I've recently joined a utilities tariff that gives me free electricity between 9am & 5pm on Saturdays. While I'm making sure to do all my laundry and DIY on Saturdays, I was also looking at things like UPS batteries to enable me to use some of that power on other days or at other times. These seem to be prohibitively expensive though, This ups would allow me to run my computer for about 10 minutes at most, and the electricity to do that would only cost 1/600th the cost of the battery!

So far I try to make the most of the offer by moving to Saturday:

  • All washing & tumble drying
  • All power-tool based DIY
  • Weekly meal prep using electric oven (ie curry or casserole for the week)

All of this means I'm using about 10% less electricity on other days of the week, but I'd like to push that even further if possible.

  • 3
    Possible duplicate of Using electricity at night
    – Chenmunka
    Oct 17, 2016 at 14:28
  • 1
    As opposed to a UPS to run a computer, why not migrate to a laptop and charge the battery during your free electricity period? You could potentially order a couple of spare batteries. Oct 17, 2016 at 16:01
  • @SteveMatthews I thought the intention in mentioning the UPS was that it's a commercial electrical storage device, but the capacity is too small for practicality; hence mention in my answer of building your own battery/inverter power storage system.
    – Zeiss Ikon
    Oct 17, 2016 at 16:05
  • I understand but surely a shift in metality towards low power devices would also help eek out as much electricity as possible. Oct 18, 2016 at 7:44
  • It's true that using less power would make storage go further. I was also wondering if moving any other (surprising) activities to Saturday would be good?
    – Martin KS
    Oct 19, 2016 at 7:21

7 Answers 7


Any energy storage is going to require investment -- and the magnitude of the investment relative to the amount of energy you can store is why power utilities almost never attempt to store energy (as opposed to offering consumers incentives to use power when they have excess capacity, as they've done with you).

Batteries aren't the only way, of course -- you can store energy by lifting weights, including pumping water into a high tank, and using the weights' fall later to generate power; by spinning up a large flywheel to very high speed, and using the flywheel to run a generator later, or by pressurizing a gas that will later be used to run a motor/generator.

Of these, only lifting a weight (either a solid mass, or liquid) is reasonably obtainable without high-tech equipment, and reasonably efficient. It will still require more investment per kilojoule (both in money and in time) than batteries and an electronic inverter to give AC output of the correct voltage. Worth noting that nearly all UPS units can be upgraded by connecting larger batteries; deep cycle lead-acid batteries are currently the most cost-effective for this, and cost significantly less than a similar capacity in the gel-cell lead-acid batteries usually found inside a UPS. You could also assemble your own such system from batteries, a charger, and an inverter, likely at lower cost than a UPS (because you're not paying for their automatic switch-over feature and hardware damage warranty). Look at systems designed to use solar power with regular mains-powered appliances for building cues.


Another energy storage mechanism is thermal storage. For example a heated pool, or creating a lot of ice to fill a large ice chest. Or more indirectly, do cooking that requires a lot of energy and eat it through the week.

If you happen to have and use a large air compressor, fill it on Sat.

You could make your own biodiesel, which requires some heating to make.

Your electric company may have fine print prohibiting the following, but you could check. Use electrical energy to create a product/service you could sale, for example biodiesel, candles, ice, dry ice, etc. Even if sale is prohibited, you could still make things for your own use.

If you do arc welding as a hobby, do it on Sat.

Bottom line, energy storage isn't the only option.


Spare laptop batteries, and maybe looking into caravan/RV electricals like 12v TVs.

"Leisure Batteries" for example; higher Amp/hour batteries. Coupled with battery chargers are real cheap (compared to inverters + batteries), and there should be ones out there that should charge close to that in less than a day for a decent price hopefully.

If you start looking into inverters, then prices can get silly real quick. Trying to squeeze 120v-240v, at even 1 watt/hour, from a 12v battery probably wouldn't last too long. (12v at 1kW is 83.33 amps...)

  • That's a fair point - If I were to shift over a couple of computers or something like that to 12v and use those batteries then I might break even after a couple of years :)
    – Martin KS
    Apr 17, 2017 at 20:58
  • @MartinKS - I think it's possible. The only downside I can see is that most electrical products are designed for mains voltage (120-240v), leaving things like cable/satellite boxes and washing machines/dryers restricted. If you get a decent selection of channels through your TV antenna only, it could be an idea. Cookers/coolers/shavers/heaters/kettles/etc. come in 12v variants and are fairly common in the outdoor/camping shops I've seen.
    – user20447
    Apr 22, 2017 at 8:35

Only being once a week is pretty limiting. Energy storage is not impossible, but it is impractical because the cost is too high- for example, the Tesla Powerwall would save a couple dollars worth per week, but costs $3K, so it would never pay for itself. An electric car would have similar economics.

Most big energy consumers in a home can't be time shifted easily (A/C, fridges, freezers, hair dryers, kettles) or don't use much value of electricity.

I suggest looking at vampire consumption such as TV set top boxes and AC adapters that are not plugged into anything, that can be turned off when not in use. This is not shifting, it's simple conservation. Anything that is warm all the time when you touch it is sucking juice continuously. You can buy products that will show the consumption of appliances such as the Kill-a-Watt. Some homes have dozens or scores of such adapters continuously powered.

  • Some electric cars come with a US federal tax incentive (credit) that can make the extra cost of the electric car a wash (it did for me).
    – Χpẘ
    Mar 27, 2017 at 23:40

If you have an electric vehicle, charge it during this period! Or you can invite your friends if they happen to have electric vehicles...

  • There are currently only 66,000 electric cars in the UK - only one in 1000 people owns one! No one I know does, and neither do they know anyone who does... Maybe in a few years time though hey :)
    – Martin KS
    Mar 30, 2017 at 13:44

A typical 12V car battery has 720Wh. Buy 5 of these, and a 1500W inverter. Charge them in the free period, and tap them through the inverter during the non-free period.

  • A good answer, but not sure of the financial sense of doing it. 1Kwh in the UK costs 12p while a battery costs £40+ so ignoring the cost of the inverter, you've got a 333 week payback... So, 7 years!
    – Martin KS
    Jun 25, 2017 at 15:56
  • 1
    @MartinKS ::Envy:: A kWh costs several pounds here.
    – user2497
    Jun 25, 2017 at 16:02
  • Yikes, I'd not considered places where it might cost that many times as much - where are you?
    – Martin KS
    Jun 25, 2017 at 20:22

A little dangerous but dissociate water to hydrogen and oxygen to power a fuel cell.


  • 4
    Extra-ordinarily dangerous, particularly given the problem in pressurising and storing hydrogen!
    – Martin KS
    Oct 17, 2016 at 14:05
  • @MartinKS You asked for hacks. It is done commercially. And even some home labs.
    – paparazzo
    Oct 17, 2016 at 17:34
  • 2
    Very true, but only by people who understand how dangerous it is if done wrong! My intention is that any future readers should be forewarned!
    – Martin KS
    Oct 19, 2016 at 7:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.