By Madeline ReedTo figure out how long a portable power station will run your camping gear, you need a simple runtime formula: divide usable watt-hours by the average watts your equipment draws. This article walks through the formula, explains losses like inverter efficiency and depth of discharge, and gives worked examples for common camping loads such as LED lights, a fan, a 12V fridge, and a laptop. You’ll also learn why cycling loads need average watts, how inverter idle draw matters, and what cold weather does to battery capacity.
The Runtime Formula
The basic runtime formula is straightforward:
Runtime (hours) = Usable Watt-Hours (Wh) ÷ Average Load (Watts)
Usable watt-hours is not the same as the power station’s rated capacity. You must account for two main losses: depth of discharge (DoD) and inverter efficiency. Few power stations publish a depth-of-discharge figure, so a reasonable planning assumption is that about 80-90% of the rated capacity is available before the battery management system shuts off output.
Additionally, if you’re running AC devices through the inverter, you lose some energy in the conversion from DC to AC. Inverter efficiency typically ranges from 85% to 90%. For DC loads like a 12V fridge plugged into the 12V socket, there is no inverter loss, only a small DC conversion loss.
Usable Capacity and Losses
To calculate usable watt-hours for AC loads, multiply the rated capacity by the depth of discharge and then by inverter efficiency. For example, a power station rated at 500 Wh with a 90% DoD and 85% inverter efficiency gives you:
500 Wh × 0.90 × 0.85 = 382.5 usable Wh for AC devices.
If you run only DC devices (like a 12V fridge or USB lights), you skip the inverter efficiency factor:
500 Wh × 0.90 = 450 usable Wh for DC devices.
Always check the manufacturer’s instructions for the recommended depth of discharge. Some power stations advertise a “usable capacity” directly, which simplifies the math. If not, assume about 85-90%.
Average Versus Peak Watts
Many camping appliances do not draw a constant wattage. A 12V fridge cycles on and off, a laptop charges at a high rate then tapers off, and a fan might have multiple speed settings. Using peak watts in the runtime formula will give you a very short, misleading runtime. Instead, use the average watts over a typical use period.
For cycling loads, measure the average power draw over several hours using a USB power meter or a plug-in watt meter. For example, a fridge might draw 60 W when the compressor runs but only run for 20 minutes per hour. That gives an average of 60 W × (20/60) = 20 W. You would use 20 W in the runtime formula, not 60 W.
For non-cycling loads like lights or a fan running continuously, the average watts equal the rated watts at that setting.
Example: Lights and Phones
Suppose you have a power station with 500 Wh rated capacity, 90% DoD, and 85% inverter efficiency. You plan to run:
- Two 5W LED lanterns for 5 hours each night (total 10 W)
- Charge two phones, each drawing 10 W for 2 hours (total 20 W for 2 hours, but you charge them sequentially)
First, calculate usable capacity for AC loads: 500 × 0.90 × 0.85 = 382.5 Wh.
LED lights: 10 W × 5 hours = 50 Wh.
Phone charging: 10 W × 2 hours per phone = 20 Wh each, total 40 Wh (if charged one after the other). If charged simultaneously, it’s 20 W × 2 hours = 40 Wh.
Total daily load: 50 + 40 = 90 Wh.
Runtime: 382.5 Wh ÷ 90 Wh per day = 4.25 days (about 4 nights).
This assumes you don’t use the inverter for anything else. If you run a laptop or fan, add those watts.
Example: A Fan Overnight
12V camping fans vary widely, from a few watts for small clip-on fans to 20-30 W for larger models on high. If you run it on medium at 15 W for 8 hours overnight, that’s 15 × 8 = 120 Wh.
Using the same 500 Wh power station with 90% DoD and no inverter loss (because it’s DC): usable DC capacity = 500 × 0.90 = 450 Wh.
Runtime for the fan alone: 450 Wh ÷ 15 W = 30 hours. That’s nearly four nights if you run it 8 hours per night.
If you use an AC fan through the inverter, the math changes. For an AC fan drawing 15 W, usable capacity is 382.5 Wh (from the earlier calculation). Runtime: 382.5 ÷ 15 = 25.5 hours, or about three nights. The inverter loss reduces runtime by roughly 15%.
Example: A 12V Fridge
A 12V fridge is a cycling load, so you need average watts. Average draw varies with ambient temperature, the set temperature and how often you open the lid. As a planning example, assume an average draw of 10-25 W over a 24-hour period.
Let’s say your fridge averages 15 W over 24 hours. That’s 15 × 24 = 360 Wh per day. This is a substantial load.
Using the same 500 Wh power station with 90% DoD for DC loads (450 Wh usable), the fridge alone would run for 450 ÷ 15 = 30 hours. That’s only a little over a day. You would need a larger power station or a solar panel to keep the fridge running continuously.
For a more accurate estimate, measure your fridge’s actual average draw with a watt meter. See our guide on running a 12V fridge while camping for tips on reducing power consumption, such as pre-cooling the fridge and its contents at home.
If you run the fridge on AC through the inverter, the usable capacity drops to 382.5 Wh, giving a runtime of 382.5 ÷ 15 = 25.5 hours. Running DC is more efficient.
Inverter Idle Draw
Even when no AC device is plugged in, the inverter itself consumes power to stay on. This idle draw typically ranges from 5 to 20 watts, depending on the power station’s design. For a power station with a 10 W idle draw, leaving the inverter on for 10 hours wastes 100 Wh. That’s significant when you have limited capacity.
To avoid this, turn off the inverter (or the AC outlet) when not in use. Many power stations have a separate button for the AC output. Use DC ports and USB ports for charging phones and lights whenever possible, as they don’t incur inverter idle draw. If you need AC power only occasionally, switch the inverter on only when you plug in a device and turn it off after charging.
Many power stations show the current output on their display; with the AC output switched on and nothing plugged in, that reading is a rough guide to the idle draw. This helps you budget for the loss.
Cold Weather
Cold temperatures reduce the usable capacity of lithium-ion batteries. Around freezing, the loss is often in the range of 10-30%, depending on the chemistry and how cold the cells actually get. The battery management system may also block charging below freezing to protect the cells.
To mitigate cold weather effects:
- Keep the power station inside your tent or RV where it stays above freezing.
- If it must stay in the cold, keeping it in an insulated bag or box between uses helps, but never cover the vents while it is running or charging.
- Charge the power station fully at room temperature before heading out, since most lithium batteries must not be charged below freezing.
- Expect shorter runtime and plan accordingly. For a 500 Wh power station in 32°F weather, usable capacity might drop to 350 Wh or less for DC loads.
If you’re camping in winter, consider a power station with a low-temperature charging cutoff or a built-in heater. Check the manufacturer’s instructions for operating temperature ranges.
Labels only give a maximum. A plug-in watt meter shows what your own devices really draw over a day, which makes runtime math far more accurate.
FAQ
How do I find the average watts of my camping gear?
Use a watt meter or a USB power meter to measure actual draw over time. For cycling loads like a fridge, measure over at least 24 hours. For lights and fans, the label wattage is usually accurate if run continuously.
Can I run a 12V fridge and lights on the same power station?
Yes, but add their daily watt-hours together. For example, a fridge averaging 15 W (360 Wh/day) plus lights at 10 W for 5 hours (50 Wh/day) totals 410 Wh/day. A 500 Wh power station with 90% DoD (450 Wh usable) would last about 450 ÷ 410 = 1.1 days. You would need a larger unit or solar recharging.
Does inverter idle draw matter if I only use DC ports?
No. Inverter idle draw only applies when the AC output is turned on. If you use only DC ports (cigarette lighter, USB), the inverter is off and draws no power. This is more efficient for DC devices.
How much does cold weather affect runtime?
Around freezing, plan for roughly 10-30% less usable capacity, and more in deeper cold. Keeping the battery warm between uses reduces the loss.
Calculating runtime for your camping power station is a matter of knowing your usable capacity and your average load. Use the formula with realistic numbers, account for inverter losses and idle draw, and adjust for cold weather if needed. For a deeper dive into building a daily watt-hour budget, see How Much Power Do You Need for Camping?. And for more appliance wattage examples, check the Camping Appliance Wattage Chart.
About Madeline Reed
Madeline focuses on practical advice for off-grid power for camping, rv & vanlife: what matters before you buy, what to skip, and how to get more out of what you already own.
