead-acid?

Bulk stage (0–80 % SOC): constant current, voltage rises freely. Absorption stage (80–100 % SOC): constant voltage at ~14.4–14.8 V, current tapers. Float stage (100 % maintenance): constant voltage at ~13.5 V, very low current to offset self-discharge. The bulk stage is fast and uses most of the panel's available current; the absorption stage is slow and bottlenecked by the battery's internal chemistry, not by your solar input.

How do I calculate solar panel charge time?

Charge time (hours of sun) = Battery Wh × DoD / (Panel W × controller efficiency × battery efficiency). For a 12V 100Ah LiFePO4 battery (1,200 Wh) at 80 % DoD with a 200W panel, MPPT controller (95 % efficient), and LFP charge efficiency (98 %): 1,200 × 0.8 / (200 × 0.95 × 0.98) = 5.16 hours of peak sun — about 1 day at 5 PSH. The calculator below does this automatically; the formula above is what's running underneath.

Why doesn't a battery charge instantly when the sun is bright?

Two reasons. First, the panel is current-limited — a 100W panel produces at most ~5.5 A, regardless of how much sun is hitting it. Second, the battery is voltage-limited by the charge controller — the controller intentionally caps the charging voltage to prevent overcharge. Energy enters the battery at the *minimum* of those two limits, which for most setups is the panel's current limit. This is why bigger panels charge proportionally faster, but only up to the point where the battery's own acceptance rate becomes the bottleneck.

Can I leave a battery on solar charge indefinitely?

Yes, with a proper charge controller. Both PWM and MPPT controllers transition into a float stage once the battery is full and hold it at a low maintenance voltage (~13.5 V for lead-acid, ~13.6 V for LiFePO4). They will not overcharge a healthy battery. The exception is poorly-designed cheap controllers (under $20 from generic Amazon sellers) which sometimes lack proper float-stage logic — these can damage batteries over weeks. Stick with Victron, Morningstar, Renogy, EPEVER, or similar reputable brands.

How much faster is a 200W panel vs a 100W panel for charging?

About twice as fast in pure power terms, but not always twice as fast in real-world charge time. During bulk stage, doubling the panel doubles the current and halves the charge time. But once the battery enters absorption (lead-acid only) or reaches its acceptance limit (some LFP packs at very high SOC), additional panel power doesn't help — the bottleneck is the battery, not the panel. For most realistic 12V 100Ah scenarios, doubling panel watts cuts the practical charge time by 40–50 %, not exactly half.

Sources

Marko Visic

Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.