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RV Refrigerator Consumption Calculator: electricity and propane usage

A 45 W compressor fridge runs about 40% of the time in summer and thus needs around 430 Wh a day – almost half of a 100 Ah LiFePO4. An absorption fridge uses about 0.8 lb (350 g) of propane daily but draws over 10 A on 12 V. This calculator shows what your fridge really consumes in each mode and how long the battery can keep up.

Units

Nameplate: compressor 40–60 W, absorption 12 V 120–170 W, absorption 120 V 125–190 W, Peltier cooler 40–60 W.

For 12 V operation of the absorption fridge: driving time per day.

Share of time the compressor or heating element runs: about 30% at 68 °F (20 °C), 50% at 86 °F (30 °C), more with frequent opening.

Battery (for the runtime)

LiFePO4 80–90%, lead-acid about 50%.

Result

Power consumption per day

432 Wh

Equals
33.8 Ah/day
Full battery lasts (fridge only)
2.4 days
Per week
3 kWh
Show calculation
  1. 1Energy: 45 W × 24 h × 40% = 432 Wh/day
  2. 2Equals: 432 Wh ÷ 12.8 V = 33.8 Ah/day
  3. 3Battery runtime: 100 Ah × 12.8 V × 80% ÷ 432 Wh/day = 2.4 days

 

How it's calculated

Compressor refrigerators cycle via thermostat: the rated power is only drawn during run phases. Consumption therefore depends on the duty cycle – and that on ambient temperature, insulation, how full it is and door openings.

Energy [Wh/day] = rated power [W] × 24 h × duty cycle
45 W × 24 h × 40% = 432 Wh

Absorption refrigerators produce cold from heat – from propane, a 12 V heating element or 120 V AC. On propane the consumption in g/h (or BTU/h) per the data sheet counts; on 12 V the heating element runs continuously without a thermostat (only while driving), on 120 V it cycles.

Propane [g/day] = consumption [g/h] × operating hours 12 V: energy = heating element [W] × driving hours 120 V: energy = heating element [W] × hours × duty cycle

Thermoelectric coolers (Peltier) run continuously and only reach about 27–36 °F (15–20 °C) below ambient temperature.

Battery runtime [days] = capacity [Ah] × voltage [V] × usable fraction ÷ energy per day

Worked example

A 3.2 cu ft (90 L) compressor fridge rated at 50 W runs about 50% of the time at 82 °F (28 °C); the house battery is a 100 Ah LiFePO4 with 80% usable.

Inputs

  • Refrigerator type: Compressor refrigerator (12 V)
  • Power draw: 50 W
  • Operating hours per day: 24 h
  • Duty cycle (thermostat): 50 %
  • Battery capacity: 100 Ah
  • Usable fraction: 80 %
  • System voltage: 12 V LiFePO4 (12.8 V)

Result

600 Wh

Power consumption per day

Equals
46.9 Ah/day
Full battery lasts (fridge only)
1.7 days
Per week
4.2 kWh

Calculation

  1. Energy: 50 W × 24 h × 50% = 600 Wh/day
  2. Equals: 600 Wh ÷ 12.8 V = 46.9 Ah/day
  3. Battery runtime: 100 Ah × 12.8 V × 80% ÷ 600 Wh/day = 1.7 days

The variables explained

Power draw (W)
Per the nameplate. Compressor 40–60 W, absorption heating element 12 V 120–170 W, 120 V 125–190 W.
Propane consumption (g/h)
Per the absorption fridge's data sheet; 14–18 g/h (≈ 670–860 BTU/h) is typical for 3–3.5 cu ft (80–100 L).
Duty cycle (%)
Run-time share of the compressor or the 120 V heating element. 30% at 68 °F (20 °C), 50% at 86 °F (30 °C), up to 70% in high summer.
Operating hours (h)
24 h when parked; for 12 V absorption fridges the daily driving time.

Common mistakes

  • Calculating a compressor fridge at rated power × 24 h – that overestimates consumption two- to threefold.
  • Running an absorption fridge on 12 V while parked: 130 W without a thermostat drains a 100 Ah battery in less than a day.
  • Neglecting ventilation: blocked vents or sun on the fridge wall raise the duty cycle considerably.
  • Underestimating freezing supplies: a freezer compartment in use can double consumption.

Assumptions and limits

  • Constant duty cycle over the day; at night the compressor usually runs less often.
  • Typical consumption values per manufacturer data (Dometic, Thetford) – your own data sheet takes precedence.
  • Absorption fridges on propane additionally need a few Wh per day for control and ignition (neglected).

Frequently asked questions

How long does a 100 Ah battery last with a compressor fridge?

At 430 Wh daily consumption and 1,024 Wh usable energy (LiFePO4, 80%) about 2.4 days – with no other loads. With an AGM (600 Wh usable) about 1.4 days.

Absorption or compressor – which is more economical?

Parked without shore power, the absorption fridge on propane uses practically no electricity and needs 0.7–1 lb (300–450 g) of propane a day. The compressor needs 300–600 Wh of electricity but also cools reliably in heat and when not level. With a solar system the compressor is usually the better choice today.

Sources and background