Before you buy a single battery or solar panel, you need to know how much electricity you'll actually use each day. This guide walks through the process step by step, with a comprehensive table of common campervan appliances and worked examples for different use patterns.
The Basic Calculation
Every electrical device has a power rating in watts (W). Multiply the wattage by the number of hours you use it per day, and you get watt-hours (Wh):
Wh per day = Watts x Hours of use per day
To convert watt-hours to amp-hours (Ah) at 12V — which is how battery capacity is measured — divide by 12:
Ah per day = Wh per day / 12
Add up all your devices and you have your total daily power consumption. This number determines what battery capacity and solar array you need.
Common Campervan Appliance Power Ratings
This table covers the most common electrical loads in a campervan. All figures are typical values — check the label or manual for your specific devices.
| Appliance | Wattage | Typical Daily Use | Daily Wh | Daily Ah (12V) |
|---|---|---|---|---|
| LED ceiling lights (4 x 3W) | 12W | 4 hours | 48 Wh | 4.0 Ah |
| LED reading light | 3W | 2 hours | 6 Wh | 0.5 Ah |
| Phone charging (x2) | 12W each | 2 hours each | 48 Wh | 4.0 Ah |
| Laptop charging | 60W | 2 hours | 120 Wh | 10.0 Ah |
| Tablet charging | 15W | 2 hours | 30 Wh | 2.5 Ah |
| 12V compressor fridge | 35–50W | 8 hours (duty cycle) | 280–400 Wh | 23–33 Ah |
| 12V cool box | 40–65W | 8 hours (duty cycle) | 320–520 Wh | 27–43 Ah |
| Water pump (Shurflo/Fiamma) | 30–60W | 0.25 hours | 8–15 Wh | 0.7–1.3 Ah |
| MaxxFan / Maxxair vent | 3–30W | 6 hours | 18–180 Wh | 1.5–15 Ah |
| Diesel heater (Webasto/Eberspacher) | 20–80W | 10 hours | 200–800 Wh | 17–67 Ah |
| TV (12V, 19–24 inch) | 20–40W | 3 hours | 60–120 Wh | 5–10 Ah |
| Starlink Mini | 25–40W | 8 hours | 200–320 Wh | 17–27 Ah |
| Hair dryer (via inverter, low) | 1,000W | 0.1 hours (6 mins) | 100 Wh | 8.3 Ah |
| Nespresso machine (via inverter) | 1,200W | 0.05 hours (3 mins) | 60 Wh | 5.0 Ah |
| Microwave 800W (via inverter) | 1,200W input | 0.17 hours (10 mins) | 200 Wh | 16.7 Ah |
Important Notes on the Table
- Fridge duty cycle: A compressor fridge doesn't run continuously. It cycles on and off to maintain temperature. A typical duty cycle is 30–50%, meaning a 50W fridge actually uses 15–25W on average. The table above accounts for this.
- Inverter efficiency: When running 230V appliances from a 12V battery via an inverter, you lose about 10–15% to inefficiency. The watt-hours shown for inverter loads account for this overhead.
- Diesel heater variation: Power draw varies enormously based on the heat setting. At full blast on startup, a diesel heater draws 80W+, but at steady-state low heat it may use only 20W.
Step-by-Step: Calculate Your Usage
- List every electrical device you plan to use in the van.
- Find the wattage from the device label, manual, or the table above.
- Estimate daily hours of use — be realistic, not optimistic.
- Multiply watts by hours for each device to get daily Wh.
- Add up all the Wh values to get your total daily consumption.
- Divide total Wh by 12 to get daily Ah consumption at 12V.
- Add 20% safety margin — real-world usage almost always exceeds estimates.
Example 1: Weekend Warrior (Moderate Use)
A couple using the van for weekends away, April to October. No heating needed. Modest electrical use.
| Appliance | Watts | Hours | Daily Wh |
|---|---|---|---|
| LED ceiling lights (x4) | 12W | 3h | 36 Wh |
| Phone charging (x2) | 24W | 2h | 48 Wh |
| Compressor fridge | 40W avg | 8h | 320 Wh |
| Water pump | 45W | 0.2h | 9 Wh |
| Vent fan (low) | 5W | 4h | 20 Wh |
| Total | 433 Wh |
Daily consumption: 433 Wh = 36 Ah at 12V. With a 20% safety margin: ~43 Ah per day.
A 100Ah lithium battery gives you over two full days of autonomy. A single 200W solar panel in summer easily replenishes this daily. This is a classic budget build scenario.
Example 2: Full-Timer (Heavy Use)
A couple living in the van full-time, year-round including winter. Working remotely (laptop + Starlink). Diesel heater for cold months. Coffee from a Nespresso machine.
| Appliance | Watts | Hours | Daily Wh |
|---|---|---|---|
| LED ceiling lights (x4) | 12W | 5h | 60 Wh |
| LED reading lights (x2) | 6W | 2h | 12 Wh |
| Phone charging (x2) | 24W | 2h | 48 Wh |
| Laptop charging (x2) | 120W | 4h | 480 Wh |
| Starlink Mini | 30W avg | 10h | 300 Wh |
| Compressor fridge | 40W avg | 10h | 400 Wh |
| Water pump | 45W | 0.3h | 14 Wh |
| Diesel heater (winter avg) | 35W avg | 12h | 420 Wh |
| Vent fan | 10W | 6h | 60 Wh |
| Nespresso (x2 coffees) | 1,200W | 0.08h | 96 Wh |
| TV (evening) | 30W | 3h | 90 Wh |
| Total | 1,980 Wh |
Daily consumption: 1,980 Wh = 165 Ah at 12V. With 20% margin: ~198 Ah per day.
This is a demanding system. A 200Ah lithium battery provides roughly one day of autonomy. You'll need 400W+ of solar, regular driving (alternator charging via the Orion-Tr Smart), and occasional hookup to sustain this. In winter, when the diesel heater runs heavily and solar is minimal, you'll rely more on driving and hookups.
Summer vs Winter: The Seasonal Factor
Your power usage changes dramatically between seasons:
Summer Increases
- Fridge works harder in heat — duty cycle increases from 30% to 50%+
- Vent fans run more often and at higher speeds
- Devices charge more (longer daylight, more activity)
Winter Increases
- Diesel heater adds 200–800Wh per day depending on climate and insulation
- Lights used for longer hours (dark by 4pm in December)
- Condensation fans may run continuously
Winter Decreases
- Fridge works less — ambient temperature is lower
- Vent fans used less
As a rough guide, expect your winter daily consumption to be 30–60% higher than summer, primarily due to heating. Meanwhile, solar production drops by 60–80% in winter. This is why full-time winter vanlifers need large battery banks, alternator charging, and occasional hookups.
Sizing Your System from Your Usage
Once you know your daily Ah consumption, you can size your system:
Battery Capacity
Aim for a battery bank that provides at least 1.5 to 2 days of autonomy without any charging input. For the weekend warrior (43 Ah/day), a 100Ah lithium battery provides over 2 days. For the full-timer (198 Ah/day), 300–400Ah of lithium is ideal.
Solar Array
In the UK, a rough rule of thumb is that each 100W of solar panel produces 30–50Ah per day in summer and 5–13Ah per day in winter. Size your array to fully replenish daily usage in summer, and accept that winter will require supplementary charging from driving or hookup.
DC-DC Charger
A 30A DC-DC charger (like the Victron Orion-Tr Smart 12/12-30) adds approximately 30Ah per hour of driving. Most campervan trips involve at least some daily driving, making this a valuable secondary charge source.
For a more detailed guide on assembling the right components, see our complete campervan electrical system guide. You can also use our system builder tool to get personalised product recommendations based on your specific usage requirements.
