Best solar power bank: the panel size decides how fast it refills
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Quick answer
A solar power bank refills only as fast as its panel allows. A small panel built into the case can take days of sun, so for outages and trips a power bank paired with a separate folding panel is more practical. Goal Zero, for example, lists its 35.52 Wh Venture 35 at 6 to 12 hours of sun from a 10 W Nomad 10, and 3 hours from an 18 W USB-C charger. Check watt-hours, not just mAh, and keep power banks in carry-on bags when flying.
Do the solar math before you buy
Charge time from the sun is roughly battery watt-hours divided by the panel's watts, before losses and clouds:
- Venture 35 with a 10 W panel: 35.52 Wh / 10 W = about 3.6 hours in perfect sun. Goal Zero lists 6 to 12 hours, which reflects real conditions.
- The same battery with a 2 W built-in panel: 35.52 / 2 = about 18 hours of perfect sun, before any losses. That is why small built-in panels are best treated as an emergency trickle.
- A bigger bank: the 71.04 Wh Venture 75 takes 11 to 22 hours on a Nomad 10, and Goal Zero lists 7 hours on its larger Nomad 50.
Look for the panel's rated watts on any solar power bank listing; if it is not stated, you cannot estimate charge time.
Two Goal Zero power banks compared
| Spec | Venture 35 | Venture 75 |
|---|---|---|
| Capacity | 35.52 Wh (9,600 mAh at 3.7 V) | 71.04 Wh (19,200 mAh at 3.7 V) |
| USB-C | 18 W in and out | 60 W in and out |
| USB-A | 2 ports, 15 W max each | 2 ports, 15 W max each |
| Wall recharge | 3 h (18 W USB-C) | 1.5 h (60 W USB-C PD) |
| Solar recharge | Nomad 10: 6 to 12 h; Nomad 20: 4 to 8 h | Nomad 10: 11 to 22 h; Nomad 20: 7 to 14 h |
| Phone recharges listed | 3 | 5 |
| Rating / weight | IP67 / 10.12 oz | IP67 / 20.35 oz |
Both list an IP67 rating, which Goal Zero says covers dust and submersion to 1 meter for at least 30 minutes, plus a 50 lumen built-in light.
Why mAh can mislead
Goal Zero rates the Venture 75 as 19,200 mAh at 3.7 V, while its cells are 6,400 mAh at 11.1 V. Both equal 71.04 Wh. Watt-hours let you compare banks with different internal voltages:
- 9,600 mAh x 3.7 V / 1,000 = 35.52 Wh (Venture 35).
- 19,200 mAh x 3.7 V / 1,000 = 71.04 Wh (Venture 75).
Use Wh when comparing solar charge times, phone recharges or airline limits.
Pairing a panel
- Panel size: Goal Zero recommends the Nomad 10 or Nomad 20 for the Venture 35, and the Nomad 20 or Nomad 50 for the Venture 75.
- Panel output: the Nomad 20 lists a USB port of 5 V up to 2.1 A, or 10.5 W, plus an 8 mm solar port. Charge the bank from the panel, then the phone from the bank.
- Placement: the panels have kickstands; aim them at the sun and move them as it moves.
- Storage: Goal Zero says to charge the Venture banks every 3 to 6 months when stored.
For panel choices in more depth, see our small solar panels guide.
Flying and battery safety
- Carry-on only: FAA says spare lithium batteries, including power banks, must travel in carry-on baggage, and must be removed from a bag that is checked at the gate.
- Size limit: FAA limits lithium ion batteries to 100 Wh each without airline approval. Both Venture models are under that, at 35.52 and 71.04 Wh.
- Heat and damage: FAA warns lithium ion batteries can overheat if damaged, overheated, exposed to water, overcharged or packed poorly. Do not leave a power bank baking in the sun beside its panel; Goal Zero lists an operating range of 32 to 113°F.
- Airline rules: FAA notes some airlines set stricter limits on how many power banks you may carry.
What to look for when you buy
Starter solar kit: Goal Zero Venture 35 + Nomad 10
Best for: Keeping a phone and headlamp charged on trips and in outages
Goal Zero lists a 35.52 Wh IP67 power bank with 18 W USB-C, three phone recharges and a 50 lumen light, plus a 10 W folding panel that refills it in 6 to 12 hours of sun.
Watch out: The 10 W panel is slow for larger banks; it needs direct sun and repositioning.
See options on Amazon (opens Amazon in a new tab)Bigger bank: Goal Zero Venture 75
Best for: Phones, tablets and USB-C laptops
Goal Zero lists 71.04 Wh, 60 W USB-C in and out, a 1.5 hour wall recharge, five phone recharges, IP67 and a built-in light.
Watch out: Pair it with a Nomad 20 or larger panel; a Nomad 10 takes 11 to 22 hours.
See options on Amazon (opens Amazon in a new tab)Panel upgrade: Goal Zero Nomad 20
Best for: Faster solar refills for either Venture bank
Goal Zero lists a 20 W folding panel with a 10.5 W USB port and an 8 mm solar port, cutting the Venture 35's solar time to 4 to 8 hours.
Watch out: The panel has no battery of its own; it only charges while in the sun.
See options on Amazon (opens Amazon in a new tab)Questions people ask
- Are solar power banks worth it?
- Built-in panels charge slowly; by simple math a 2 W panel needs about 18 hours of perfect sun to fill a 35.52 Wh bank, so a separate folding panel is faster.
- How long does a solar power bank take to charge?
- Goal Zero lists 6 to 12 hours for its Venture 35 on a 10 W Nomad 10, and 4 to 8 hours on a 20 W Nomad 20.
- Can I take a solar power bank on a plane?
- In carry-on only, per FAA, and under 100 Wh without airline approval.
- Should I compare mAh or Wh?
- Wh; it accounts for voltage. 9,600 mAh at 3.7 V is 35.52 Wh.
Sources
- Venture 35, Goal Zero
- Venture 35 + Nomad 10 Kit, Goal Zero
- Venture 75, Goal Zero
- Nomad 20, Goal Zero
- PackSafe: Lithium Batteries, Federal Aviation Administration