How many amp/h do I need for 3KW for 15 hours?

[mlewis_apc]

I have a 10KW solar installation to offset the cost of a 3KW server room.

I am thinking of installing some batteries to supplement a solar installation for night time use.
The solar install was larger than I needed in order to offset low wattage days, or, when the sun didn't shine as strongly if you will. All I need is about 3KW from the install.

Anyhow, I am trying to calculate what I would need in terms of amp/h in order to never kill my batteries, yet be able to run the full load off of them during night time hours.

So, that equates to about 15hrs at 3KW/h. It's probably simpler than I am making it in my mind but I've confused myself so figured, time to post and ask pros

Thank you!

[Anonymous user]

HI in order to attempt to answer your question a bit of additional information would be helpful:

What is the size/capacity and efficiency of the Inverter/Solar Charge Controller you chose for the solar backup power system ?

Are you using the solar system as a "B" power source to reduce the hydro power drawn by the servers (IE the servers all have dual power supplies) ?

Or is it a complete switchover to the battery/inverter at night via a transfer switch of some sort ?

I gather you are needing / thinking deep cycle batteries if you are going to run them down significantly overnight every single night ?

[mlewis_apc]

All of the servers have redundant two and three power supplies and everything is on an APC UPS which is then tied to the grid along with a transfer switch for generator backup.

The solar setup is a 10KW system, I can't give you it's efficiency really and there is no inverter at the moment, I would need to add one.

As mention in my original post however, I only need 3KW but wanted to make as sure as I could that even on low sun days, I might have a good chance of getting at least what I need.

Right now, the setup simply feeds back to the power company which in turn credits our account. The panels have inverters built in so the array puts out 240V at x watts depending on sun, all A/C. I would add batteries in order to run the server room in evening hours off of the batteries. I would use more battery than I need to make sure they never get fully depleted. I haven't thought about how I would switch the power in or out yet but probably an automated switch which can be time controlled. That way, even if there isn't enough power, it would know to switch back to utility. All of this would be pre-UPS.

[Anonymous user]

I did a back of the envelope calculation based on say 90% inverter efficiency (which is a bit on the bottom end I suppose), and utilizing 12 volt cells you would need just over 4,000 Amp-Hours worth of batteries to go 15 hours at 3,000 watts:

3kW x 15 Hrs = 45 kWH  .... divide that by 12 volts = 3750 AH ...divide that by .90 for efficiency gets you 4,200 AH.... if you take the average car battery at say 60 to 75 AH capacity you would need the equivalent of 55 to 60 of them to last the night.

[mlewis_apc]

I had a setup which had 20 12VDC marine batteries hooked to it's own smart charging system which was being used with a 18K USP a few years back.

At one point, it ran on battery for over 7hrs until the power came back. Those 20 batteries were a pain to maintain so I can't imagine 60 batteries.

However, maybe I could look at very large solar batteries, maybe 48VDC or something with lots of HA, combined or something.

This is interesting and doesn't sound very doable now.

[Anonymous user · Accepted solution]

You need 3KW power for 15 hours. Right?

To estimate the AH rating of the battery system, Kindly share the DC operating voltages of the system in use please.