Calculate fridge consumption
Computes consumption from physics instead of a duty cycle: heat ingress through the cabinet, door openings and the compressor cooling capacity give the real duty cycle. For a van in the shade at 20 °C that is 328 Wh a day instead of the 378 Wh the usual 35 % rule of thumb claims — and 75 kWh over the year at the site. Plus the inrush current that defeats inverters.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-07-15
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Ambient temperature at the location | T_out + ΔT_solar | 25 °C | assumed |
| Heat ingress | UA * (T_amb - T_in) + Q_door | 27.333 W | exact |
| Duty cycle | Q_in / P_cool | 0.3037 | exact |
| Consumption per day | P_el * 24 h * duty | 328 Wh/d | exact |
| Inrush current | I_cont * f_inrush | 18.75 A | exact |
- Formula
duty cycle = UA · (T_ambient − T_interior) / P_cooling- Valid for
- Fixed-speed compressor fridge, steady operation, stocked compartment.
- Not covered
- Defrost cycles, variable-speed inverter compressors, the pull-down after loading warm food, evaporator icing. The heat-gain factor per location is an experience value and can be overridden.
- Data sources
- Aggregate of manufacturer datasheets, the EU energy label database EPREL and measurement series from off-grid practice · retrieved 2026-07-15
Frequently asked questions
How much power does my compressor fridge use per day in a van?
The calculator derives the duty cycle from ambient temperature: heat ingress UA · (T_ambient − T_interior) plus door openings, divided by the cooling capacity. Daily consumption is then electrical power times 24 hours times the duty cycle. A fridge in a hot van in the sun cycles roughly twice as often as the same unit in the shade — and uses twice the energy.
Why is the standard 0.35 duty-cycle assumption wrong?
Because it ignores ambient temperature — in summer it is off by about a factor of two. The calculator shows what the fixed assumption would give as a comparison figure and warns when the deviation gets large. The heat-gain figure per location (vehicle in sun, in shade, indoors, cellar) is a clearly labelled experience value you can override.
Why does my inverter trip when the fridge compressor starts?
The compressor's start-up current exceeds its running current by a start factor and can trip the inverter when the battery's state of charge is low — the calculator reports that current and warns about it. Defrost cycles, variable-speed inverter compressors, the pull-down after loading warm food and evaporator icing are outside the model.
Why does the same fridge cycle twice as often in a van as in a cellar?
Because heat ingress grows linearly with the temperature difference: at 12 °C cellar temperature and 5 °C interior it is 7 K; in a sun-exposed van at 45 °C installation temperature, 40 K — nearly six times as much. The duty cycle is heat ingress divided by cooling capacity, and daily consumption scales directly with it. The calculator computes this from your installation spot instead of assuming a fixed value.
Where do I get my fridge’s UA value?
Either from the datasheet (heat transfer in W/K) — or back-calculated from a measurement: measure daily consumption at a known ambient temperature, then UA ≈ consumption × COP chain / (24 h × temperature difference). Typical compressor coolers sit at 1–2 W/K. The measurement route is more accurate because it includes door openings and insulation ageing.
When does the calculator report that the compressor cannot keep up?
When heat ingress reaches cooling capacity: the compressor would have to run continuously (100% duty) and the interior temperature still rises. This really happens in a closed vehicle in blazing sun. Remedy: shade and ventilate the installation spot — that cuts consumption more than any more efficient unit.
How much electricity does a fridge use per year?
It depends on the placement and the climate, and the calculator now derives both from the annual outdoor temperature profile. For a 12 V compressor fridge in a van parked in the shade in the Berlin region it comes to about 75 kWh a year. The daily figure above applies only to the design case you entered — in high summer it is markedly higher, in winter markedly lower. Sizing follows the design case; running cost follows the year.
Which outdoor temperature should I enter for sizing?
The worst case the system still has to cover — not the annual average. The calculator checks your entry against the warmest month at your location and speaks up when the two are more than 5 kelvin apart. In the Berlin region the warmest month averages 21.7 °C; anyone travelling in southern Europe has to assume considerably more. And a vehicle parked in the sun adds a further double-digit number of kelvin on top.
How much does a shaded parking spot save?
More than any insulation retrofit. Heat ingress grows linearly with the temperature difference between surroundings and cabinet — and a vehicle parked in the sun heats up by a double-digit number of kelvin compared with shade. The calculator carries this uplift as its own visible value per placement. Parking the van in the shade cuts fridge consumption more than any change of appliance.
Why is the annual figure lower than the daily figure suggests?
Because the daily figure applies to the design case, and that does not hold all year. 328 Wh a day would extrapolate to roughly 120 kWh a year; the actual result is 75 kWh, because the fridge cycles far less in the cool months. That difference is exactly why sizing and running cost are two separate calculations — and why rules of thumb miss both.
Is a 100 Ah battery enough for the fridge?
For the fridge alone yes, but the question is framed wrongly. At 328 Wh a day and 12 V that is about 27 Ah daily. A 100 Ah lead battery releases only around 50 Ah of that usefully, a LiFePO4 about 80 Ah — so barely two or three days without recharging, and that is before any other device is connected. The 18.8 A inrush current is the second hurdle: it sizes cables and inverter, not capacity.