What Size Power Station Do I Need? Use Our Calculator and Guide
Use our watt-hour worksheet to find your capacity target — add up your devices, pick your days off-grid, and get a number before you look at a single product.
Every power station question is secretly the same question: how many watt-hours do you burn per day, and how many days pass between recharges? Answer that and the right size falls out. Skip it and you’ll either overspend by $1,000 or run flat on night two.
This page is the worksheet. If you’d rather it did the arithmetic for you, the sizing calculator uses exactly the formula below and shows its working as you go.
What size power station do I need?
Take each device’s wattage, multiply by the hours you’ll run it in a day, and add them together. Multiply that daily figure by the number of days between recharges, then divide by 0.85 to cover inverter losses:
daily Wh = Σ (watts × quantity × hours)
capacity Wh = daily Wh × days ÷ 0.85A van-life kit running a 12V fridge, a CPAP, two phones and two lights comes to 1,180 Wh per day. Over a single night that’s 1,180 ÷ 0.85 = 1,388 Wh, which puts you in the 1,400–1,750 Wh band once you add sensible headroom.
That’s the whole method. Everything below is detail on getting the inputs right.
Why is rated capacity more than usable capacity?
Capacity is quoted at the battery. You draw power at the outlet, and the inverter that converts DC to AC takes 10–15% along the way. A “1,000 Wh” unit delivers roughly 850 Wh of AC energy.
We use 85% throughout this site — it’s the conservative end of the typical range, and inverter efficiency sags at very low loads, which is where small devices spend their time.
Two consequences worth knowing:
- DC output skips the inverter. Running a 12V fridge or a CPAP from the DC socket rather than an AC plug recovers most of that 15%. If your device came with a 12V cable, use it.
- Cold weather costs you more. Lithium cells lose usable capacity below freezing — LiFePO4 typically won’t charge at all below 32°F, though it will discharge. Winter campers should add headroom beyond the 0.85.
How do I find a device’s wattage?
In order of reliability:
- The label or nameplate. Most appliances state watts, or volts and amps — multiply those for watts. A 120 V device drawing 1.5 A is 180 W.
- The EnergyGuide sticker, for fridges and large appliances. It gives annual kWh; divide by 365 for daily watt-hours. This already accounts for duty cycling, which makes it the best input available.
- A plug-in energy meter. About $20, and it tells you what the device actually uses in your home rather than what the label claims.
Common loads, real numbers
| Device | Draw | Daily energy |
|---|---|---|
| Phone charge | 10 W | 15–20 Wh |
| Laptop | 60 W | 60–120 Wh |
| LED lantern | 5 W | 30–50 Wh |
| 12V camping fridge | 45 W cycling (13.5 W avg) | ~324 Wh |
| CPAP, DC, no humidifier | 10–15 W | ~100 Wh/night |
| CPAP, AC, heated humidifier | 40–90 W | ~520 Wh/night |
| Full-size home fridge | 120 W cycling (48 W avg) | ~1,150 Wh |
| Starlink standard | 50–75 W | 1,200–1,800 Wh (24h) |
| Coffee maker | 900 W | 45 Wh per brew |
| Microwave | 1,000–1,500 W | 25 Wh per minute |
| Space heater | 1,500 W | don’t — use propane |
Why does my fridge use less than its label says?
Because compressors cycle. A fridge runs until the compartment is cold, shuts off, and waits — typically running only 30–50% of the time. The label states the compressor’s draw while running, so it overstates real consumption by roughly half.
Size compressor devices from average draw, not nameplate:
120 W nameplate × 40% duty cycle = 48 W average
48 W × 24 h = 1,152 Wh per dayGetting this wrong is the most expensive mistake in sizing. Using 120 W instead of 48 W would tell you to buy a 3,000 Wh unit where a 1,000 Wh one suffices. The full treatment is in how long will a power station run a fridge.
Watts and watt-hours are different questions
This trips up more buyers than any other single thing.
- Watt-hours (Wh) = how long. Capacity. The size of the tank.
- Watts (W) = what at all. The inverter’s continuous rating. The size of the pipe.
A 2,000 Wh unit with a 1,000 W inverter will not run a 1,500 W microwave for one second, despite having plenty of capacity. Conversely a 300 Wh unit with a 1,800 W inverter could start the microwave and then die twelve minutes later.
Check both. Find your single largest load, confirm the inverter’s continuous rating exceeds it, then size capacity separately.
What about startup surge?
Anything with a motor or compressor — fridges, pumps, power tools, air conditioners — draws three to seven times its running wattage for a fraction of a second at start-up. A fridge that runs at 120 W can spike past 800 W.
Surge doesn’t meaningfully affect capacity; it’s over in milliseconds. It affects whether the inverter trips. Assume 5× the running watts unless the manufacturer publishes a locked-rotor amperage figure, and check the unit’s surge rating against that.
If an appliance runs fine but trips the inverter on start-up, this is why.
Capacity targets by scenario
| Scenario | Target capacity |
|---|---|
| Weekend tent trip (devices + lights) | 300–500 Wh |
| Van weekend (+ 12V fridge) | 1,000–1,500 Wh |
| Boondocking, 3+ nights | 2,000 Wh + 200 W solar |
| CPAP, camping, DC, no humidifier | 300–500 Wh |
| CPAP, outage, AC with humidifier | 1,000 Wh per night |
| Home backup, fridge + phones, one day | 1,500–2,000 Wh |
| Home backup, multi-day | 3,600 Wh expandable, plus solar |
| Off-grid cabin base | 3,000 Wh + expansion batteries |
When does solar change the answer?
Past two or three days off-grid, panels matter more than a bigger battery, because battery capacity is a fixed budget and solar is an income.
A 200 W array realistically returns 600–800 Wh on a clear summer day — treat 60–70% of rated wattage over a good day as your planning figure, not the rated number. That covers most van-weekend loads indefinitely in good sun, and approximately nothing under forest canopy in October.
Match panel wattage to your unit’s maximum solar input. Exceeding it doesn’t break anything; the charge controller simply caps, and you’ve wasted money.
Two ways people get this wrong
Overbuying capacity. The 2 kWh class looks like insurance, but it’s 50–62 lbs and twice the price. If your honest daily figure is 500 Wh, a 1 kWh unit is the right answer and the extra $700 buys you nothing you’ll use.
Underbuying inverter watts. People fixate on watt-hours and forget the continuous rating, then discover their kettle or microwave won’t start. If you intend to run anything with a heating element, you need 1,500 W minimum, and capacity is a secondary concern.
Does buying a “solar generator” change the sizing?
No — and it’s worth being clear about this, because the two terms send people down different shopping paths for identical hardware.
A portable power station is the battery, inverter and ports in one box. A solar generator is that same box sold in a bundle with one or more portable panels. There is no difference in the unit itself. Sizing the battery therefore works exactly the same way whichever term you searched for.
What panels change is the days term in the formula. Without solar you size for the whole trip:
5 days × 494 Wh ÷ 0.85 = 2,906 WhWith 200 W of panels returning roughly 700 Wh on a clear day, your net daily draw drops from 494 Wh to a small deficit, and you can size for two or three days of buffer instead of five:
3 days × 494 Wh ÷ 0.85 = 1,744 WhThat’s the difference between the 3 kWh class and the 2 kWh class — around $1,000. Panels are frequently cheaper than the battery capacity they replace, which is the real argument for them.
The caveat is that solar income is not guaranteed. Plan the battery so that a run of cloudy days is uncomfortable rather than dangerous, especially if a medical device is on the list.
How long will each capacity class actually last?
A quick sanity check once you have your number:
| Class | Usable AC | 12V fridge | Household fridge | CPAP (DC, no humidifier) |
|---|---|---|---|---|
| 300 Wh | 255 Wh | 19 hrs | 5 hrs | 2.9 nights |
| 500 Wh | 425 Wh | 31 hrs | 9 hrs | 4.8 nights |
| 1,000 Wh | 850 Wh | 63 hrs | 18 hrs | 9.5 nights |
| 2,000 Wh | 1,700 Wh | 126 hrs | 35 hrs | 19 nights |
If the class your arithmetic landed on doesn’t cover the load you care about most in that table, you’ve made an error somewhere — go back and check the duty cycle assumptions first, because that’s where errors usually hide.
Next steps
Run your actual devices through the calculator — it applies this formula and shows each step. Then take the capacity band it gives you to our picks by class.
If your deciding load is a fridge, a CPAP, or a campsite, those have their own pages: fridge runtime · CPAP sizing · camping.
Frequently asked questions
- How do I calculate what size power station I need?
- Multiply each device's watts by the hours you'll use it per day and add them up. Multiply that daily total by the number of days between recharges, then divide by 0.85 to account for inverter losses. The result, in watt-hours, is your minimum battery capacity.
- Is a 1,000 Wh power station enough?
- For most weekend campers, yes. It delivers about 850 Wh at the AC outlet, which covers a 12V fridge for roughly two and a half days, or a CPAP for five nights, or a household fridge for 18 hours. It is not enough for home backup beyond about a day.
- What can a 300 Wh power station run?
- Phones (roughly 15 full charges), a laptop (about four charges), camera batteries, LED lights, and a fan — a weekend of device charging. It cannot meaningfully run a fridge, a kettle, a heater, or anything else with a compressor or a heating element, because its inverter tops out around 300 W.
- Why is my power station's usable capacity less than its rating?
- Capacity is measured at the battery, but you draw power at the outlet, and the inverter that converts DC to AC loses 10–15% along the way. A 1,000 Wh unit therefore delivers roughly 850 Wh of AC energy. Running from the DC sockets bypasses the inverter and recovers most of that loss.
- How much headroom should I add when sizing?
- Dividing by 0.85 already covers inverter losses. Add a further 20–30% on top if you camp in cold weather (lithium capacity drops below freezing), if the unit is more than a couple of years old, or if you're sizing for an emergency where running out has real consequences.
- Do I need to size for watts or watt-hours?
- Both, and they answer different questions. Watt-hours decide how long you can run things. Watts decide what you can run at all — if your device draws 1,500 W and the inverter is rated for 1,000 W, capacity is irrelevant because it won't switch on.