Power sizing for overlanding trips.

The first number a serious buyer can write down is not the watt-hours on the spec sheet — it is the watt-hours the trip actually consumes across a single twenty-four-hour window, with a margin for the things the catalogue didn’t list. Begin by naming every electric load on the trip: the fridge, the lights, the comms stack, the camera batteries, the laptop at night, the water-pump if the rig carries one, the air compressor if the rig runs one, the cooking appliance if the cooking appliance is electric. Each one gets a wattage figure pulled from its maker, not from the catalogue’s headline number, and each one gets a duty cycle — the hours per day the device is actually drawing, not the hours it is sitting on the bench.

Multiply wattage by duty cycle and sum across the load list. The number that falls out is the gross daily consumption in watt-hours; the number a serious buyer plans against is 1.4 to 1.6 times that, because the catalogue-compiled watt hours are usually best-case and the trip is rarely best-case. A fridge that draws an average of forty watts over twenty-four hours is a 960-watt-hour line item, but a fridge that opens thirteen times on a hot afternoon draws sixty watts averaged across the window and rounds up to fourteen-hundred-and-forty watt-hours. The honest budget sits at the second number, not the first.

The next decision is how much of the load you plan to run on the alternator while driving, on shore power on the nights you find it, and on the station itself overnight. That fraction is what the station actually has to deliver: it is the budget, not the headline number. A buyer who plans to drive six hours on a travel day and lets the rig charge while rolling has cut the budget by a quarter; a buyer who plans two hours of driving and eight hours of camp has not. The honest answer to “do I have enough station” is rarely the watt-hours figure on the box; it is the budget minus the alternator minus the shore-power nights, plus the margin.

Cooking on an induction plate is a category of its own and the one most buyers underestimate. A two-burner induction head running on a low simmer draws around eight hundred watts averaged across the cycle; running both heads at a rolling boil will pull eighteen hundred watts continuous and spike higher on startup. A kettle is one thousand to fifteen hundred watts for the four minutes it actually runs, but those four minutes are the longest sustained load the station will see all day and they will dictate the station’s inverter spec more than any other item on the list. The wrong kit on the wrong cooking decision is a station that closes the kettle halfway through because the inverter has tripped, and that trip is the trip you remember.

Refrigeration is the load that runs all day, every day, and the load the station is sized around once cooking is sized separately. A compressor fridge in a thirty-degree ambient draws roughly forty to sixty watts averaged over twenty-four hours, depending on insulation and door-opening discipline; a cheap absorptive fridge on propane is the same daily figure but on a different fuel. The relevant question is not “how big is the fridge” but “what is the daily duty cycle from cold soak to cold soak”. A station that comfortably carries a compressor fridge for three days but only barely for four is a station whose four-day margin is the wrong margin to live with — because the fourth night is when you actually need it.

Lighting and comms are the long tail of the draw list, but they add up across a multi-day trip. A pair of LED camp lights on a five-hour duty cycle draw perhaps forty watt-hours between them; a Starlink terminal drawing sixty watts averaged across its window is two-hundred-and-eighty watt-hours; a two-radio VHF/UHF setup on standby is a rounding error. The long tail rarely breaks a sizing budget, but the long tail is also the part of the budget a buyer is tempted to dismiss as “negligible” — and the dismissal catches up with the buyer on the night the comms stack is the only thing keeping the trip safe. Plan the long tail explicitly; round it up, not down.

The inverter is the part of the station that most often fails to size, and the failure is the same shape on every trip: a load that fits inside the station’s continuous wattage but sits above the inverter’s actual continuous wattage once the load is wired through it. A station published at two thousand continuous watts may carry an inverter rated at fifteen hundred watts continuous and thirty-five hundred peak; a kettle that pulls eighteen hundred watts continuous will trip the inverter on the second boil of the evening, and the station’s two-thousand-watt label will be the line the buyer remembers as misleading. Ask the maker what the inverter’s continuous rating is, separately from the station’s continuous rating, because the two numbers are not always the same and they are the numbers that decide whether the kettle boils.

Peak surge is the second inverter number that bites. Motors — fridge compressors, water pumps, air compressors — draw three to seven times their rated wattage for the few hundred milliseconds they take to start. An inverter that can sustain fifteen hundred watts continuously and surge to three thousand for half a second will start a fridge compressor; an inverter that can sustain fifteen hundred watts continuously and surge to twenty-five hundred will not. The cycle repeats across every motorized load on the trip, and it is the inverter that decides whether the fridge stays cold or whether it clicks off mid-afternoon. The wrong kit on the wrong compressor is the compressor that won’t restart, and the food that doesn’t make it to dinner.

Headroom is the third rule. The conservative sizing stance is to plan the largest continuous load to sit at no more than seventy to eighty per cent of the inverter’s continuous rating, with a margin against the inverter’s thermal-derating curve once the unit is warm. An inverter that has been running for an hour at fifteen hundred watts in a hot cargo bay will derate before it reaches its published figure, and a kettle that started cleanly on the first boil will be the kettle that won’t start on the fifth. The right posture is the one that picks the inverter with the next-step headroom, not the one that picks the station with the largest headline figure — the station is a battery, the inverter is the gate, and the gate is the part that decides the trip.

The recharge side of the budget is the side the catalogue tends to flatter, and the side the trip tends to expose. A two-hundred-watt rigid panel rated at twenty-three per cent efficiency, in full midday sun, perpendicular to the sun, will produce around one-hundred-and-seventy watts for an hour or two — and that is the best figure the panel will ever produce on the trip. The same panel at forty-five degrees off-axis early in the morning will produce sixty; the same panel on a dash mount through the windshield will produce forty; the same panel at three in the afternoon with a hot panel surface will derate even further, because panel output drops as cell temperature climbs and a panel baking in desert sun is running fifteen to twenty-five degrees above ambient.

Curve shape matters more than peak figure. Solar output on a moving trip is not a four-hour window of peak production; it is a bell curve that rises through mid-morning, peaks briefly near solar noon, and falls back through mid-afternoon. A buyer who plans a one-hundred-and-twenty watt-hour daily deficit against two-hundred watts of panel is planning against a four-hour peak that the day never actually delivers. The honest number is the watt-hours the trip accumulates across the entire sun window — and that number, on a clear day with cable losses and angle losses baked in, sits at roughly fifty to sixty per cent of the panel’s published peak wattage. The rest is the published figure that didn’t survive the actual sun.

That number is also where a kit either closes or doesn’t. A trip that needs six-hundred watt-hours per day from the station, against two-hundred watts of panel delivering one-hundred-and-ten averaged across eight hours, runs a five-hundred-watt-hour daily deficit and never quite closes the cell bank at the end of the trip. The honest sizing math — the math a serious buyer should do on paper before the catalogue tab opens — usually reveals that the right station is the next tier up, not the headlined tier, and the right panel array is the one sized to refill the next tier, not the headline tier. The StoneWave Power Station is the kit a serious overland buyer walks toward once the sizing math has actually been run; the sizing math is what the catalogue page won’t do for you.

Read the kit: StoneWave Power Station— the kit a serious overland buyer walks toward once the sizing math has closed.

Panel output is also the part of the system most exposed to the field, and the field is rarely the clean glass of the catalogue photo. Cloud cover drops panel output meaningfully — diffuse-light conditions under a high overcast run at twenty-five to forty per cent of rated output; partial cumulus with moving shadows is even worse, because the panel spends half the cycle in a shadow band and the inverter sees the drop as a series of transient faults. A higher-altitude trip with intermittent afternoon cloud is a panel output curve that looks like the clear-day bell curve with the middle sawn out, and the daily recharge falls into the four-hundred-watt-hour range rather than the eight the catalogue copy implies.

Dust is the slower, quieter loss on long rotations over unpaved track. A clean panel in dry, dusty air will lose five to eight per cent of output to surface scatter over the first day; a panel that sees a week of desert track without a clean will lose fifteen to twenty-five. The honest field practice is to wipe the panel with a soft cloth at every camp, a discipline that costs five minutes and recovers a meaningful fraction of daily yield. The honest kit stance is the one that buys one panel class larger than the catalogue math suggests and accepts that the panel will spend its life dirty, not clean, and that the diagram on the catalogue page is not the diagram the trip delivers.

High-altitude UV is a third loss that long-trip buyers underestimate. UV intensity rises with altitude, but panel output does not — the same panel at the same angle in the same sun produces the same watts. The feeling that the trip is producing more, because the sun is more intense, is partly a perception artefact and partly a real gain from cooler cell temperatures at altitude. The right altitude practice is to angle the panel for morning sun a bit more aggressively than the catalogue maths suggests, because the cold early-morning panel is the panel that produces the most watts per dollar and the cold late-afternoon panel is the panel that closes the day’s budget before sunset.

Cold weather is the loss the catalogue explicitly excludes from its rated figures and the loss the overland buyer describes most often in the second-trip conversations. Lithium chemistry derates at low temperatures — the cell’s internal resistance rises, the usable capacity drops, and the charge acceptance curve flattens. A station that delivers a hundred per cent of its rated capacity at twenty-five degrees will deliver eighty-five to ninety per cent at zero, seventy-five to eighty-five at minus ten, and a step-function drop below minus fifteen depending on the chemistry. LiFePO4 is more tolerant than NMC at low temperatures, but the tolerance is a curve, not a flat line, and the curve passes through the camp temperature on a winter trip.

Charging in the cold is the worse failure mode. A lithium cell that can safely discharge at minus ten is, on most chemistries, a cell that should not be charged below zero without a low-temperature cut-off in the BMS. A station plugged into a panel in the cold morning sun is asking the BMS to push current into a cold cell, and the honest BMS will refuse; the cheaper BMS will accept the charge and degrade the cell across the cycle. Ask the maker what the published low-temperature charge window is, whether the BMS enforces it, and what the maker does at the boundary — the answer tells you whether the station was designed for the winter trip or whether the winter trip is the trip the maker didn’t design for.

Field posture is the third decision. A station carried inside the vehicle, in the cabin, in winter, is a station that holds a usable percentage of its rated capacity across the trip; a station carried in an uninsulated cargo bay, exposed to overnight lows, is a station that derates every night and recovers by mid-morning. The same station, the same trip, the same chemistry, gives two different sizing outcomes depending on where it lives. The right answer is to size for the station’s worst-case position on the trip — the cold-soak position, the cargo-bay position, the position the catalogue doesn’t describe — and to buy the next-tier-up station if the worst-case position is what the trip is going to deliver. The honest catalogue posture is the one that admits this, and the honest kit posture is the one that plans for it.