# Battery sizing for inverter load

I have got to run a 240 volt 60 watt air pump for a domestic sewage treatment plant. There is no mains electric on site at all so generators run at evening time for power.

I am now thinking that the only way is a battery bank and a 12 to 240 inverter.

My question is what size battery bank would be needed to power the 60 watt 240 volt load for 24 hours?

• Welcome to EE.SE, Ray, but please use proper punctuation, sentence construction and capitalisation as in standard English and so that it is understandable by an international audience. I've tidied it up for you. – Transistor May 19 '18 at 18:29
• Thank you for comments on punctuation etc etc try istalling it in my BRAIN after suffering a stroke not everything comes out in the right order etc – Ray May 20 '18 at 19:27
• It is not possible to size the battery unless the real load is known. You say it is a 60W air pump, but does it really use 60W? If you havne't measured it, there is no way to know for sure. – mkeith May 21 '18 at 7:35
• Consider using a 48V inverter instead. The higher voltage inverters are often a bit more efficient. – mkeith May 21 '18 at 7:40

60 W * 24 h = 1440 Wh. 1440 Wh / 12 V = 120 Ah. 120 Ah assumes no losses in the inverter, no voltage drop from the battery and very deep depth-of-discharge of the same (short lifespan). If you double that to 240 Ah, you should have margins.

• Thank you 240 is the target then I will work on that - the generator des run around 6 hours a day so that should charge them well and also give plenty of back up – Ray May 20 '18 at 19:26
• This is a good answer assuming the pump really is 60W. I do suggest you (the OP, I mean) measure the actual pump power consumption if at all possible. – mkeith May 21 '18 at 7:38

More important is the type of deep discharge battery.

Be sure to use an efficient inverter. Cost to run on batteries is highly dependent on efficiency.

Use a high quality CV/CC battery charger. See Charging Lead Acid

Is the pump running 100% of the time?

the generator does run around 6 hours a day so that should charge them well and also give plenty of back up

Six hours may be insufficient to charge a fully discharged (50-80%) lead acid batteries. You will need at least 12 hours or use more batteries in parallel to reduce the depth of discharge (DoD). Lower DoD requires less charge time and gives you more charge cycles (longer life).

Shoot for a maximum 50% discharge. See How to Prolong Lead-acid Batteries

I would say two 100 Ah would be the minimum. Three or four may give you the needed charge time and longer overall lifespan. Budget \$1000 per year.

For fastest charge time an Absorbent Glass Mat (AGM) would likely be best.

• That six hour will definitly be a separate problem. You will never be able to reatch or at least not end the absorption phase of the charging in that time. A sufficiently large battey pack will still do the job if you cycle it between 50-85% DoD, but I would try to bring it up to 100 % DoD every three months or so to extend the lifespan of it. Li-Ion would not suffer from this but the initial purchase price will of course be much higher. – winny May 21 '18 at 7:54