Solar watts vs. solar amp-hours per day
A 400W solar array does NOT generate 400W * 8 hours = 3,200 watt-hours per day. Real-world output is typically 25-40% of nameplate due to angle, panel temperature, partial shade, and clouds.
Conservative rule: 400W solar generates roughly 80-130 amp-hours of 12V charge per typical sunny day. 800W solar generates 160-260 amp-hours.
A van that runs a fridge + lights + occasional laptop charging draws roughly 60-90 amp-hours per day. So 400W solar covers it in sunny weather.
Cloudy weather can drop solar generation by 70% — plan a battery bank big enough to bridge 2-3 cloudy days.
Battery bank size — the real constraint
Lithium iron phosphate (LiFePO4) is the standard for modern van builds — 80-100% usable depth of discharge, 3,000-5,000 cycle life, no memory effect.
Common sizes: 200 amp-hour (small build), 300 amp-hour (medium), 400-600 amp-hour (large). Each 100 Ah of lithium is roughly 30-40 lb and $400-800 in 2026.
A 200 Ah lithium bank stores roughly 2.4 kWh usable — that's 1.5-2 days of low-load operation without solar input.
A 400 Ah lithium bank stores roughly 4.8 kWh usable — that's 3-4 days of low-load operation, or 1-2 days of running a microwave + induction cooktop.
#vanlife field note
Renter wants to know if a van's 400W solar means they can stay off-grid for 5 days running a fridge + lights + laptop.
What 'off-grid days' actually means
An honest off-grid days estimate combines solar + battery + load profile. Hosts who quote '5 days off-grid' are usually assuming sunny weather and conservative loads.
PickRV's van card heuristic estimates off-grid days from solar wattage (capped at 10 days regardless). The number is a rough planning aid, not a guarantee.
Real-world: 600W solar + 400 Ah lithium + conservative loads = 7-10 days off-grid in sunny weather. The same setup in 3 days of rain = 3-4 days.
Things that drain batteries faster than people expect
Roof fan (Maxxair, Fantastic): 1-3 amps continuous. Running 8 hours overnight = 16-24 Ah.
Diesel heat (Webasto, Espar): runs on diesel fuel but the glow plug + fuel pump draws 1-2 amps when on. Overnight cycling: 8-12 Ah.
Inverter idle draw: 0.5-1 amp continuous even with nothing plugged in. Many users leave inverter on 24/7 — wastes 12-24 Ah daily.
Refrigerator: 2-5 amps when compressor runs, 30-50% duty cycle. Typical daily draw 30-50 Ah.
Laptop charging via inverter: 2-5 amps for 2 hours = 4-10 Ah per charge.
What to ask the host before booking
Actual solar wattage installed (not 'roof has solar')
Battery bank: lithium or lead-acid? amp-hours?
Inverter wattage and whether it's pure-sine-wave
What appliances draw from the inverter (microwave + induction cooktop are huge loads; coffee maker + laptop are modest)
Diesel heat or propane heat or electric heat (impacts winter off-grid viability)
