How many volts are in 150 watts

Standard power consumption, device runtime and so

When planning or modifying the electrical system in a mobile home or wowa or when buying electricity-hungry consumers, the question arises again and again: how long will I have electricity when the device is in operation

The ostensible answer
Battery capacity in Ah: The current consumption of the device (s) in A is unfortunately not correct, as is so often the case.

To understand the following, it is necessary to know about the following units





Electric tension




Electric electricity 




Electric power








Battery capacity



Amp hours




In order to be able to calculate the operating time, the current consumption I (A) of the loads must be determined beforehand. either using the information on the data sheet or by measuring.

Convert watts to amperes

If no current (A) but only the power (W) of the device is specified, this must be converted.

I = P / U
The power consumption of a load with 55 watts is calculated as I = 55 watts / 12 volts = 4.6 amps

Why 12 volts?

Because that corresponds to the approximate mean value for the power consumption in the womo / wowa. 13 volts has a full battery, at 11 volts the discharge should approximately end. The mean value is therefore 12 volts.

Battery operating time

Now that the required current has been calculated in this way, it can be happily divided by the available capacity (Ah) of the batteries and this is how the operating time is obtained.

Invoice in the form of
h = Ah / A

for "available" 70 Ah and a load of 55 watts, the operating time would be as follows:
h = 70 Ah / 4.6A = 15.2 hours.

How many Ah are now available in the on-board battery. To cut it short: There are always fewer Ah than you think.

A battery is only fully charged if the charge is regulated by means of a charging characteristic (usually 4 to 7 levels) and with temperature compensation, whereby the temperature sensor should ideally be on the battery.

The battery is only fully charged when it is charged
- the LiMa if it has installed a good B2B controller (sterling etc)
- the charger if it fulfills the above conditions
- the solar panel if the solar charge controller fulfills the above conditions

In all other cases (especially if there is no temperature compensation) the battery will
- too strongly charged at temperatures above 30 degrees and soon damaged by cell corrosion
- too little charged at temperatures below 20 degrees and long-term damage due to sulphation

At temperatures around zero degrees and below, the insufficient charge is clearly noticeable. Without temperature compensation, the battery is only charged to approx. 75% to 80%, regardless of how long it is on the charger.

How much capacity is available therefore depends on the charging technology.

As a guideline you can count at

- Characteristic curve chargers WITHOUT temperature compensation assume 80% charge in winter and 90% charge in summer

- For chargers without a characteristic curve and without temperature compensation (or when charging via the alternator), assume 70% charge in winter and 85% charge in summer.


Due to its design, a battery must never be completely discharged. Depending on the type and type of battery, at least 20% to 40% of the total capacity must remain in the battery in order to prevent deep discharge and damage to the cells. We reckon with 25% capacity that must or should remain at the end of unloading.

Calculation example:

There are lavish 2 x 110 Ah batteries,
Charging using a characteristic curve charger without temperature compensation or LiMa
last is a cool box with an output of 150 watts
how long is the operating time with fully charged batteries in winter at approx. + 5 ° degrees

Calculation of the current:
150 watts / 12 volts = 12.5 amps

Calculation of the capacity:
80% of 220 Ah therefore 175 Ah are available

Calculation of the remaining capacity:
25% of 220 Ah, therefore 55 Ah, should be left over as the remaining capacity

Calculation of the "available" capacity
175 Ah - 55 Ah = 120 Ah are "available"

Calculation of the running time:
120Ah / 12.5A = 9.6 hours

If several consumers are connected, simply add the currents before the start of the calculation and then carry out the calculation

With these formulas or those derived from them, with known consumer currents and switch-on times, the required battery capacity can also be calculated back in order to be able to be self-sufficient for a certain period of time. But this is another story.

A simple abbreviated calculation to do this

Whenever it is more complicated, the following example shows that it is also easier to do with arithmetic.

W.att: V.olt x 1.8 = A.mper /H

the resulting value is simply divided by the total capacity of the battery. It's really quick and easy.

To stick with the example.

150W: 12V = 12.5A x 1.8 = 22.5Ah
220 Ah: 22.5 = 9.7 hours