Boondocking is just camping without hookups, and the whole thing lives or dies on one balance. Each day you pull a certain number of amp hours out of the battery, and each day the sun needs to put a similar amount back. Get that balance right and you can stay out for weeks.
The math is not hard. It only feels hard because nobody writes down real numbers. Let us do exactly that.
Step one: measure your daily draw
Start with what you actually use. The honest way is a battery monitor with a shunt, watched over a few normal days. It counts every amp hour in and out and tells you the truth. Setting one up is covered in installing a battery monitor shunt.
If you are still planning, estimate. A 12 volt compressor fridge might use 30 to 50 amp hours a day. Lights and phone charging add 10 to 15. A water pump and a vent fan add a handful more. That puts a simple setup around 60 amp hours a day before any inverter loads.
The quick version
- Measure or estimate daily use in amp hours at 12 volts.
- Size solar to replace that on an average day, with margin for clouds.
- Size the lithium bank for two to three days of use as a buffer.
- Keep one backup charge source for long gray stretches.
Step two: size the solar
A rough field rule is that each 100 watts of solar returns roughly 30 amp hours on a good day, less in winter or heavy cloud. So 300 watts might return around 90 amp hours on a bright day and perhaps 40 on a poor one.
Match that to your draw. If you use 60 amp hours a day, 300 to 400 watts gives comfortable margin on average and keeps you in the black most of the time. Light users escape with 200 watts. Anyone running an inverter for a laptop, coffee, or induction cooking should think 600 watts and up. The panel and controller choices behind those numbers are in the pillar on solar charging for lithium.
Step three: size the battery
Solar handles the average day. The battery handles the gap between sunny and cloudy days and covers your evenings when the panels are asleep.
Size the bank for two to three days of use. At 60 amp hours a day, a 200 amp hour lithium bank gives roughly three days of buffer, since lithium lets you use around 90 percent of the rated capacity. That is the real advantage over lead acid, which practically halves its usable number. The reason is explained in why lithium gives you more usable power.
A worked example
Picture a couple in a van. Their monitor shows 75 amp hours a day, including some laptop time through a small inverter. They camp in mixed sun.
They fit 400 watts of solar, which returns about 110 amp hours on a bright day and 45 on a cloudy one. They run a 200 amp hour lithium bank. On sunny days they finish full with room to spare. On a run of gray days they lean on the battery buffer, and if it stretches too long they drive for an hour and let the DC to DC charger top things up. That combination has kept them out for two weeks at a time.
Tip: Build in margin. Sizing solar and battery to exactly your average leaves no cushion for a cloudy week or a guest charging three devices. A little extra is cheap insurance for peace of mind.
When solar cannot keep up
Even a good setup meets a stretch of storms. That is where a backup earns its place. A DC to DC charger pulls power from the engine when you drive, and a small generator can top the bank in an hour if you are parked. Running solar and engine charging together is covered in running solar and DC-DC together.
Do the math once, write your numbers on a card, and boondocking stops being a guessing game. It becomes a simple budget where the sun pays the bill and the battery covers the nights.
Trimming your daily draw
Sizing solar and battery is only half the battle. The cheapest amp hour is the one you never use, so a boondocker who trims consumption needs less of everything.
The fridge is usually the biggest single load. A well insulated 12 volt compressor fridge might sip 30 amp hours a day, while a poorly vented one in a hot van can double that. Giving the compressor breathing room and shading the rig cuts the number noticeably.
Inverter phantom loads add up too. An inverter left on all night can quietly burn 10 to 20 amp hours doing nothing useful, so a switch or a timer pays for itself. Swapping any remaining halogen or incandescent bulbs for LED trims a few more, and running the water heater and cooktop on propane instead of electric keeps the heavy loads off the battery entirely.
Small habits matter as much as gear. Charging phones and laptops during the sunny middle of the day, when solar is producing anyway, means the battery never has to cover them. A few changes like these can drop a 90 amp hour day to 60, which is the difference between needing 600 watts of solar and getting by comfortably on 400.
A common boondocking mistake
The mistake I see most often is sizing the system around a perfect sunny day and then being surprised by the first cloudy week. Averages hide the bad days, and the bad days are exactly when you run out of power.
The fix is to plan around a below average day, not a great one. If a bright day returns 110 amp hours and a gray one returns 45, build your power budget on something closer to the 45 and treat the sunny days as a bonus that refills the buffer. That single change in mindset separates people who stress about their battery monitor from people who barely glance at it.
The second habit worth building is checking your numbers on real trips rather than trusting the spreadsheet. Watch the monitor for a week of the kind of camping you actually do, in the season you do it, and adjust. A setup that looks perfect in a summer planning session can fall short in November, when the sun is low and the nights are long. Measured reality beats an optimistic estimate every time, and it costs nothing but a little attention.
