# Math check on battery usage

solomon_says
Posts:

**6**Registered Users ✭✭
12V x 100Ah = 1200W

I want to find out total watts for a single 12V 100Ah battery assuming 100% efficiency. Measuring a 60 W bulb for one hour, I can run the bulb for 20 hours?

Do I measure bulb usage in watts or watt hours? I don't need a sizing, I just need to know the math.

I want to find out total watts for a single 12V 100Ah battery assuming 100% efficiency. Measuring a 60 W bulb for one hour, I can run the bulb for 20 hours?

Do I measure bulb usage in watts or watt hours? I don't need a sizing, I just need to know the math.

0

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## Comments

298Registered Users ✭✭✭units.12 Volts x 100 Amp-Hours = 1200

Watt-Hours

You must use the correct "units".Also,

Watt-Hours =Watts x HoursYes, you can run the bulb for 20 hours, but it will be very dim.

Typically, we run for 10 hours or less, then we stop.

Typically, we run until a specific "Low Voltage" is hit, then we stop.

EDIT: fixed typo!

6Registered Users ✭✭118Solar Expert ✭✭✭6Registered Users ✭✭If I know 1 battery has 1200 watt-hours, is it correct to calculate run time of a 60W light bulb if 1200Wh ÷ 60W? I read somewhere that watt-hours are found by V x Ah ÷ 100 so now I'm very confused.

118Solar Expert ✭✭✭reallyexpensive,reallyfast. You should never plan on discharging any rechargeable lead acid battery more than 50%, and if you want the batteries to last a lot longer, you only want to discharge them 20-25%, meaning you'd need to divide 100aH by either 4 or 5, to find out how many ampere hours you can use from them, before you'll need to fire up your generator to recharge them, instead of replacing them..2,205Solar Expert ✭✭✭✭✭You are getting confused by mixing a time based figure with a value that has no time base.

Example

A 12V, 50 W lamp is required to operate for 10 hours from a 12v battery, what would the recommended battery capacity be, for a maximum 30% depth-of-discharge ?

Current is power divided by voltage, so 50/12 = 4.167 amps

Ampere hours would be current multiplied by time in hours, 4.167 × 10 = 41.67 AH

Battery size for 30% depth-of-discharge 100/30 × 41.67 = 138.9 Ah

Personally I prefer to do the electrical calculation first then involve the time, working with Ah and Wh together is confusing, batteries are sized in Ah. so easier to transpose.

So the 60W, 100Ah battery question.

60W/12V = 5 A

The 100 Ah battery is really 30Ah, for a 30% depth-of-discharge

Therefore 30Ah/5A = 6 hours running time to 30% DOD.

Of course this is a rough calculation, assuming full capacity etcetera, voltages would be slightly higher, therefore power higher, it's always better to be conservative, by increasing battery capacity, or reducing the run time.

Hope this helps.

6Registered Users ✭✭298Registered Users ✭✭✭