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  1. Amp Hours Vs Reserve Capacity
  2. Amp Hours Calculator
  3. Amp Hours Units
  4. Amp Hours Calculator
  5. What Is Meant By Amp Hours
  6. Watt Hours To Amp Hours
ampere hour
Rechargeable batteries
Top: AA battery (2500 mA⋅h)
Bottom: AAA battery (1000 mA⋅h)
General information
Unit systemNon-SI metric unit
Unit ofElectric charge
1 A⋅h in .... is equal to ..
SI units3600 C
Electrostatic units1.079253×1013 statC
Electromagnetic units360 abC
Gaussian units1.079253×1013 Fr

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An ampere hour or amp hour (symbol: A⋅h or A h; often also unofficially denoted as Ah) is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for one hour, or 3,600 coulombs.[1][2]


Shop, read reviews, or ask questions about WEST MARINE Group 4D Dual-Purpose AGM Battery, 198 Amp Hours at the official West Marine online store. Since 1968, West Marine has grown to over 250 local stores, with knowledgeable Associates happy to assist. Shop, read reviews, or ask questions about WEST MARINE Group 27 Dual-Purpose AGM Battery, 92 Amp Hours at the official West Marine online store. Since 1968, West Marine has grown to over 250 local stores, with knowledgeable Associates happy to assist. For example, a 6 watt navigation light bulb in a 12 volt system will draw 0.5 amps - which, if it's switched for ten hour each day when underway will have consumed 5 amp-hours (Ah). Continuing in this vein for each item of equipment will produce a table much like that shown below, which incidentally, is the one I did for my boat Alacazam.

The commonly seen milliampere hour (symbol: mA⋅h, mA h, or unofficially mAh) is one-thousandth of an ampere hour (3.6 coulombs).


The ampere hour is frequently used in measurements of electrochemical systems such as electroplating and for battery capacity where the commonly known nominalvoltage is dropped.

A milliampere second (mA⋅s) is a unit of measure used in X-ray imaging, diagnostic imaging, and radiation therapy. It is equivalent to a millicoulomb. This quantity is proportional to the total X-ray energy produced by a given X-ray tube operated at a particular voltage.[3] The same total dose can be delivered in different time periods depending on the X-ray tube current.

To help express energy, computation over charge values in ampere hour requires precise data of voltage: in a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval.[4] Generally, the battery voltage varies during discharge; an average value or nominal value may be used to approximate the integration of power.[5]

Other measures of electric charge[edit]

Amp Hours

The Faraday constant is the charge on one mole of electrons, approximately equal to 26.8 ampere hours. It is also used in electrochemical calculations.


  • An AA sizedry cell has a capacity of about 2 to 3 ampere hours.
  • An average smartphone battery usually has between 2.5 and 4 ampere hours of electric capacity.
  • Automotive car batteries vary in capacity but a large automobile propelled by an internal combustion engine would have about a 50 ampere hour battery capacity.
  • Since one ampere hour can produce 0.336 grams of aluminium from molten aluminium chloride, producing a ton of aluminium requires transfer of at least 2.98 million ampere hours.[6]

See also[edit]


  1. ^'electric charge (Symbol Q). IEV 113-02-10'. electropedia.org. International Electrotechnical Commission (IEC). 2020. Retrieved 2020-09-20. Note 7 to entry: The coherent SI unit of electric charge is coulomb, C. The unit ampere hour is used for electrolytic devices, such as storage batteries: 1 A·h = 3,6 kC.
  2. ^Thompson, Ambler; Taylor, Barry N. (2008). Guide for the Use of the International System of Units (SI). NIST Special Publication 811(PDF) (2nd ed.). Gaithersburg: National Institute of Standards and Technology. p. 45. To convert from ampere hour (A·h) .. to coulomb (C) .. Multiply by 3.6 E+03
  3. ^X-ray Safety Handbook, 9.0 Terms and Definitions, VirginiaTech Environmental, Health and Safety ServicesArchived July 23, 2007, at the Wayback Machine
  4. ^Efty Abir, Najrul Islam (2016). 'How to Calculate Amp Hours – Learn of Convert Watts to Amps'. Leo Evans. Retrieved 8 December 2016.
  5. ^National Research Council (U.S.) (2004). Meeting the energy needs of future warriors. National Academies Press. p. 27. ISBN0-309-09261-2.
  6. ^T. L. Brown, H. E. Lemay Jr, 'Chemistry the Central Science', Prentice-Hall, 1977 ISBN0-13-128769-9 page 562

Retrieved from 'https://en.wikipedia.org/w/index.php?title=Ampere_hour&oldid=1014628263'

The subject of boat electrics is a complex one, but the bottom line is that the current draw, battery bank capacity and charging regime must all be matched for the 12volt system to function satisfactorily.

Here we deal with the first part of that equation; calculating the current draw of your of the domestic circuit of the boat electrics over a typical 24 hour period.

Once this is known it's straightforward to assess the required size of the domestic battery bank.

And with that knowledge we can readily calculate the battery charging regime necessary to prevent undue strain on the batteries and keep the whole electrical system ticking over.

Boat Electrics ~ Assessing the Daily Current Draw

Amp Hours Vs Reserve Capacity

To calculate our daily domestic electrical requirement we must first make a list of all electrical equipment on board, and apply a current rating to each item.

If you've got a battery monitor installed in the system and capable of being switched to read amps - like the one shown here - you'll be able, by turning on one item at a time, to read the actual current draw for each item - otherwise you'll have to use a multimeter, or work it out.

Ratings can usually be found on equipment nameplates or in their manuals, and will be expressed in terms of power (measured in watts) or current draw (measured in amps). The relationship between power and current is expressed as:
Power (W) = Current (A) x System Voltage (V)
To derive amps from watts, simply transpose this equation and divide the wattage by the system voltage.

For example, a 6 watt navigation light bulb in a 12 volt system will draw 0.5 amps - which, if it's switched for ten hour each day when underway will have consumed 5 amp-hours (Ah).

Continuing in this vein for each item of equipment will produce a table much like that shown below, which incidentally, is the one I did for my boat Alacazam.

This calculation though, remains an estimate. For example:

  • in cold weather the fridge will draw less power than in hot weather;
  • in rough weather the autopilot would use more power than when it's calm;
  • hours of darkness will vary with latitude and time of year, affecting current draw for navigation and domestic lighting;
  • you'll use the watermaker more when you've got guests aboard etc, etc;
  • plus there are start-up currents and other losses that have been ignored.

So it's approximate, but indicates that you'll need to replace around 325Ah each day when you're sailing and 211Ah when you're at anchor.

My ip private address. The underway current consumption clearly presents the worst case scenario, with more power being consumed during the night than during the day. In this example the domestic battery bank will be drawn down by 175Ah during the hour night-time hours - an average discharge of around 14.6A over 12 hours.

So what's the difference between amps (A)
and amp-hours (Ah)?

The best way to explain it is by example..

If an appliance drawing 5A was to run for 1 hour, its consumption would amount to 5Ah.

This would be the same as an appliance drawing 1A running for 5 hours - again the consumption would be 5Ah.

So amp-hours are simply the (average) amperage drawn multiplied by the time in hours.

Boat Electrics ~ Power Conservation

In our example there're several things that could be done to reduce the daily consumption:

  • LED (light emitting diode) lights. These draw a fraction of the current taken by a standard incandescent light and have an exceptionally long service life. I reckon if the anchor light, tricolour, cockpit light and cabin lights were replaced with LED's then at least 15amps could be shaved off the underway consumption and a similar amount off when at anchor. A further benefit of a combined anchor/tricolour LED light is that you won't have to scoot up the mast to change a blown bulb - a prospect I view with increasing dismay these days.
  • The autopilot. If you had windvane self-steering it wouldn't use any power at all, reducing the daily drawdown by a whopping 31%.
  • The freshwater pump. Turn it off on passage and use the hand pumps.

Amp Hours Calculator

Amp Hours Units

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Amp Hours Calculator

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  • Will a Marine Solar Panel Make Much Difference to Battery Charging?

    Fitting a marine solar panel isn't always the most cost effective 'green energy' method of charging your boat's batteries. If you're thinking of fitting them you should read this first

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    A Deep Cycle Marine Battery is definitely the way to go, but which type is best? A liquid lead acid boat battery or one of the Valve-Regulated Lead Acid (VRLA) types? Find out here!

  • Daily Current Draw Calculator

    Calculate your Amperage Requirement with this Daily Current Draw Calculator and use this information in selecting the Appropriate Battery for your Boat

What Is Meant By Amp Hours

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