# Are batteries efficient enough to store cheap night electricity for use in the day? **URL:** \ **Category:** Factual Questions\ **Created:** [December 11, 2011, 9:00pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847 "2011-12-11T21:00:55Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Dahu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dahu/32/3416_2.png) [@Dahu](https://boards.straightdope.com/u/Dahu)\ **Post date:** [December 11, 2011, 9:00pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/1 "2011-12-11T21:00:55Z")
I have an acquaintance here in the UK who is in the middle of a project aimed at storing cheaper night time electricity in batteries to be used for lighting the house during the day. He doesn’t seem to have done any detailed calculations to show if this is feasible, and to be honest I’m dubious (why doesn’t everyone do it if it works)? I don’t know how big his house is, but say it’s an average UK three bedrooms. Night-time electricity here is called Economy7, you get 7 hours per night of reduced rate, and the cost is typically 1/4 that of daytime power. I don’t know what sort of batteries he’s intending on using, only that “about 2” would be enough. And the scheme will pay for itself in “about 2” years. So dopers, is this going to work? ---
**Author:** ![guizot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guizot/32/3636_2.png) [@guizot](https://boards.straightdope.com/u/guizot)\ **Post date:** [December 11, 2011, 9:24pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/2 "2011-12-11T21:24:22Z")
So, he wants to charge batteries at night that he can use to power his house in the day-time? He’ll probably need more than two batteries, and pretty big ones. (Use a notebook computer as a gauge to consider size, power output, and battery-life.) ---
**Author:** ![The\_Evil\_Prince\_Zorte](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@The\_Evil\_Prince\_Zorte](https://boards.straightdope.com/u/The_Evil_Prince_Zorte)\ **Post date:** [December 11, 2011, 9:30pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/3 "2011-12-11T21:30:05Z")
I don’t know about general electric usage, but they do this for air conditioning in commercial buildings. They have systems that use the cheap nighttime electricity to make ice, and then use the ice to cool the building during the day. ---
**Author:** ![sitchensis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sitchensis/32/4892_2.png) [@sitchensis](https://boards.straightdope.com/u/sitchensis)\ **Post date:** [December 11, 2011, 9:44pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/4 "2011-12-11T21:44:05Z")
A guy I work with, who lives off the grid, spends about 6 grand on batteries every 10 years. He charges his batteries with a gas generator (60%) and a pelton wheel (40%). I believe there would be some major upfront costs involved. I also believe that if it was truly cost effective, your power company would already be doing it. ---
**Author:** ![tellyworth](https://avatars.discourse-cdn.com/v4/letter/t/977dab/32.png) [@tellyworth](https://boards.straightdope.com/u/tellyworth)\ **Post date:** [December 11, 2011, 9:46pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/5 "2011-12-11T21:46:25Z")
> [@Dahu](#): > > to be used for lighting the house during the day. He can’t open the curtains? ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [December 11, 2011, 9:47pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/6 "2011-12-11T21:47:29Z")
> [@sitchensis](#): > > A guy I work with, who lives off the grid, spends about 6 grand on batteries every 10 years. He charges his batteries with a gas generator (60%) and a pelton wheel (40%). I believe there would be some major upfront costs involved. I also believe that if it was truly cost effective, your power company would already be doing it. Ours does something thats kinda a variation on the theme. The article is old and laments issues that are not current > **[Top PG&E; Hydro Plant Hampered by Demand](https://www.latimes.com/archives/la-xpm-2001-jan-24-mn-16302-story.html)** > > Grid: Water supply is so low that the massive Helms powerhouse is almost 'sucking mud,' officials say. And it needs electricity itself to operate. in a nutshell, pump water uphill to a resovoir at night when power demands are lower and run it back down through the generators during the day. ---
**Author:** ![gaffa](https://avatars.discourse-cdn.com/v4/letter/g/e68b1a/32.png) [@gaffa](https://boards.straightdope.com/u/gaffa)\ **Post date:** [December 11, 2011, 9:47pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/7 "2011-12-11T21:47:43Z")
> [@The\_Evil\_Prince\_Zorte](#): > > I don’t know about general electric usage, but they do this for air conditioning in commercial buildings. They have systems that use the cheap nighttime electricity to make ice, and then use the ice to cool the building during the day. There are [two](http://maps.google.com/maps?q=Chicago&hl=en&ll=41.876213,-87.635712&spn=0.000866,0.002064&sll=37.0625,-95.677068&sspn=59.119059,135.263672&vpsrc=6&hnear=Chicago,+Cook,+Illinois&t=h&z=20) [buildings](http://maps.google.com/maps?q=Chicago&hl=en&ll=41.879695,-87.627109&spn=0.000866,0.002064&sll=37.0625,-95.677068&sspn=59.119059,135.263672&vpsrc=6&hnear=Chicago,+Cook,+Illinois&t=h&z=20) in downtown Chicago that do exactly that - makes ice all night long when electricity is cheap, and supply 33 degree water to surrounding buildings. I did the animation about the project for Commonwealth Edison (now Exelon). Chicago’s downtown is uniquely suited for a system like this because of the series of tunnels connecting each building in the Loop at the sub-basement level, making it cheap and fairly easy to run the pipes. ---
**Author:** ![lisiate](https://avatars.discourse-cdn.com/v4/letter/l/f05b48/32.png) [@lisiate](https://boards.straightdope.com/u/lisiate)\ **Post date:** [December 11, 2011, 9:59pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/8 "2011-12-11T21:59:56Z")
I remember night store heaters. Which were basically a large block of concrete, heated during the night on the night rate to radiate heat during the day. Oh and your friend needs to factor in the energy losses due to converting the AC mains power to DC (rectifing) for storage and back to AC (inverting) to use in the house. And the cost of the device(s) to do this as well. I’m not sure what efficiency can be expected, but at a guess I’d say even in a well designed system the total loss would be around 30-40%. So he’d have to buy this much more power at night to make up the losses. And have enough batteries to store this much more power as well. On the battery question I think the rule of thumb is you need a large battery for each kilowatt hour you want to store. And an average house uses around 50 kilowatt hours per day. So he’s looking at 30 to 40 batteries minimum I’d say. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [December 11, 2011, 10:38pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/9 "2011-12-11T22:38:54Z")
This set up is common in Haiti, though the reason is they only have grid power for 1/3 of the day. While it was more then 2 batteries it was not all that big a battery pack, but they don’t use much power during the ‘on battery’ times either. A few lights, especially LED’s or CF’s should be OK to run off of a couple of batteries. Though for such small power loads is it worth it at all. One thing however, with such a setup it is easy to add solar or small scale wind/hydro, which may be a fun project. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [December 11, 2011, 10:41pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/10 "2011-12-11T22:41:23Z")
> [@tellyworth](#): > > He can’t open the curtains? What part of ‘in the UK’ do you not get? 😉 ---
**Author:** ![lisiate](https://avatars.discourse-cdn.com/v4/letter/l/f05b48/32.png) [@lisiate](https://boards.straightdope.com/u/lisiate)\ **Post date:** [December 11, 2011, 11:16pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/11 "2011-12-11T23:16:12Z")
Lookiing around further it seems I oversated average consumption, here in New Zealand it’s more like 20 to 25 kwH per day for an average household. Still looking at around 20 car-size batteries though. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [December 11, 2011, 11:23pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/12 "2011-12-11T23:23:38Z")
During the California energy crisis in 2000/2001 there was a lot of discussion about battery backed solar on an email list I subscribe too. The general consensus that the cost of lead acid batteries used correctly cost 17 cents a KWh. So the cost of electricity from lead acid batteries is the cost of electricity to charge them plus 17 cents a KHw plus the inefficiencies of charging the batteries. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [December 11, 2011, 11:38pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/13 "2011-12-11T23:38:04Z")
A lot depends on the specifics of the differential in peak versus off-peak electricity pricing in a specific market, and the cost (over its predicted lifetime) of the electrical energy storage unit (be it battery or flywheel or some other system). Utilities have been starting to use these systems some. The first use is for very short term shifting, i.e. dealing with stabilizing the grid and buffering it from the sudden spikes in demand. Some see a viable second life for decommissioned electric vehicle batteries as stationary load shifting batteries, below their 80% capacity mark so no longer useful in the vehicle, but able to be connected together into larger stationary units. These could be sold either for residential or for utility grade purposes. Meanwhile companies like A123 are working with utilities on [purpose designed devices](http://www.sandia.gov/ess/docs/pr_conferences/2010/hawk_dte.pdf). Finally some conceive that parked and plugged in EVs could be used in a vehicle to grid (V2G) manner to provide some grid stabilization. But that veers off subject some. ---
**Author:** ![lisiate](https://avatars.discourse-cdn.com/v4/letter/l/f05b48/32.png) [@lisiate](https://boards.straightdope.com/u/lisiate)\ **Post date:** [December 12, 2011, 12:22am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/14 "2011-12-12T00:22:02Z")
> [@gazpacho](#): > > During the California energy crisis in 2000/2001 there was a lot of discussion about battery backed solar on an email list I subscribe too. The general consensus that the cost of lead acid batteries used correctly cost 17 cents a KWh. So the cost of electricity from lead acid batteries is the cost of electricity to charge them plus 17 cents a KHw plus the inefficiencies of charging the batteries. Is that including the cost of getting the batteries in the first place or just your ongoing maintenance cost? Seventeen cents US is roughly what I pay for power in NZ (24.37 cents NZ per KwH). Looks like batteries are out for me, (unless the efficiencies are more than 100% :)) ---
**Author:** ![Dahu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dahu/32/3416_2.png) [@Dahu](https://boards.straightdope.com/u/Dahu)\ **Post date:** [December 12, 2011, 12:24am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/15 "2011-12-12T00:24:00Z")
> [@Derleth](#): > > What part of ‘in the UK’ do you not get? > > 😉 Yup, I put that in deliberately. This far north at this time of year, 7.5 hours of daylight. Not taking into account the weather.☹ ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [December 12, 2011, 12:32am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/16 "2011-12-12T00:32:06Z")
> [@lisiate](#): > > Is that including the cost of getting the batteries in the first place or just your ongoing maintenance cost? Seventeen cents US is roughly what I pay for power in NZ (24.37 cents NZ per KwH). Looks like batteries are out for me, (unless the efficiencies are more than 100% :)) 17 cents a KWh is the cost of the batteries only. It does not include the circuitry that goes around them or any ongoing maintenance. Batteries wear out and need to be replaced the data on lead acid batteries is pretty extensive so people have a good idea on how to charge and deplete them without premature wear out but even with that they wear out. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [December 12, 2011, 12:32am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/17 "2011-12-12T00:32:50Z")
> [@The\_Evil\_Prince\_Zorte](#): > > I don’t know about general electric usage, but they do this for air conditioning in commercial buildings. They have systems that use the cheap nighttime electricity to make ice, and then use the ice to cool the building during the day. The cost of the ranks and the piping would be a lot lower than the cost of batteries to do the same job. An ice system has a added advantage in origional construction. Our church building has a ice system. At night the main system makes ice during off peak hours. During the week days on off peak hours the main system cools the building and during peak hours the ice system cools the building. On weekends during service both the ice system and the main system run providing the cooling for the larger load. In origional construction it cost less to put in the dual system than it would have for a large system. ---
**Author:** ![jasg](https://avatars.discourse-cdn.com/v4/letter/j/f0a364/32.png) [@jasg](https://boards.straightdope.com/u/jasg)\ **Post date:** [December 12, 2011, 2:13am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/18 "2011-12-12T02:13:33Z")
Regarding the OP, GM is investigating [repurposing](http://media.gm.com/content/media/us/en/gm/news.detail.html/content/Pages/news/us/en/2011/Jul/0720_reuse) Chevy Volt batteries for offline storage of power for household use. The article in the link says : _The system could store electricity from the grid during times of low usage to be used during periods of peak demand, saving customers and utilities money. The battery packs could also be used as back-up power sources during outages and brownouts._ ---
**Author:** ![Hairy\_Bob](https://avatars.discourse-cdn.com/v4/letter/h/ecc23a/32.png) [@Hairy\_Bob](https://boards.straightdope.com/u/Hairy_Bob)\ **Post date:** [December 12, 2011, 10:52am UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/19 "2011-12-12T10:52:10Z")
> [@lisiate](#): > > Lookiing around further it seems I oversated average consumption, here in New Zealand it’s more like 20 to 25 kwH per day for an average household. Still looking at around 20 car-size batteries though. I’m a UK doper, happen to have an electricity monitor which tells me I use an average of about 100kwh a week (14.3/day), in a 3-bedroom house with a wife and 2 kids, electric cooker and shower, but gas central heating. We live fairly energy efficiently, have A rated appliances and don’t leave things like tv turned on all day long, but I don’t go around religiously turning all the sockets off when I’m not using something. Basically, I use a bit more than I could get away with, but probably less than average. I would WAG the UK average to be about 15-20kwh/day, for people that have to earn money. We’ve come across a few single mums that essentially run everything electrical they own (tv, kettle, washing machine, dishwasher, extra lamps all over etc.) pretty well the whole time they are in the house. I have tried explaining the savings that could be made by say, doing a load of washing when you have a load to do, rather than every time junior dirties a set of clothes, or closing the fridge door while you make the tea, or only boiling the water you need, rather than filling the kettle every time whether for one cup or five. The answer is inevitably, “What’s the point? I don’t pay for it.” These households, I imagine, may bring the average up somewhat… ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [December 12, 2011, 2:27pm UTC](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847/20 "2011-12-12T14:27:33Z")
> [@lisiate](#): > > I remember night store heaters. Which were basically a large block of concrete, heated during the night on the night rate to radiate heat during the day. > > Oh and your friend needs to factor in the energy losses due to converting the AC mains power to DC (rectifing) for storage and back to AC (inverting) to use in the house. And the cost of the device(s) to do this as well. I’m not sure what efficiency can be expected, but at a guess I’d say even in a well designed system the total loss would be around 30-40%. So he’d have to buy this much more power at night to make up the losses. And have enough batteries to store this much more power as well. > > On the battery question I think the rule of thumb is you need a large battery for each kilowatt hour you want to store. And an average house uses around 50 kilowatt hours per day. So he’s looking at 30 to 40 batteries minimum I’d say. Are the losses really that large? Hydro-Quebec now transmits a lot of the power they generate at 730 KV DC to cut radiative loss. Since they are not generating 730 KV (at least I don’t think they are) whatever they are generating, presumably AC at a lower voltage, they are transforming it to 730 KV, rectifying it, transmitting it up to 1000 miles, then chopping it to get AC again. There is one possible exception. Aluminum smelting is big here and uses DC (but does it use 730 KV?) So I really wonder whether the losses can be anywhere near that large. [Next page](https://boards.straightdope.com/t/are-batteries-efficient-enough-to-store-cheap-night-electricity-for-use-in-the-day/605847.md?page=2)