How Many Amp-Hours Do You Actually Need?
Battery sizing gets argued about like it's a matter of taste. It isn't. There's a number, you can work it out in an afternoon, and almost all of it comes down to one appliance. Guess low and you're rationing power by Saturday night. Guess high and you've paid for capacity that rides along as ballast. Here's the arithmetic, and the tradeoff that decides where in the range you land.
Start With the Fridge
A 12V compressor fridge is the only thing in a teardrop that draws power all night, every night, whether you're awake or not. Ours costs roughly 25 to 35 amp-hours over a full day, depending on ambient temperature and how often the lid gets opened. That single number is most of your answer — everything else in the build is rounding error next to it.
Note what that figure is not. The spec sheet rating — 7.5A in our case — is the input draw while the compressor is actually running, not what it pulls around the clock. A compressor fridge duty-cycles: it runs, hits the setpoint, shuts off, and coasts. Multiply the rated draw by 24 and you'll size a bank three times bigger than you need, and pay for it.
If you don't own the fridge yet, take the manufacturer's stated daily consumption and add a third. Published figures come from a bench at a mild ambient with the lid shut. August in Tennessee is neither.
Then Add the Small Stuff
Everything else is small, but it isn't nothing, and it's the part people forget until they're watching the shunt drop. Rough daily figures for the way we actually camp:
Roof Fan
The biggest of the small loads. On low overnight it's a few amp-hours; running hard on a hot afternoon it climbs. Call it 5–10 Ah on a summer day, less in shoulder season.
Interior Lights
LED strips and puck lights pull almost nothing — an amp-hour or two across an evening. Not worth optimizing.
Water Pump
Runs in seconds-long bursts. Even a week of dishes and hand-washing barely registers.
Phones and Cameras
Two phones and a headlamp on a USB charger is a couple of amp-hours a night. Add a laptop or a drone battery and it stops being trivial.
Inverter Idle
The one that surprises people. An inverter left switched on draws power doing nothing at all. Ours stays off unless something actually needs 120V.
The Shunt Itself
Monitoring costs a trickle. Worth every milliamp — you cannot manage a bank you can't see.
Those are our numbers, from our build, camping the way we camp. Treat them as a starting shape, not a spec. The only figures that matter for your trailer are the ones your own shunt reports after a real weekend.
A Weekend, in Amp-Hours
Put it together for a two-night trip in warm weather, arriving Friday evening and leaving Sunday morning:
A hundred amp-hours of usable capacity covers that weekend with nothing going in. Stretch to four or five days, or camp somewhere hot, and you want either more bank or a panel that's actually earning.
Work Out Your Own Number
Everything above is arithmetic on someone else's trailer. Here's how to replace it with a measurement from yours. You need a shunt-based monitor — not the voltage readout on a solar controller, which tells you almost nothing useful about state of charge on lithium.
1. Start Full
Charge to 100% at the house and let it sit until the charger tapers off. A partial start makes every number after it meaningless.
2. Reset the Counter
Zero the consumed amp-hours on the shunt as you pull out of the driveway, so the trip is the only thing being measured.
3. Camp Normally
Don't ration. The point is to find out what an ordinary weekend costs, not what a careful one does.
4. Read It Each Morning
Note consumed amp-hours before the sun gets to work. Overnight is the honest window — nothing is going back in.
5. Note the Weather
Ambient temperature drives the fridge, which drives everything. A 55°F night and a 90°F afternoon are different trips.
6. Do It Twice
One weekend is an anecdote. A hot trip and a cool one give you a range, and the range is what you size to.
Take the highest daily figure you recorded, multiply by the longest stretch you camp without a charge, and add about 20% for the trip that runs hotter or longer than planned. That's your usable requirement. Anything smaller and you'll meet the ceiling eventually; anything much larger is weight and money riding along for a day that never comes.
Installed Bank, or a Power Station?
Once you have a number, the next question is what shape it takes: a LiFePO4 bank wired into the trailer, or a portable power station you lift in and out. Both are legitimate, they fail in different places, and the choice is less obvious than build forums make it sound.
Most teardrop builds land between 100 and 300 amp-hours installed. At the daily figures above, 100 Ah covers a weekend comfortably and a long weekend if the sun cooperates; 300 Ah is a week off-grid with the fridge running and no arithmetic at bedtime. Portable units are rated in watt-hours instead, and the useful range runs roughly 1,000 to 2,000 Wh — call it 80 to 160 usable amp-hours at 12V.
Weight is where people expect the portable option to win, and it doesn't, quite. A 300 Ah Renogy Core Mini is around 55 pounds and holds roughly 3,800 watt-hours. A Jackery Explorer 1000 v2 is around 23 pounds and holds 1,070. More than three times the energy for under two and a half times the weight — the installed bank is the denser option. It's just 55 pounds you can never take out.
So density isn't the argument. The installed bank is wired in: it charges from the roof panel and the tow vehicle without anyone remembering to do anything, and it isn't taking up galley counter space. The power station comes inside on a cold night and can be replaced in an afternoon without touching the trailer's wiring. On a first build, that last point is worth more than it looks.
Rated Capacity Is Not Usable Capacity
This is where the chemistry decides how much battery you're actually buying. A 100Ah lead-acid or AGM battery does not give you 100 amp-hours. Draw it below about half and you start taking life off it — so a 100Ah AGM is realistically a 50Ah battery that weighs twice what it should.
LiFePO4 will hand back most of its rated capacity without complaint, charges faster, and weighs roughly a third less for the same usable energy. It costs more up front and it doesn't like charging below freezing without a heater or a low-temp cutoff — which is a real consideration here in winter, not a footnote.
The practical version: match on usable amp-hours, not the number on the label. Our weekend needs about 100 usable. That's one lithium battery, or two AGMs and about sixty extra pounds on the tongue.
Putting It Back
A bank only has to cover the gap between charges, so how you recharge changes how big it needs to be.
Solar
A roof panel under a Smokies canopy is not a roof panel in Arizona. Shade, angle and season all cut the number, so size the bank for a cloudy stretch rather than a brochure figure.
Shore Power
A 20A inlet and an inverter/charger refill the bank fast on the nights you have a pedestal. Elkmont and the other GSMNP campgrounds have none, which is exactly why the bank has to stand alone.
The Tow Vehicle
A DC-DC charger off the alternator puts real amp-hours back on a driving day. If your trips involve moving most days, it does more work than solar does.
Pre-Charging
Free. Top the bank at the house and pre-chill the fridge on wall power the night before, and the first day costs you almost nothing.
If you want a figure for solar rather than a feeling, it's the same shape of arithmetic. Panel watts divided by 12 gives you amps in perfect conditions. Multiply by the hours of genuinely useful sun — four is optimistic in summer, two is realistic under trees — then take about 70% of that for controller losses, panel temperature and imperfect angle. A 100W panel that looks like 8A on paper is realistically putting back 10 to 20 amp-hours on a good day, and close to nothing on a wet one under a canopy.
Which is the honest argument for sizing the bank properly instead of buying your way out with panels. Solar extends a well-sized bank. It does not rescue an undersized one, and in the Smokies it rescues it least on exactly the weekends you needed it most.
Cold Changes the Math
LiFePO4 will discharge in the cold quite happily. Charging it below freezing is the problem — do it and you plate the cells, permanently, and the battery never tells you at the time. This is not a theoretical concern in East Tennessee. Nights in the mountains drop below freezing well before the calendar suggests they should, and a sunny morning will have a solar controller trying to push current into a battery that's still at 28°F.
Three ways out, in ascending order of cost: buy a battery with a built-in low-temperature cutoff, which simply refuses the charge until it's warm enough; buy one with a self-heating element, which draws a little power to warm itself before accepting a charge; or keep the bank somewhere that doesn't get down to ambient, which in a teardrop usually means inside the cabin rather than a vented bay.
Capacity also sags in the cold — expect to see less out of the same battery on a January trip than a July one, even before the fridge stops working as hard. Size for the season you actually camp in, and if that includes winter, the cutoff is not an optional feature.
In the Bay
When to Size Up
Where People Get It Wrong
Plan the Whole System
Our wiring diagram builder takes the components you're considering and sizes the battery, solar and wire gauge around them — then prints a parts list you can shop from.
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The monitor is the one thing not to skip — every number here is a guess until a shunt measures your own trailer. Sizes should follow your load math, not ours.
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