Sizing is the step almost everyone gets wrong, in one of two directions. Undersize and you're rationing power by day three, glaring at a dead battery while the sun sets. Oversize and you've wasted real money on panels and capacity you'll never use. We've sized systems for everything from a one-panel kayak-camping rig to a full-time RV setup, and the same five-step worksheet works every time. Grab a notepad — this takes about fifteen minutes.
Step 1: Add up your daily energy use
Everything starts with watt-hours per day. For each device you'll run, multiply its wattage by the hours you'll use it daily. If a device only lists amps, multiply amps by volts to get watts (a 12V device drawing 5A is 60W).
Here's a realistic weekend-camping example:
| Device | Watts | Hours/day | Wh/day | |---|---|---|---| | LED string + lantern | 10W | 5h | 50Wh | | Laptop | 60W | 3h | 180Wh | | Two phones | 10W | 2h | 20Wh | | 12V fridge (duty-cycled) | 45W avg | 24h | ~700Wh | | Fan | 25W | 6h | 150Wh | | Total | | | ~1,100Wh |
A few honest notes from the field. Compressor fridges are almost always the biggest line item, and their draw swings wildly with ambient temperature — a fridge that averages 30W in mild weather can average 60W+ parked in the sun in July. Wattage labels show maximum draw, not average; a "60W" laptop charger averages far less once the battery tops off. When in doubt, round up.
The single best cheap purchase you can make before buying anything is a plug-in watt meter (the classic Kill A Watt style). Measure your actual devices for a few days at home. Measured numbers beat estimated numbers every time, and they're usually lower than you fear.
Step 2: Add 30% for system losses
Power gets lost at every stage between the panel and your device:
- Charge controller conversion: roughly 2–5% with MPPT, more with PWM
- Battery round trip (charge in, discharge out): roughly 5–15% for lithium
- Inverter conversion: roughly 10–15% for AC loads
- Wiring and connectors: 2–5%
Rather than agonizing over each line, multiply your daily total by 1.3:
1,100Wh × 1.3 = ~1,430Wh of generation needed daily.
One refinement worth knowing: inverter loss only applies to AC loads. If you run mostly 12V DC devices (fridge, lights, fans, USB), your real losses are smaller and 1.2 is fair. If everything goes through the inverter, stick with 1.3.
Step 3: Find your peak sun hours
"Peak sun hours" is the equivalent number of hours of perfect, full-strength sun your location gets per day. It's not daylight hours — a 10-hour overcast day might deliver only 2 peak sun hours.
Reasonable planning numbers for the US:
- Southwest (AZ, NM, NV, SoCal): 5.5–7 hours
- Southeast and southern Plains: 4.5–5.5 hours
- Midwest, Mid-Atlantic, Northeast: 4–5 hours
- Pacific Northwest and northern tier: 3–4 hours
- Winter, anywhere: cut your number by 30–50%
NREL publishes free solar resource maps if you want your exact location, but for sizing purposes these brackets are fine. The critical decision: size for the worst month you'll actually use the system. A summer-only camping rig can use generous numbers; a year-round cabin must be sized for December, which often doubles the panel requirement.
Step 4: Calculate panel wattage
The formula:
Panel watts = daily Wh needed ÷ peak sun hours ÷ 0.75
That 0.75 derating reflects reality: panels rarely deliver their sticker rating outside a lab. Heat, haze, imperfect angles, dust, and early/late sun all shave output. Flat-mounted panels (RV roofs) lose another chunk versus tilted ones.
Our example, camping in the Midwest at 4.5 sun hours:
1,430Wh ÷ 4.5h ÷ 0.75 = ~425W of panel
So a 400W array is the sensible target — two 200W folding panels, or four 100W rigid panels on a roof. If that number shocks you, remember the fridge: drop it and the same camper needs barely 150W.
Renogy
Renogy 400W Portable Solar Suitcase
Serious harvest for big stations: 400W folding suitcase that can refill a 1kWh unit in about three hours of decent sun.
Step 5: Size the battery bank
The battery has to cover you from sunset to sunrise plus however many bad-weather days you want to ride out. We size for 1.5 to 2 days of autonomy for most uses — one full cloudy day plus margin. Remote cabins where a generator isn't an option justify 3 days.
With LiFePO4, you can safely use 80–90% of rated capacity, so:
Battery Wh = daily Wh × days of autonomy ÷ 0.85
Our example: 1,430Wh × 1.5 ÷ 0.85 = ~2,500Wh, which maps neatly to a pair of 12V 100Ah LiFePO4 batteries (about 2,560Wh) or one 2kWh-class power station plus margin discipline.
If you're shopping lead-acid (we'd talk you out of it, but still): you can only safely use about 50% of rated capacity, so double everything above.
Worked examples by use case
The weekend camper (no fridge): Lights, phones, a laptop, a fan — around 400Wh/day. After losses: ~520Wh. A 100–200W panel and a 500–1,000Wh power station covers this comfortably with room for a cloudy day.
The car camper / overlander (with 12V fridge): Our worksheet example. ~1,100Wh/day, ~1,430Wh after losses. Wants 400W of panel and 2,000–2,500Wh of storage. A 1kWh power station can work here if you also charge from the alternator while driving — vehicle charging effectively substitutes for several hundred watt-hours of solar.
Anker
Anker SOLIX C1000 Gen 2
Anker’s refined 1kWh station: 1,024Wh LiFePO4 pushing 2,000W (3,000W peak) with ultra-fast recharging and a quieter thermal design.
The full-time RVer: Realistic daily use runs 1,500–3,000Wh once you add a TV, residential-style fridge, coffee maker, and work-from-anywhere electronics. After losses: 2,000–4,000Wh. That means 600–1,000W of panel and a 4,000–6,000Wh battery bank. This is firmly DIY-component territory — see our RV solar installation guide for the build.
The off-grid cabin: 3,000–6,000Wh/day depending on whether you run a well pump and full-size fridge. Sized for winter sun, that's 1,200–2,500W of panel and 8,000–15,000Wh of storage. At this scale, get the loads measured precisely first — every 500Wh/day of error costs real money.
Quick-reference chart
| Daily use (after losses) | Panel array | Battery bank (LiFePO4) | |---|---|---| | 500Wh | 150–200W | 1,000–1,500Wh | | 1,000Wh | 300–400W | 2,000–2,500Wh | | 2,000Wh | 600–800W | 3,500–5,000Wh | | 3,000Wh | 900–1,200W | 5,000–7,000Wh |
These assume roughly 4.5 peak sun hours. Sunnier? You can trim panels 20%. Pacific Northwest or winter use? Add 30–50% to the panel column, not the battery — extra panels rescue you from gloomy stretches far more effectively than extra storage you can't refill.
Mistakes that wreck the math
- Sizing for average days instead of bad days. Averages include the perfect days. Your system fails on the worst ones.
- Forgetting phantom loads. Inverters draw 5–25W just being on. Left on 24/7, that's up to 500Wh/day — sometimes more than your actual devices. Buy an inverter with a low-power standby mode or switch it off.
- Believing the power station's sticker. A "1,000Wh" station delivers roughly 800–850Wh to AC devices after inverter losses. Plan with the real number.
- Maxing out the charge controller on day one. Buy a controller with 25–50% headroom over your initial array. Everyone adds panels eventually; nobody enjoys buying the same controller twice.
- Ignoring panel orientation. Flat-mounted roof panels harvest 10–25% less than tilted ones, and the gap widens in winter when the sun sits low. Portable panels you can aim partially offset a smaller array.
Don't forget the inverter and controller
Panel and battery sizing get all the attention, but two more components need numbers before you order.
Inverter sizing is about peak simultaneous load, not daily energy. Add up the wattage of everything you'd realistically run at the same moment — say, a 600W microwave plus 100W of laptop and lights — and buy an inverter rated 25–50% above that, with surge capacity at least double for anything with a motor or compressor. Two warnings from experience: first, resistive appliances (kettles, toasters, hair dryers, space heaters) draw 1,000–1,800W and will dictate your inverter size all by themselves, so decide early whether they're really coming off-grid with you. Second, don't buy a 3,000W inverter "to be safe" for a 500W lifestyle — big inverters idle at 15–25W, and that standby draw runs around the clock against the daily budget you just calculated.
Charge controller sizing has two checks. Output side: panel watts ÷ battery voltage gives maximum charging amps (400W ÷ 12V ≈ 33A, so a 40A controller fits). Input side, for MPPT: your array's open-circuit voltage — corrected upward for the coldest morning it will ever see, since panel voltage rises as temperature drops — must stay under the controller's maximum input voltage. Then add our standing advice: buy 25–50% more controller than today's array needs. Controllers are the component people outgrow first, and the upgrade means redoing wiring you just finished.
Common sizing questions, answered quickly
Can I just buy a big power station and skip the math? You can, and for casual use it works — a 1kWh-class station with 200W of panel covers most weekend scenarios. But the math takes fifteen minutes and routinely saves people from buying double the station they need, or half — and either mistake is an expensive one at power-station money.
Should I oversize panels or battery first? Panels, almost always. Extra battery capacity you can't refill is dead weight; extra panel harvest shortens recovery after cloudy days and extends your usable season into spring and fall. The exception is quiet-hours-heavy usage (CPAP, overnight fridge in hot weather), where the battery does the heavy lifting.
How much does winter really change things? More than people expect. Between shorter days, lower sun angles, and weather, December harvest in the northern half of the US runs 40–60% below July. A year-round system sized only for summer will spend the winter disappointing you. Size for your worst month of actual use, or plan a backup charging source (vehicle alternator, generator, or grid) for the dark months.
Do I need to derate for panel aging? A little. Quality panels lose roughly 0.5% output per year — about 10% over two decades. It's worth a mental note for permanent installs, but the day-one derating for heat and angle (our 0.75 factor) matters far more.
Sanity-check before you buy
Run your final numbers through this checklist: Can the battery cover one fully cloudy day? Can the panels refill the battery from half-empty in one decent day? Is the inverter rated above your largest simultaneous AC load, with surge headroom for anything with a motor? Does the charge controller have room for one more panel? If you answered yes four times, buy with confidence.
Where to go next
See our best portable power stations for pre-built systems matched to the sizing brackets above, or back up to the beginner's guide to off-grid solar if any component along the way felt unfamiliar.