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Three example setups, calculated
These use the same math as the calculator above with lithium batteries, 12 V, 4 peak sun hours and 2 days without sun.
| Setup | Energy per day | Battery | Solar | Inverter | Controller |
|---|---|---|---|---|---|
| Weekend camper | 565 Wh | 105 Ah | 190 W | none | 20 A |
| Remote worker | 1,507 Wh | 279 Ah | 510 W | 200 W | 55 A |
| Full-time with cooking | 2,084 Wh | 386 Ah | 700 W | 2,900 W | 75 A |
- Weekend camper: 12 V fridge (45 W, 9 h), LED lights (10 W, 4 h), two phones (10 W, 2 h each), fan (20 W, 4 h). All DC, so no inverter.
- Remote worker: fridge, lights, a laptop and Starlink on the inverter (60 W each, 6 h), two phones, fan and water pump.
- Full-time with cooking: the remote worker setup plus a 1,000 W microwave for 15 minutes and a 1,200 W kettle for 12 minutes a day. The two short cooking loads barely change the energy total, but they set the inverter size.
How the numbers are calculated
- Daily energy (Wh) = watts × quantity × hours. Loads on the inverter are divided by 0.85 to cover conversion losses.
- Battery (Wh) = daily Wh × days without sun ÷ usable depth of discharge (90% lithium, 50% lead-acid). Amp-hours = Wh ÷ system voltage.
- Solar (W) = daily Wh ÷ (peak sun hours × 0.75). The 0.75 covers heat, wiring, dirt, panel angle and charge controller losses.
- Inverter (W) = total AC watts × 1.25, assuming every AC appliance can run at once. Lower it if some never overlap.
- Charge controller (A) = solar watts ÷ system voltage × 1.25.
It does not model start-up surges (fridge compressors, pumps and air conditioners pull more for a moment), shading, or charging from the alternator or shore power. Treat the result as a starting point and confirm cable and fuse sizes with an installer.
Typical appliance wattages
Use these as starting values, then check the label or measure with a plug-in watt meter.
| Appliance | Typical power |
|---|---|
| 12 V compressor fridge | 35–60 W while running, often a third to a half of the day |
| LED lights | 5–15 W per fixture or strip |
| Laptop charging | 45–100 W |
| Phone or tablet charging | 5–20 W |
| Roof vent fan | 10–40 W |
| Water pump | 40–70 W while running |
| Diesel heater | 10–40 W once running, more at start-up |
| CPAP machine | 30–60 W, more with a heated humidifier |
| Starlink | Mini about 20–40 W, standard dish about 50–100 W |
| Microwave | 1,000–1,500 W |
| Electric kettle | 1,000–1,800 W |
| Induction cooktop | 1,000–1,800 W at high power |
| RV roof air conditioner | 1,200–2,000 W running, with a larger start-up surge |
Common sizing mistakes
- Counting the fridge at full watts for 24 hours. It cycles on and off, so enter the hours it really runs.
- Forgetting the inverter’s own use. Many inverters draw 10–30 W just by being on, so switch yours off when you don’t need AC.
- Using summer sun in winter. Pick the sun level for the season you travel in, not the best month.
- Treating a 100 Ah lead-acid battery as 100 Ah. Only about half of it is safely usable.
- Buying a huge inverter “just in case.” Oversized inverters waste more power at idle. Size it to the loads that actually run together.
Common questions
How many solar panels do I need for a van?
It depends on what you run, not on the size of the van. A weekend setup with a fridge, lights and phones needs about 190 W of solar, while a remote worker with a laptop and Starlink needs about 510 W. Cooking with a microwave or kettle pushes that toward 700 W, which is close to the limit of most van roofs.
What is a peak sun hour?
One peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square meter. It is not the number of daylight hours. A place with 12 hours of daylight may only give 3 to 4 peak sun hours on an average day, and fewer in winter or under cloud.
How long will a 100 Ah battery run my 12 V fridge?
A compressor fridge that draws 45 W and runs 9 hours a day uses about 405 Wh, or roughly 34 Ah at 12 V. A 100 Ah lithium battery holds about 1,080 Wh of usable energy, so it runs that fridge alone for about two and a half days. A 100 Ah lead-acid battery has only about 600 Wh usable, so closer to a day and a half.
Is lithium worth the extra cost over AGM?
For most van and RV owners, yes. Lithium can use about 90% of its rated capacity while lead-acid should stay near 50%, so you need roughly half the rated Ah for the same usable energy. It also weighs far less and lasts many more charge cycles. Check that the battery can charge in the cold, because many lithium batteries refuse to charge below freezing unless they have built-in heating.
Can I run an air conditioner on solar and batteries?
Rarely in a van. A 1,500 W air conditioner running 4 hours uses about 7 kWh a day once inverter losses are counted, and replacing that on a 4-sun-hour day takes around 2,350 W of solar. That is far more than most vehicles can carry, so air conditioning usually relies on shore power or a generator.
Should I choose a 12 V, 24 V or 48 V system?
As a rule of thumb, 12 V suits small and medium setups with an inverter up to about 1,500 to 2,000 W. Above that, 24 V or 48 V keeps the current lower, so you can use thinner cables and smaller fuses. The calculator shows the battery size in amp-hours for whichever voltage you choose.
Related calculators
Using a portable power station instead of a built-in system? Try the power station runtime calculator to see how many hours it will run your devices.
Ready to wire it up? The wire gauge and fuse calculator shows the cable size and fuse rating for each circuit.
Want to know how long a given battery will last? Try the battery runtime calculator.
Pumping water on a farm or homestead? See the solar water pump calculator.
Choosing an inverter or wondering how long panels take to charge your battery? Try the inverter size calculator and the solar charge time calculator.
Cutting costs or camping? See the appliance energy cost calculator, the RV amp load calculator, the alternator charge time calculator and the battery cost calculator.
Typical appliance watts are in the appliance wattage chart, and the terms are explained in the off-grid power glossary.