Voltage used to charge a UPS-Inverter and its usage

[BillP]

I'm new to this area and know very little about electrons and how electrical components work. I'm always curious about Inverter/UPS usage.

I'm from China  and we have an  inverter at home. It is attached with two batteries which will supply the power for my home for 8 hours (TV, 4 ceiling fans, 4 lights). When there is interruption in power supply I drain out the inverter completely and when power comes back it gets charged fully in 2 hours.

I was always thinking that if it can charge up fully in 2 hours and supply power for 8 hours then I can save electricity for 6 hours which I'm paying to supplier by disconnecting power supply after my inverter charge up completely.

I know this is silly thinking but I want to have some explanation from experts saying that my thinking is incorrect, because if that happens then everyone in this world can save lot of power.

[Brad_C]

If your inverter is running from Lead Acid batteries, then you are not charging them fully in 2 hours. You might put a good surface charge on, but you can't fully charge a lead acid battery in such a short time.

Additionally, even if you could you are not saving power. There are efficiency losses in the whole chain. Your battery charger is probably ~80% efficient (for a good one), your batteries will only store about 80% of what you put in there, and your inverter is probably only 85% efficient. In short if you work it that way you are probably consuming in the order of twice the amount of required power. So no, nobody in the world can save any power at all doing it that way.

Sorry to be the bearer of bad news.

Lets pretend the batteries *can* actually be charged in 2 hours and run some ballpark numbers. Say you are powering 800W load for 8 hours. You'll need 8000W/h of battery to deliver that (1000W into the inverter to deliver 800W out). So you'll want to charge that in 2 hours, meaning you'll be putting about 4.7KW for 2 hours into the battery (as you need 15% more than what you put in due to battery inefficiency). That's going to require about 5882W fro the wall (charger is only 80% efficient). So you are using 11,764W/h to deliver 6,400 W/h of power. Your total chain is about 54% efficient.

Now, sure you can buy more efficient chargers and inverters, and you could use Lithium-ion batteries (which *can* be charged in 2 hours). A tesla power wall can do those sort of numbers, but you are still wasting power for no real gain.

[voidstar_apc · Accepted solution]

In addition to Brad's comment about the inefficiency of using a UPS, most electrical suppliers charge on energy usage, not amount of time. So consuming much more energy in a shorter period of time won't save money.