Estimated runtime in hours equals usable battery energy in watt-hours divided by average power draw in watts. The calculation is useful only when the energy and power figures describe the same part of the system. A panel's wattage, a charger's rating and a battery's mAh label are not substitutes for those two inputs.
Use this guide to make a transparent estimate, understand its limits and record a practical runtime check without opening the fan or battery.
If your question is electricity use or the bill rather than battery endurance, use the fan electricity and running-cost calculator. It uses your own input watts, running hours and electricity rate.
Start with energy and power
Watts (W) describe the rate of electrical power use. Watt-hours (Wh) describe an amount of energy. A hypothetical load drawing a constant 6W for four hours uses 24Wh. The U.S. Energy Information Administration's electricity measurement guide explains this distinction.
For runtime, ask for:
- The usable battery energy available under the stated test conditions.
- The average power draw at the chosen speed, with any permitted lights or USB loads included.
- Where the power was measured and which losses the usable-energy figure already includes.
The basic relationship is:
Runtime (hours) ≈ usable energy (Wh) ÷ average load (W)
If you only have a nominal battery label, first establish the complete-pack voltage and capacity. The Wh versus mAh guide covers that step. Nominal label energy is a starting figure, not a measurement of all energy available during operation.
A worked example with clearly hypothetical values
Suppose a test establishes 36Wh of usable energy at the same electrical boundary used to measure the loads below. The following values illustrate the calculation; they are not specifications for any Farseen model.
| Hypothetical average load | Calculation | Estimated runtime |
|---|---|---|
| 6W | 36Wh ÷ 6W | 6 hours |
| 9W | 36Wh ÷ 9W | 4 hours |
| 12W | 36Wh ÷ 12W | 3 hours |
This shows how a change in average load affects duration when usable energy is held constant. It does not assign those wattages to low, medium and high speed. Actual fan loads and usable energy must come from the relevant specification or a suitable test.
If an approved accessory combination increases the total average draw from 6W to 9W in this example, the estimate changes from six hours to four. Do not insert an invented bulb wattage or assume a USB device always draws its port's maximum rating.
Account for losses once
Distinguish these three starting points:
| Starting information | How to use it |
|---|---|
| Usable energy measured at the load side | Divide by load power measured on that same basis |
| Usable energy measured leaving the battery | Divide by the complete system's battery-side power draw |
| Nominal battery label energy only | Identify the unavailable energy and conversion losses before presenting a usable-energy estimate |
For example, if an illustrative 40Wh nominal figure is adjusted to 36Wh usable using an assumed 90% availability factor, do not apply that same 90% reduction again. The assumption must be stated and must not be presented as a verified Farseen efficiency.
Battery condition, temperature, control electronics and the device's stopping threshold can affect the relationship between nominal and usable energy. If those inputs are unknown, give a conditional estimate or explain what is missing. Extra decimal places do not make uncertain inputs reliable.
Which wattage should you use?
Use the fan system's average draw under the operating conditions being estimated. These common numbers answer different questions:
- Solar-panel watts describe a rated source, not the fan's consumption.
- Charger ratings do not reveal the battery-powered motor's average load.
- USB output ratings describe an output limit, not every connected device's actual consumption.
- An outlet meter used while charging measures mains-side charger input, including charging behavior and losses. That is not automatically the fan's battery discharge power.
Do not open the battery or modify the wiring to obtain a reading. If suitable manufacturer data is unavailable, a normal-use elapsed-time check is more appropriate than an improvised electrical measurement.
What the current Farseen figures can establish
The current supplier-stated runtime ranges are 5–9 hours for 198F, W01 and S600, and 5–7 hours for T02. The available records do not give matched speed, lighting and USB-load conditions or verified fan watts by speed.
Those ranges therefore cannot validate the hypothetical table above or guarantee a full night at your chosen setting. The records also do not establish simultaneous solar charging and operation for these models. Do not subtract panel watts from the fan load to claim continuous operation.
Record a useful runtime check
Follow the model's charging and operating instructions. Keep the charger disconnected during a battery-only runtime check, and use only approved accessory combinations.
| Record | Why it matters |
|---|---|
| Exact model and battery configuration | Keeps different versions separate |
| Charging method and starting charge indication | Makes the starting condition visible |
| Fan speed | Defines the operating setting |
| Lights and USB device, if permitted | Identifies additional loads |
| Start time and stopping condition | Distinguishes elapsed runtime from a guess |
| Room conditions and battery history | Helps interpret differences between checks |
Stop if there is damage or abnormal operation; this is not a battery stress test. Repeat under comparable normal-use conditions only if the first result needs confirmation.
For related questions, see how speed changes battery life and whether lights and USB use affect runtime. Use the calculation to explain your assumptions, then use the actual configuration and operating conditions to judge whether the fan meets your needs.
Farseen Editorial Team
The Farseen Editorial Team publishes practical guides and company updates about solar rechargeable fans, off-grid appliances, and product sourcing. Where an article discusses product-specific details, refer to its cited sources or contact Farseen for current information.