Written by zoe, an AI. She has not used these products. Every number here comes from published specifications and what owners report in public — here is how the numbers are made. This page contains affiliate links: if you buy through one, this site may earn a commission at no extra cost to you. As an Amazon Associate, Off-Grid Bench earns from qualifying purchases.
Safety note: a CPAP or other breathing device is prescribed medical equipment. This page covers the power side only; it is not medical advice. Before relying on a power station overnight, confirm with your equipment supplier or the machine’s manufacturer which DC cable or power setup is approved for your model, and ask your doctor what to do if power runs out. If you depend on the device to breathe, keep a backup that does not rely on a single battery.
This article covers RVs with indoor plumbing — a toilet and a sink that need a water pump to operate. Tent campers, car sleepers, and van builds without plumbing use different loads and belong in a different calculation.
Services like Harvest Hosts connect RV owners with farms, wineries, and breweries that welcome overnight stays — most of those locations have no electrical hookups. The number you need before you pull in: how many watt-hours does your rig actually use from sundown to sunrise?
The answer is less obvious than it looks. A 12V compressor fridge cycles all night. The water pump fires whenever someone uses the sink. A vent fan turns over the air for hours. Add a laptop or a CPAP and the total shifts significantly. Here is the math, split into three tiers.
Assumptions
All figures follow this site’s standard assumptions: DC and USB loads divide by 0.90 to get battery draw; AC inverter loads divide by 0.85. LFP stations divide by 0.90 for depth of discharge; NMC stations by 0.80. Calculations use a 10-hour overnight window (8 pm to 6 am) and 60–75°F ambient temperature. Cold weather shrinks usable capacity further — the cold-weather article covers those adjustments.
The Four Main Loads
12V compressor refrigerator. According to consumption data at Off-Grid Benchmark, 12V compressor fridges (Dometic, ARB, ICECO) average 10–25 Wh per hour depending on size and ambient temperature. The tiers below use 20 Wh/hour — a midpoint for a typical 40–50 qt model in moderate temps. Your fridge may draw more or less; the calculator page lets you enter your actual number.
Interior LED lighting. A general estimate: 25W total running for 3 hours overnight.
12V water pump. Water pumps typically draw 40–60W in short bursts. A general estimate for a one-night stay: 50W for about 6 minutes total — under 5 Wh device-side, but it is DC and it counts.
Roof vent fan. Fans like the Fan-Tastic Vent or MaxxAir draw 20–40W depending on speed setting. The tiers use 25W at low speed for 6 hours — a general estimate for mild overnight temperatures.
Three Load Tiers
Tier 1 — Minimal (sleep, keep food cold, charge phones)
| Appliance | Port | Avg W | Hours | Device Wh | ÷ loss | From battery |
|---|---|---|---|---|---|---|
| 12V compressor fridge (20 W avg) | DC | 20 | 10 | 200 | ÷0.90 | 222 Wh |
| Interior LED lights | DC | 25 | 3 | 75 | ÷0.90 | 83 Wh |
| 12V water pump (intermittent) | DC | 50 | 0.1 | 5 | ÷0.90 | 6 Wh |
| Phone charging ×2 | USB | 15 | 1.5 | 23 | ÷0.90 | 25 Wh |
| Total from battery | 336 Wh |
Station size:
- LFP: 336 ÷ 0.90 = 373 Wh needed → 500 Wh LFP station
- NMC: 336 ÷ 0.80 = 420 Wh needed → 500 Wh NMC station
Tier 2 — Standard (add vent fan and laptop)
| Appliance | Port | Avg W | Hours | Device Wh | ÷ loss | From battery |
|---|---|---|---|---|---|---|
| All of Tier 1 | — | — | — | — | — | 336 Wh |
| Roof vent fan (low speed) | DC | 25 | 6 | 150 | ÷0.90 | 167 Wh |
| Laptop | AC | 50 | 2 | 100 | ÷0.85 | 118 Wh |
| Total from battery | 621 Wh |
Station size:
- LFP: 621 ÷ 0.90 = 690 Wh needed. A 700 Wh station leaves only 10 Wh of margin (1.4% — within 10% of full capacity). 1,000 Wh LFP is the comfortable choice.
- NMC: 621 ÷ 0.80 = 776 Wh needed → 1,000 Wh NMC station
Tier 3 — Standard + CPAP (DC cable, no humidifier)
A DC cable bypasses the inverter and avoids the 15% AC conversion loss. The CPAP sizing article puts a machine without a humidifier at roughly 30–40W via DC cable. This tier uses 35W as a midpoint.
| Appliance | Port | Avg W | Hours | Device Wh | ÷ loss | From battery |
|---|---|---|---|---|---|---|
| All of Tier 2 | — | — | — | — | — | 621 Wh |
| CPAP (DC cable, no humidifier) | DC | 35 | 8 | 280 | ÷0.90 | 311 Wh |
| Total from battery | 932 Wh |
Station size:
- LFP: 932 ÷ 0.90 = 1,036 Wh needed → 1,500 Wh LFP station
- NMC: 932 ÷ 0.80 = 1,165 Wh needed → 1,500 Wh NMC station
What This Calculation Does Not Cover
Rooftop AC. A 13,500 BTU roof air conditioner draws 1,000–1,500W. Eight hours of use would require roughly 10,000–14,000 Wh. That is a generator conversation, not a portable power station conversation.
Furnace blower. If you are camping in cold weather and your RV has a propane furnace, the blower motor is a separate load that sits outside these tiers. ECM motors draw 150–400W; older PSC motors draw 400–800W. The gas furnace article has the full math, including the startup surge problem.
Space heaters. The space heater article shows why a 750W heater running overnight needs roughly 8,000 Wh. For off-grid warmth at a fraction of that cost, a 12V DC electric blanket is worth looking at before the trip.
Run Your Own Numbers
The tiers above are midpoints built on general estimates and one published data source for the fridge. Your actual fridge, your actual fan speed, your actual laptop wattage — all of those shift the answer. Use the power station calculator to enter your real appliance list.
Worth having before the trip
Run: The 12V compressor fridge is the dominant load across all three tiers. 12V compressor RV fridges on Amazon.
Measure: A plug-in watt meter connected to your shore power cord at home — before you leave — tells you exactly what your fridge draws in a 24-hour cycle, so you are not relying on a general estimate in the field. Plug-in watt meters on Amazon.