An Eight-Hour Parking AC Energy Budget: A Worked Example

Estimate parking AC runtime from usable energy divided by average electrical demand. Use a measured demand for the conditions you expect. The lowest advertised current or the cooling-output rating is not an adequate basis for an overnight promise.

Start with energy, not amp-hours alone

For a simple planning estimate, nominal battery energy in watt-hours is nominal voltage multiplied by amp-hours. Two banks with the same amp-hour number can store different amounts of energy if their voltages differ. Use the battery maker's rated watt-hour value when available and check its operating limits.

Not all nominal energy is available to the AC. The permitted discharge range, reserve, battery condition, temperature and conversion losses can reduce the usable amount. The installation's protection settings may stop cooling before the energy estimate is exhausted.

A transparent eight-hour illustration

The following inputs are hypothetical planning assumptions, not Vethy product specifications or a field result:

Input	Illustrative value
Battery nominal voltage and capacity	24 V × 200 Ah
Nominal stored energy	4,800 Wh
Assumed usable fraction after reserve and losses	80%
Assumed energy available to all loads	3,840 Wh
Other loads over eight hours	40 W × 8 h = 320 Wh
Remaining allowance for AC	3,520 Wh
Average AC input permitted by that allowance	3,520 Wh ÷ 8 h = 440 W

If the AC averages 600 W instead, eight hours requires 4,800 Wh for cooling alone, plus 320 Wh for the other assumed loads. That exceeds this example's available energy. The same battery label therefore cannot establish both outcomes.

Replace assumptions with a route test

Measure a complete rest period, including the initial cooldown. Record starting battery condition, ambient temperature, humidity, solar exposure and cabin setpoint. Log other electrical loads rather than attributing the whole battery discharge to the AC.

Recent parking-cooler launches, including Cooltronic G3, give buyers reasons to compare new configurations. They do not remove the need for this installed-system energy check. A cooling-capacity claim should not be substituted for measured electrical input.

What the result means

This calculation is a screening tool. It does not size cables, protective devices or charging equipment, and it does not guarantee comfort. Use it to identify whether the proposed energy budget is plausible, then confirm the full installation and the required operating period through the supplier's documentation and a representative trial.

Build your own energy budget in five steps

The worked example above uses invented numbers so the arithmetic stays visible on the page. Replacing them with your own figures takes one evening and makes the result genuinely yours. Follow the steps below in order and write every value down, because the numbers you skip are exactly the ones that surprise you halfway through the hottest night of the year.

Keep the units consistent from the first line. Energy belongs in watt-hours, power in watts and time in hours, and mixing them is the most common way a budget looks fine on paper and fails in practice. If a document gives you a different unit, convert it once, note the conversion beside the figure, and use the converted value everywhere else in the worksheet. One conversion, written down once, prevents the most embarrassing class of arithmetic error.

Find the battery's rated watt-hours; if only amp-hours are given, multiply by nominal voltage and note where the number came from.
Write down the ambient conditions you are planning for, so the finished budget states what it assumes.
Choose a usable fraction from the battery documentation, not from habit, and reduce the nominal energy by it.
List every other load that shares the bank overnight and estimate its energy across the whole period.
Subtract the other loads; what remains is the allowance the air conditioner may use.
Divide the allowance by the number of hours you need cooling, which gives the permitted average input.
Compare that average input with a measured demand for your conditions from a route test or the supplier.
Decide in advance how much margin the plan needs, and write that number on the same line.
File the completed worksheet with its date, so the next revision starts from history instead of memory.
State the intended operating period beside the result, because a budget for eight hours says nothing about twelve.

If the allowance sits comfortably above the measured demand, the plan is plausible. If the two are close, treat the plan as fragile until a full trial says otherwise, and add a margin rather than optimism. A margin is cheaper than a night without cooling. If the allowance is below demand, no rating label will rescue the configuration; the battery, the duty or the charging plan has to change instead, and the earlier that becomes visible the cheaper the fix.

Redo the budget after any change to the system: a new battery, an added refrigerator, a route that moves south or a setpoint habit that drifts colder. The steps take an evening, the arithmetic never changes, and the alternative is relearning the same lesson during the hottest week of the year. Keep the previous worksheet so the change appears as a difference, not a discovery. Fleets that keep their worksheets spot the drift years before it becomes a complaint.

Adjust the illustration for real conditions

The example uses a flat average demand, but real nights are not flat. The initial cooldown after a hot drive draws the most power, the middle of the night draws less, and a late-afternoon rest in the sun draws more again. A single average hides all of that, which is acceptable for screening a plan but never for promising an outcome.

Several conditions push real demand above a mild estimate, and most of them are visible in advance. Where your routes regularly include the ones below, expect the measured average input to sit visibly above the figure that looked sufficient on the datasheet, and plan the allowance accordingly.

Afternoon sun on the roof and through the glass before the rest period starts.
High humidity that keeps the compressor loaded for longer than a dry night would.
A setpoint chosen well below the outside temperature rather than a few degrees below it.
Cooking heat and body heat retained in a closed cabin at the start of the night.
A hard drive immediately before parking, which leaves the cabin at its hottest.
Parking on asphalt in full sun rather than in shade, which adds load through the roof for hours.

Pre-cooling changes the picture in your favor. Cooling the cabin while driving, or while connected at the depot, costs the overnight budget nothing and removes the hardest hour of the night. The same battery then supports a longer comfortable period, and the budget calculation covers a gentler duty than the raw numbers suggested. Close the blinds and park in shade where possible; the cheapest watt-hour is the one the cabin never receives. Pre-cooling also shifts demand away from the hour when the battery is least able to spare it.

Season matters too, and so does geography. A budget validated in spring can fail in the first heat wave, not because anything broke but because the assumptions moved. Note the conditions beside every measured result, so that next summer's version of you knows whether the number came from a mild night or a demanding one. A number without its conditions is only half a measurement.

Five errors that break the calculation

The arithmetic in the example is deliberately simple, which is why the errors around it are so common in real quotations and forum plans. Each of the mistakes below makes the budget look healthier than reality, and each has a straightforward correction you can apply at the planning stage. Check the list before trusting any overnight claim, yours or anyone else's.

Notice that none of the errors involves difficult mathematics. They are all failures of definition: the wrong figure, the wrong unit or the wrong starting point. That is good news, because definitions can be fixed on paper in minutes, while the consequences of a wrong definition are usually discovered outdoors, at night, at the least convenient hour possible.

Using the lowest advertised current: that figure belongs to a mild condition; substitute a measured demand for your hardest realistic night.
Counting nominal energy as available: reserve, protection settings and conversion losses shrink the usable amount; apply the documented fraction.
Forgetting the other loads: fans, a refrigerator and lights silently consume part of the bank; include them exactly as the example does.
Comparing amp-hours across voltages: two banks with the same amp-hour label store different energy; convert everything to watt-hours first.
Ignoring the initial cooldown: the first hour can dominate the average, so a budget built on the quiet middle of the night is not an overnight plan.
Quoting peak instead of average: starting current belongs to wiring and protection sizing, not to eight hours of energy arithmetic.
Treating a mild test as proof: a result from a cool spring night describes that night; the budget must also survive the demanding ones.

A budget built with these corrections is conservative, and conservative budgets are the ones that still look correct in August. Optimistic budgets are rediscovered at two in the morning, when the protection setting disconnects the load and the cabin starts warming while everyone is asleep. Recompute after every change, because stale worksheets are where optimism hides.

If the corrections change the outcome of a purchase decision, say so explicitly in the quotation. The honest version costs nothing at this stage and prevents the costly version, in which a fleet discovers the arithmetic only after the equipment is installed and the first long weekend has already gone wrong. A quotation that survives these corrections is worth signing; one that avoids them is not.

Log the numbers so they can be checked

A budget earns trust only after a measured trial on your own vehicle. The logging does not require special equipment: a battery monitor that counts energy, a notebook or a phone, and a few representative nights are enough to replace every assumption in the example table with a number you can defend. Log the same fields every night so the comparison means something. Irregular logs produce anecdotes; consistent ones produce a budget.

Choose the nights deliberately. One mild night and one demanding night tell you more than three average ones, because the spread between them is what the budget has to survive. Include at least one night with the busiest normal load list, so that the other loads in the calculation are honest rather than theoretical.

Starting and ending battery state for each rest period.
The time the unit actually spent cooling, not just the hours it was switched on.
Ambient temperature and, where possible, humidity during the night.
The cabin setpoint actually used, not the one intended.
Other loads that ran, even briefly, with their purpose.
Any protection event, with the time and the setting that triggered it.
Anything unusual about the day, from a detour to a borrowed trailer refrigerator, that could distort the numbers.
The duration of any pre-cooling before the rest period began.
Which worksheet version the night was run against, if the plan changed mid-week.

After three to five nights, compute the average energy the air conditioner actually used and compare it with the allowance from your budget. A stable result validates the plan and the arithmetic behind it. A result that drifts worse each night points to a charging shortfall rather than a sizing error, and it needs a different kind of investigation. Drift across nights is a signature, and the article on batteries that run short between shifts describes where it usually leads.

Keep the logs with the vehicle file, dated and labeled. When the battery ages or the routes change, last year's numbers are the fastest way to see what moved. Undated numbers are folklore; dated ones are maintenance data, and they cost only the discipline of writing them down. A phone photograph of the monitor at the end of each night is usually enough.

When the budget says a configuration is plausible

Treat the calculation as a filter with three outcomes. Plausible means the measured demand sits below the allowance with a clear margin, verified on representative nights. Fragile means the numbers touch, and every hot night becomes an experiment. Implausible means the allowance is below demand even under mild conditions, and the configuration needs a different battery, a different duty or a different charging plan.

Define the margin in advance rather than discovering it afterwards. A margin of roughly one fifth absorbs the cold night you did not predict, the battery a year older than the worksheet and the extra fan hour someone ran without mentioning it. Smaller margins are possible; they simply turn every unplanned event into a test of the plan instead of a routine. A margin you did not choose is not a margin, it is hope.

Measured demand at least twenty percent below the allowance.
Verification across representative mild and demanding nights.
No protection events during normal duty.
Other loads documented rather than assumed.
Every assumption in the original worksheet either confirmed or replaced by a measured value.
The worksheet, the logs and the assumptions filed together as one record.

Share the outcome, not just the conclusion, with everyone who touches the system. A driver who knows the budget is fragile will close the blinds and pre-cool; a driver told only that it works will run every fan at full speed and blame the battery when the night ends early. The budget is a communication tool as much as a calculation. People protect a plan they understand far more reliably than a rule they were merely given.

Revisit the budget whenever something changes: a new battery, an added refrigerator, a route that moves south or a setpoint habit that drifts colder. Old worksheets make the update a ten-minute job, which is another reason to keep them filed rather than rewritten from memory each season. The worksheet is a living document, not a one-time homework assignment.

Terms used in the arithmetic

The worked example uses a handful of terms that appear throughout battery and air-conditioner documentation, often with slightly different meanings between brands. The short definitions below fix how each one is used in this article, which keeps the calculation honest and makes quotations from different suppliers directly comparable. Ask a supplier to confirm any term used differently in their documents. Suppliers who answer that question precisely are usually the ones whose other numbers can be trusted.

Watt-hour (Wh): a unit of energy; nominal battery energy is nominal voltage multiplied by amp-hours.
Usable capacity: the energy the battery may actually deliver within its permitted discharge range and settings.
Depth of discharge: how much of the capacity is used; deeper discharge shortens battery life, which is why a reserve is kept.
Average input: electrical power averaged over the whole period, which is what an energy budget constrains.
Duty cycle: the share of time the compressor runs versus idles; milder conditions lower it and stretch runtime.
Protection event: an automatic disconnection by a setting or device; it is data about the budget, not a fault to reset away.
Nominal versus usable: the label describes the chemistry's stored energy; the budget spends only what the settings and losses leave over.

When a datasheet or a quotation uses one of these terms for something slightly different, stop and ask before multiplying anything. Most overnight disappointments begin with two parties using the same word for two different quantities, and a one-line clarification costs far less than a battery that was never the right size in the first place.

Keep the definitions beside the worksheet itself. A budget whose terms are defined on the same page can be checked by anyone, including the person who has to defend it in a fleet meeting two months later. Self-contained documents survive; loose assumptions do not. That is also why the example table above labels every line explicitly, right down to the assumption that everything shares one bank.

Next step

Compare the available configurations in the Vethy product catalog, then request the current model-specific documentation before selecting a unit.

Sources and method

Eberspächer — New generation Cooltronic · 2026-05-11

Frequently asked questions

How do I calculate how long a parking air conditioner can run on a battery?

Divide the energy actually available to the loads by the average electrical demand of the system under your conditions. Available energy means the battery's rated watt-hours reduced by the permitted discharge range, reserve and conversion losses, minus the energy other loads will consume overnight. Average demand should come from a measured rest period on your own vehicle, because the initial cooldown and hot, humid conditions raise it well above the lowest advertised figure, and an estimate built on that figure fails on the first demanding night.

What average power draw should I use for an overnight estimate?

Use a measured value from a representative night on your own vehicle, including the initial cooldown, your usual setpoint and your real other loads. If no measurement exists yet, treat any published current as a mild-condition reference only and plan the budget around the allowance your battery can actually support. The first route test will then replace the estimate with a number you can defend, and the worksheet shows immediately whether the plan survives it.

Why does my parking AC use more energy than the specification suggests?

Specifications state electrical input under defined conditions, and real nights are rarely those conditions. A cabin that soaked in the sun, high humidity, a low setpoint and open blinds all raise the average input, and the initial cooldown alone can dominate the first hour. Log a few nights with a battery monitor, compare the measured average with the specification, and the gap usually becomes easy to explain rather than mysterious.

Can a 200 Ah battery run a parking air conditioner for eight hours?

It depends on voltage and demand, which is why the calculation should use watt-hours rather than the amp-hour label alone. At 24 V, 200 Ah is roughly 4,800 Wh nominal, and after reserve and losses around 3,840 Wh may be available, matching the example above. Subtract other overnight loads; what remains, divided by eight hours, gives the average input the air conditioner may use. Whether that is enough depends entirely on measured demand for your conditions, not on the label.