How Many Kwh Can I Expect

Discussion in 'Solar Power' started by Robert Clark, May 9, 2015.

1. Robert ClarkFunsterLife Member

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Hi Guys

Is it possible to calculate roughly how many Kilowatt Hours of energy you would get from two 115A leisure batteries when converted by an inverter to 240V?

Once I know this, I can calculate how many days I can manage 'off grid' based on our anticipated power consumption;

1 hour of 250w towel rail = 250w
5 minutes of 800w toaster = 67w
10 minutes of 1200w hair dryer = 200w
10 minutes of 1200w Nespresso coffee maker = 200w
Daily Total = 717w (assuming I am calculating this correctly?)

Thanks
Robert

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3. pappajohnFunsterLife Member

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2 hours

717w ÷ 12v = 59.75ah × 2hrs =119.5amps....or near enough 50% of your total capacity.

50% discharge is the recommended limit without causing battery damage.

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4. JeanLucFunster

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You have calculated watt-hours correctly at 717Wh. As this is to be supplied by an inverter running off 12V, you divide the Wh by 12 to get Ah.
717 / 12 = 59.75 Ah.
Then allow for inverter inefficiency - say 10% loss so divide the previous answer by 0.9
59.75 / 0.9 = 66.4 Ah

So allowing for no more than 50 % discharge (unless you have gels batteries in which case you can go to 80% discharge), your two batteries will support 115 Ah of use (2 x 115 x 50%) so you have some spare capacity - but not a lot if you want to run things like blown air heating, lighting, TV etc as well as your inverter. You would be able to last 1.7 days assuming no other consumption apart from the inverter (115/66.4).

Last edited: May 9, 2015
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5. BartyfixeditFunster

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The numbers you are quoting make this a nonstarter.

To take the towel rail alone. 250 W divide by 240 V is roughly 1 amp. One amp out of the inverter will require 20 A in. 20 A on a 230 amp hour bank will pull the voltage down very quickly. I'm pretty sure you will find that load alone will shut the inverter down within an hour.

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6. JimbohorlicksFunster

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Why would you want a 250w towel rail on for an hr per day-it wont dry the towels or put out any significant heat?

Would you be putting anything back into your battery bank? by solar or aqternator for example?

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7. pappajohnFunsterLife Member

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The towel rail will run for 5 hrs and consume approx 110amps inc loses.

With a reserve of 115ah the voltage may still be 12.2/12.3v......far above the inverters shut down voltage which may be as low as 10.5v under load.

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Easiest way to calculate AC Wattage into DC Amperage is divide by ten for a rough idea, without looking for the calculator.

Don't forget to anticipate only half of your battery capacity is usable, and that you will also have your regular DC consumption, things like lights, pump, heating, TV etc

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9. Robert ClarkFunsterLife Member

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Thanks Guys for your input- yes the daily total quoted was the maximum we would expect to use per daily session.

We already have the Inverter fitted (2kw) and tested everything on the list (not at the same time).

Started off by turning on the towel rail (which does seem to work on 1 hours elec as its quite small). In reality the towel rail will be probably be running when we're driving in the mornings, so this won't affect the leisure batteries as we have a battery to battery charger.

Ran all the other appliances listed for roughly the time quoted and the voltage showing on the inverter was still over 12V (was showing 12.1v) after the test.

If we can get 2 days from the batteries then this would be great, as we tend to tour rather than stay in the same place for more than a couple of days.

Furthermore we are conscious of our power consumption and manage our usage accordingly.

Thanks again for the answers, can always reply on MH Fun for solving life's little mysteries

Regards
Robert

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10. BartyfixeditFunster

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20A on a 230 amp hour bank is a C/11.5 load. The capacity at that load is going to be much reduced hence me saying less than an hour. It may be more but not very much. As to 10 minutes at 1200 W which is a C/2.3 load then in my opinion it will wreck the batteries before long.

All of my van electrics run off an inverter and I have a battery monitor so I am speaking from experience.

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11. hilldwellerFunsterLife Member

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12V is generally considered flat.

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12. Robert ClarkFunsterLife Member

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When I turned the appliances off the voltage reading on the inverter/charger increased to over 12.5

13. hilldwellerFunsterLife Member

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Which is fully charged. Good batteries !

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14. pappajohnFunsterLife Member

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True Brian....but that indicated 12.1v was under load not at rest.

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15. hilldwellerFunsterLife Member

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>> the voltage showing on the inverter was still over 12V (was showing 12.1v) after the test.

16. Robert ClarkFunsterLife Member

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Sorry for not being clear, it was 12.1 at the end of the test when the load was still connected

17. BartyfixeditFunster

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If you want to work it out, here is a handy calculator.
http://www.csgnetwork.com/batterylifecalc.html
You need the peukert No for your battery for really accurate readings, which you probably won't have. Probably about 1.2 if you read the instructions to the calculator.
Have a play with different values, it's quite an eye opener.
I actually got from 1.5 to 3.8 hours to flat(50%) for the towel rail depending on the Peukert No.

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Good link. Peukerts Law explains to us that as the battery voltage diminishes, the amperage will need to increase. Most good battery computers will automatically work this out for you, so the displayed results are acurate

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19. hilldwellerFunsterLife Member

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No problem, this is MotorhomeFun not a technical forum.

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20. funflairFunsterLife Member

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Watts=volts x amps, you can only use half your battery and you loose a bit through the inverter so let's call 2 115 ah batteries 1380 wh and you will get less if it's 240 volts through the inverter.

Keep it simple for us thicko's.

Martin

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