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A simple UPS (uninterruptible power supply - like APC, cyberpower, etc.) will charge itself while simultaneously powering the loads on the other side. If utility power is cut, the batteries will automatically kick in (a UPS has an automatic transfer switch built-in, I guess). When power returns, the UPS passes it through to the loads again, and slowly recharges itself - no intervention required.

If I install a bank of home power batteries - I am looking at Blue Planet Energy - can I configure them to behave for my entire house in the same way ? The auto-transfer is easy - I just need to wire an automatic transfer switch in between utility power and my main breaker panel ... but if I transfer away from utility power, the batteries won't ever charge.

How can I pass utility power through my battery bank and use it like a UPS ?

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  • the answer appears to be obvious ... you have to connect a utility powered charger to the batteries – jsotola Jul 8 '20 at 5:49
  • A UPS is always connected, while an SPS switches and that auto switching time can cause problems for many electronic clocks, fire alarms computers etc so which are you using? You name one and describe the other. – Solar Mike Jul 8 '20 at 6:07
  • "where it constantly self charges" If you can invent a self charging battery then the world will beat a path to your door. (or to your commerce website.) – Alaska Man Jul 8 '20 at 22:57
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Yes you can, this is common in places where there are frequent power failures. When the grid is available it is used to charge or maintain batteries, and when it goes out the batteries are used to power the home.

A simplified overview of the system is as follows:

enter image description here

When the grid is available, the transfer switch will be set to power the loads directly from the grid, and the grid will also power a charger to keep the battery topped off:

enter image description here

When the grid goes down, the transfer switch shifts the loads to the inverter (quickly so there is no interruption), so now it works as follows:

enter image description here

When the grid recovers, the transfer switch will go back to its original state as explained beforehand, and the charger will kick in to recharge the battery.

However installing a system that provides backup power for your entire house is going to be very expensive as you need a big inverter and a huge battery bank to handle the load. It is more practical to have some essential circuits on backup power (such as lighting and some appliances), and to manage to live without those which are not essential during the brief outage (such as maybe air conditioners or water heaters).

The overview above is rather simplified, and making such a system yourself is IMO not going to work out well by just buying a transfer switch and a separate inverter and charger. What you may look into buying is something called an inverter \ charger. Magnum, Schneider, Victron, Voltronic etc. make them.

Edit: I noticed that you mention Blue Planet Energy, I am not familiar with them but maybe it is not just a battery and has required components already built in like a Tesla Powerwall, you should ask them about it.

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    Note that in North America, your options are limited to Schneider, Outback, or a couple of Victron models due to the need for the inverter to be UL 1741 compliant (NEC 690.4(B) requires inverters and other hardware to be listed for the PV application) – ThreePhaseEel Jul 8 '20 at 23:33
  • I believe your second and third images are swapped from the context. – psaxton Jul 8 '20 at 23:55
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You’re wildly overthinking this

... because you’re trying to chintz out on service panels (and only have one service panel). Our #1 advice around here is “don’t chintz out on service panels” :) Experience has shown people always regret short-changing themselves on panel spaces. (Other than that, we love to help you save money!)

You need 2 panels.

First, is the “utility-side” AC panel which gets mains power directly onto it “like normal”. All the circuits you don’t want on battery power go on this panel.

What also goes in this panel is a battery charger whose job is to keep your battery bank at full charge and also power any loads on the battery-side system, so just like your car’s alternator, it runs lights, radio and also recharges the battery. This battery charger is always-on (except when utility power is down, obviously).

Second is the inverter-side AC panel which gets its power from the inverter. All the circuits you want to work on battery go in this panel. And yeah, that can be “pretty much all your circuits” and that is fine, if the provisioning calcs support that.

You might also find it worthwhile to have a third DC panel for loads which are able to run on battery voltage directly. Common AC panels from Home Depot have excellent quality, fitment and price; but only the Square D “QO” type can support DC. Dirt-common QO breakers can handle up to 48 VDC; for up to 120VDC talk to a competent Square D/Schneider dealer, the kind that’s open 5 days a week and is open 7-5.

Simply run off the inverter 24x7

You can use a transfer switch if you really want to, but it sounds like you’re using off-grid/home-power gear here. That stuff is designed to run 24x7. So you might as well.

At that point, you don’t need a transfer switch, since your inverter-side AC panel is simply always on inverter.

However this arrangement requires all power used in the home to come through the battery charger mentioned up top. It’ll need to be sized for that, but that’s not as bad as it sounds: The inverter can borrow from the battery itself when load exceeds charger capacity. The charger will simply “catch up” when the peak load abates. The charger only needs to keep up with average loads. So if your average load is 500W and your peak load is 6000W, a 1500W charger will be plenty.

The huge downside to a transfer switch is a) you get smacked with utility-side power spikes when on utility; and b) no power whatsoever for a significant amount of time while the transfer switch is being thrown and the inverter is spinning up. By “significant” I mean your desktop PC will crash, you’ll need to reset the clock on your microwave, etc. I suppose you could put the PC on a UPS, but now you have a PC tier UPS being fed by a whole house UPS... and that’s just wasteful.

The other thing about “always-through-inverter” is it will absolutely suppress any power spikes from outside. The battery charger will take the hit, but the DC-AC conversion makes it impossible for them to reach your inverter-side panel.

For your internal power spikes (refrigerator starting etc.), you’re on your own. For that, fit a whole-house surge suppressor.

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  • Depending on the switch, a desktop PC will not crash. – UmH Jul 8 '20 at 16:13
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    Experience has shown people always regret shorting themselves on panels if they live to tell the tale. – manassehkatz-Moving 2 Codidact Jul 8 '20 at 19:09
  • @UmH -- most residential automatic transfer switches (at least, in North America) have a delay built in to avoid nuisance switchovers due to momentary fluctuations and to give the generator a chance to start as well. – ThreePhaseEel Jul 8 '20 at 23:38
  • The "run off the inverter 24x7" strategy is also known as a "double conversion" UPS. It's actually not all that hard to find whole-building sized systems designed for this, because nearly every data center in the world uses double-conversion UPSes. – Nate S. Jul 9 '20 at 16:17
  • @NateS. Or they eliminate the second conversion altogether and simply run the PCs directly off the facility’s large battery. Google has been doing that for at least a decade, they had custom PC power supplies that inputed X volts DC. Half the server wants 12V anyway for the hard drives, and the CPU itself already does a DC-DC conversion; doing it from 12 instead of 5 isn’t far out of its way lol. – Harper - Reinstate Monica Jul 9 '20 at 18:25

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