Why a 50W fridge doesn't draw 50W for 24 hours

What a compressor fridge's nameplate watts actually measures

A 12V compressor fridge doesn't run its compressor continuously. It cools the box down to a thermostat setpoint, shuts the compressor off, and lets the box slowly warm back up before switching on again — the same cycle a household refrigerator or a car's air conditioning runs, just on a smaller scale. The nameplate wattage is the draw while the compressor is actively running, not a continuous, always-on figure. A fridge rated at 50 W draws close to 50 W during an "on" cycle and close to 0 W during an "off" cycle, and the fraction of time it spends in the "on" state is its duty cycle.

Multiplying rated watts by 24 hours silently assumes a 100% duty cycle — the compressor running nonstop, all day, every day. No compressor fridge actually does this in normal use; if one did, it would mean the box could never reach or hold its setpoint, which is closer to a fault condition than an efficient appliance.

The formula

Daily Wh = rated watts × duty cycle × 24 hours

Rated watts comes off the fridge's own spec sheet. Duty cycle has to come from the manufacturer's own measured energy or duty-cycle figure — this project's catalog only records a duty_cycle value when one has actually been published, per its sourcing rule that no value gets estimated, guessed, or borrowed from a similar-looking model. A fridge without a manufacturer-published duty cycle stays without one in the database rather than getting a plausible-sounding placeholder.

A concrete example: the gap in real numbers

The Dometic CFX3 35 is rated at 50 W and has a manufacturer-derived duty cycle of 23.52% in this project's catalog. Run both calculations on the same fridge.

The nameplate-only figure overstates real daily consumption by 4.25x. Sizing a battery bank or array around 1200 Wh/day for this one appliance means the fridge portion of the system is more than four times larger than it needs to be — real money spent on panels, battery capacity, and wiring ampacity that a correctly read spec sheet would never have required.

The gap isn't unique to this one model. Other fridges in this project's catalog carry their own published duty cycles, and every one of them shows the same pattern — a nameplate figure several times higher than the real 24-hour average, because every compressor fridge cycles, and none of them run at 100% duty in normal conditions:

Notice the Dometic CFX3 100 draws more rated watts than the CFX3 35 (73 W vs. 50 W) but a lower real daily total (253.0 Wh vs. 282.2 Wh), because its duty cycle is under 15% — a larger, more efficient compressor and better insulation mean it needs to run far less often to hold temperature. Rated watts alone, read as a 24-hour draw, would rank these fridges in the wrong order entirely.

Why duty cycle itself varies — and why this project won't guess it

Duty cycle isn't a fixed property of a compressor; it's the outcome of a thermal balance between how fast the box gains heat and how fast the compressor can remove it. That balance shifts with several real, independent factors: ambient temperature (a fridge in a hot vehicle interior in July cycles far more than the same fridge in a cool garage in October), insulation quality and box size (a smaller, better-insulated box sheds less heat and needs fewer "on" cycles), how full the fridge is (a fully loaded box — especially with frozen or pre-chilled contents — holds cold better than a mostly empty one, reducing cycling), and the thermostat setpoint (35°F cycles far more than 45°F for the same box in the same conditions).

Because duty cycle depends on all of these at once, it can only be measured, not derived from rated watts — and it's only ever correct for the conditions it was measured under. A manufacturer's published duty-cycle or energy figure is normally measured under a specific standardized ambient temperature, which is why this project's sourcing rule (§ data/manual/README.md) requires that number to come from the manufacturer's own testing rather than a rule-of-thumb percentage applied to every fridge in the catalog. A guessed 25%-for-everything shortcut would have been directionally right for the Dometic CFX3 35 in this example and off by more than 2x for the CFX3 100 — a guess that happens to work for one model is not a substitute for a real measurement on the next one.

How this project actually computes it

This project's dailyLoad() function takes a list of appliances, each with its own rated watts, duty cycle, hours of use per day, and quantity, and multiplies them through exactly as shown above — duty cycle defaults to a full 100% (no correction) only when an appliance genuinely has no cycling behavior to account for, such as an LED light that's simply either on or off for its stated hours. For a fridge, plugging in a real manufacturer duty cycle is what turns "rated watts times 24 hours" into an honest daily energy figure instead of a multiple-times-too-high overestimate.

Why this mistake is easy to make even for a careful buyer

Nothing about a nameplate figure announces that it's a running-state number rather than an average. A fridge's spec sheet lists "50 W" the same way a light bulb's spec sheet lists "10 W" — but a bulb really does draw 10 W for every hour it's switched on, so multiplying by hours-of-use is the correct method for a load with no cycling behavior. A compressor fridge looks like the same kind of appliance on a spec sheet — one wattage number, presumably describing the same "while it's running" state as everything else on the page — and the natural instinct is to apply the same multiplication that works correctly for lighting, electronics, and every other steady-draw appliance in a system's load list. The cycling behavior that makes this wrong for a fridge isn't visible anywhere on the spec sheet unless the manufacturer separately publishes a duty cycle or a measured daily/annual energy figure — which is exactly why this project's catalog treats that number as a distinct, separately sourced field rather than assuming it can be inferred from rated watts alone.

What happens when the wrong number gets used

Sizing a load list off nameplate-times-24 for a fridge doesn't just make the fridge line item too large — it pulls every downstream calculation along with it. A daily load figure inflated by roughly 900 Wh (the CFX3 35's gap between 1200 Wh and 282.2 Wh) increases the required battery bank capacity by that same 900 Wh times the autonomy days the system is designed around, and increases the required array wattage by the same proportion through arrayBalance(). For a system where the fridge is the single largest continuous load — common in a van or cabin build with otherwise modest lighting and device charging — this one unit conversion error can be the difference between a bank and array sized correctly for the actual house and one oversized by a large fraction of its total cost, all traceable to reading a nameplate watt figure as if it were a continuous draw.

Try it with a real duty cycle

Enter a fridge's rated watts and its manufacturer-published duty cycle to see both the nameplate-only estimate and the real daily energy this project's dailyLoad() function computes for a single fridge running 24 hours a day.

What to actually size against

Three practical consequences follow from this, not from a rule of thumb:

1. Never multiply a compressor fridge's rated watts by 24 hours directly. That number describes the compressor's draw while running, not the fridge's continuous average draw — it will always overstate real consumption for any fridge that isn't running 100% of the time, which in practice is every compressor fridge.
2. Use a manufacturer-published duty cycle or measured daily Wh figure, not a guessed percentage. Duty cycle depends on ambient temperature, insulation, fill level, and thermostat setting all at once — a shortcut percentage that happens to fit one model can be off by more than 2x on the next, as the CFX3 35 vs. CFX3 100 comparison above shows directly.
3. Expect real-world duty cycle to run higher than a lab figure in hot conditions. A manufacturer's published duty cycle is measured at a specific ambient temperature; a fridge living in a hot vehicle or an uninsulated van in summer will cycle more than its spec sheet implies, which is a real margin worth building into a sizing decision even when the base figure is correctly sourced.

Every published duty cycle used in these calculations comes from the same appliance catalog this site's checker draws on, so a fridge's real daily draw is a lookup, not an estimate, whenever the manufacturer has published one.