Power Station Charging Time Calculator

The charging time formula

Charging time follows the same kind of energy/power math as runtime, just run in the opposite direction — instead of asking how long stored energy lasts, it asks how long it takes to add energy back in:

Charging Time (hours) = (Capacity (Wh) × (1 − Current Charge %)) ÷ (Input Power (W) × Charging Efficiency)

The numerator is simply the energy still missing from the battery — a 1000 Wh power station sitting at 20% charge needs 800 Wh (1000 × 0.8) to reach full. The denominator is how fast that energy actually arrives: your charger's (or panel's) input wattage, reduced by charging efficiency to account for conversion and battery losses. This is exactly what the calculator above computes as you change any of the four inputs.

Worked example

Take a 1000 Wh power station currently at 20% charge, plugged into a charger rated for 300 W of input, with charging efficiency set to this site's 85% default.

Energy still needed: 1000 × (1 − 0.20) = 800 Wh. Charging time: 800 ÷ (300 × 0.85) = 800 ÷ 255 ≈ 3h 8m. Change the input power to 600 W instead and the same 800 Wh gap closes in roughly half the time — charging speed scales directly with input watts, all else equal.

AC (wall outlet) charging

Plugging into a standard AC wall outlet is, for most portable power stations, the fastest widely available charging method — and the one where the manufacturer's stated maximum input watts matters most.

How much input power a given unit actually accepts varies enormously by model — some accept a modest input, others considerably more via multiple charging ports or a proprietary fast-charging mode. There is no single "typical" figure that applies across the category, so always check your specific power station's spec sheet or charger label for its actual rated AC input watts rather than assuming a number. The table further down uses a few input-power values purely as illustrative examples to plug into the formula above, not as claims about any real product.

Solar charging

Charging from solar panels uses the same formula, with the panel's real-world output — reduced further for clouds, angle and charge-controller losses — standing in for a wall charger's input watts. Because solar output fluctuates with weather and sun position rather than staying constant like a wall outlet, it deserves its own fuller treatment.

See the Solar Generator Runtime Calculator for the dedicated solar recharge-time breakdown, including a worked example and a "sunlight factor" you can apply here as the efficiency input.

12V / car charging

Charging from a vehicle's 12V outlet (cigarette lighter socket) while driving is the slowest of the three common methods, and for one clear reason: those outlets are commonly fused around 10 A, which caps available input power at roughly 120 W at 12V — well below what most AC chargers can deliver. Check your own vehicle's owner's manual for its actual fuse rating before relying on this figure.

That low ceiling makes 12V charging most useful for topping up a partially depleted battery over a long drive, rather than taking a power station from empty to full — plug the same reduced wattage into the calculator above to see how it plays out for your own capacity and starting charge level.

Why charging is not perfectly linear

The formula above assumes a constant charging rate from start to finish, which is a useful planning approximation — but not exactly how most lithium batteries charge in practice. Battery management systems commonly slow the charge rate ("taper") as the battery approaches 100%, similar to how a phone or EV charges quickly to around 80% and then noticeably slower for the last stretch.

That means the calculator's estimate tends to be reasonably accurate for charging from a low starting point up to somewhere in the 80–90% range, and can understate the time needed for that final slice up to 100%. Treat the result as a solid planning average, not a to-the-minute guarantee.

Illustrative charging time examples

The figures below are example calculations — an input power value chosen for illustration, run through the same formula above at this site's 85% efficiency default, from an empty (0%) starting charge to full. They are not specifications for any real power station or charger; use the calculator above with your own unit's actual numbers.

• 500 Wh capacity, 100 W input (example): 500 ÷ (100 × 0.85) ≈ 5h 53m
• 1000 Wh capacity, 300 W input (example): 1000 ÷ (300 × 0.85) ≈ 3h 55m
• 2000 Wh capacity, 500 W input (example): 2000 ÷ (500 × 0.85) ≈ 4h 42m
• 3000 Wh capacity, 500 W input (example): 3000 ÷ (500 × 0.85) ≈ 7h 4m

Other factors that affect real-world charging time

Beyond the four inputs the calculator asks for, a few other things can shift actual charging time in practice:

• Temperature: many lithium batteries charge more slowly, or restrict charging altogether, at very low or very high ambient temperatures.
• Battery age/health: an older battery pack can accept charge less efficiently than when new, stretching the taper phase discussed above.
• Simultaneous ("hybrid") charging: some power stations can combine AC and solar input at once for a faster total charge — check whether your unit supports this and what its combined input limit is, since it's not simply the sum of each method's individual maximum.
• Partial vs. full charges: topping up from 60% to 100% takes meaningfully less time than 0% to 100%, and disproportionately less if most of that stretch falls in the faster, non-tapering part of the curve.