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Economy 7 power is significantly cheaper in the UK than peak power. Charging overnight in the E7 period could save money long term depending on the lifetime of

Economy 7 power is significantly cheaper in the UK than peak power. Charging overnight in the E7 period could save money long term depending on the lifetime of the battery.

IanCal 11y ago
I did a quick bit of calculation on this. Assuming you use 10KWh per day and a perfect battery: Economy 7 KWh cost : 8.4p (Best deal I saw was 9.4 for me, but this was a figure on MSE) Standard tariff KWh cost : 14.5p (Best deal I saw was ~12 for me, but this was a figure on MSE) That'd be a £306/year cost on economy 7 and £530 on a standard, taking some more extreme figures. A $3500 (~£2300) battery would then have a payoff time of about 10 years. Using the other figures (9.4 and 12) gives a £95/year saving or a ~25 year payoff. That's also assuming the batter is perfect, no extra costs in installation and you use exactly 10KWh per day. Economy 7 tariffs are about 22p/unit during the day so any overrun would quickly eat into savings. I do think this is all very interesting, but with current offers in the UK it doesn't look too beneficial. However, if someone were to offer cheaper rates on a more granular level (rather than always midnight-7am) then this could work out better. Perhaps an energy company owned battery would be a good idea, they offer a flat rate back to you knowing they can smooth things out.
- aidos 11y ago
  Oh interesting, I hadn't considered the aspect of charging overnight while it's cheaper. Of course, the moment it starts to get popular the energy companies will be eating that particular cake too, I'm sure. Sounds like right now there's not a huge margin anyway. I guess the energy companies have a financial incentive to run a more consistent supply if it could reduce the peak load. Just reminded me of a brilliant BBC documentary from a while back - The Secret Life of the National Grid. I think you probably have to piece it together from clips on youtube these days.
- CHY872 11y ago
  If you get 90% charge/discharge efficiency, you're multiplying the economy 7 tariff by 1/0.81 = 1.23456790 (so close) so it's more like 12.5 years, and a bit less compelling.
  - lsaferite 11y ago
    That is of course ignoring the other aspects, like battery backup for the house, better for the environment (?), the ability to do solar tie in, and any possible government incentives.
    - CHY872 11y ago
      In the UK power cuts are very rare in most places (and generators are cheap), so it's not a huge advantage. The enviroment factor is at best tenuous due to 20% of energy stored lost as heat and high energy requirements for producing the batteries; and in any case unilateral green action makes no difference. The solar tie in is quite nice, but current feed-in tariffs are huge in the UK (30p per kWh perhaps), so you're getting more out of paying it back than you would for storing. Government incentives obviously change the calculations, but there aren't any at the moment.
- danielweber 11y ago
  You are planning to charge the thing overnight and use it exclusively during the day, nearly exhausting it each time? The battery is going to wear out well before 10 years. If you oversize your battery so you are only pulling out half the charge in the battery[1], you can probably get 1500 cycles out of it[2]. That's still less than 5 years. (This is a common pattern if you are trying to price-compare an electric car to a gas car. For the electric car, the electricity cost is a rounding error compared to the capital cost of the battery, so you can just about assume electricity is free, and worry instead about how long the battery will last.) You will get a much bigger bang-for-the-buck by demand-shifting: if the biggest load is cooling your building in the heat of the day, run the A/C overnight to superchill a heat sink that can be accessed during the day. No battery installation needed. [1] Toyota did this with the Prius. They purposefully did not use the full range of the battery because they wanted the battery to last 7+ years. [2] http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries http://batteryuniversity.com/learn/article/how_to_prolong_li...
  - kbenson 11y ago
    It comes standard with a 10 year warranty, with an optional 10 year extension. If they don't have different numbers than you, I don't see why they would even attempt that.
    - danielweber 11y ago
      Warrantied to what, though. That you can still get 20% capacity levels in year 10, or that it will still turn on? This might be considered "normal wear and tear." 3000 deep cycles is really pushing what the industry knows to be state of the art. Maybe they are just taking the economic chance that most people won't be deeply cycling these batteries, and planning to do replacements for those who actually put it through its paces.
      - kbenson 11y ago
        > Warrantied to what, though. That's a good question, and it makes a big difference. > That you can still get 20% capacity levels in year 10, or that it will still turn on? This might be considered "normal wear and tear." Given that you can optionally warranty it for an additional 10 years, I doubt it's nearly that bad. If people are confused as to how they how they hope achieve 10 years reliability, and they are willing to warranty 20 years, they must have something up their sleeves. > 3000 deep cycles is really pushing what the industry knows to be state of the art I suspect that the powerwall's true capacity is higher than it's rating, and it uses that reserve so it's not doing deep cycles, similar to another commenter's assertion to how the Prius gets it's 7+ year rating, > Maybe they are just taking the economic chance that most people won't be deeply cycling these batteries, and planning to do replacements for those who actually put it through its paces. I think a combination of most users not fully cycling every day, extra reserve capacity to keep it from deep cycling, and some subset of people not using failing warranty conditions may all contribute.
deleted 11y ago
[deleted]

Economy 7 power is significantly cheaper in the UK than peak power. Charging overnight in the E7 period could save money long term depending on the lifetime of the battery.

IanCal 11y ago

I did a quick bit of calculation on this. Assuming you use 10KWh per day and a perfect battery: Economy 7 KWh cost : 8.4p (Best deal I saw was 9.4 for me, but this was a figure on MSE) Standard tariff KWh cost : 14.5p (Best deal I saw was ~12 for me, but this was a figure on MSE) That'd be a £306/year cost on economy 7 and £530 on a standard, taking some more extreme figures. A $3500 (~£2300) battery would then have a payoff time of about 10 years. Using the other figures (9.4 and 12) gives a £95/year saving or a ~25 year payoff. That's also assuming the batter is perfect, no extra costs in installation and you use exactly 10KWh per day. Economy 7 tariffs are about 22p/unit during the day so any overrun would quickly eat into savings. I do think this is all very interesting, but with current offers in the UK it doesn't look too beneficial. However, if someone were to offer cheaper rates on a more granular level (rather than always midnight-7am) then this could work out better. Perhaps an energy company owned battery would be a good idea, they offer a flat rate back to you knowing they can smooth things out.

aidos 11y ago
Oh interesting, I hadn't considered the aspect of charging overnight while it's cheaper. Of course, the moment it starts to get popular the energy companies will be eating that particular cake too, I'm sure. Sounds like right now there's not a huge margin anyway. I guess the energy companies have a financial incentive to run a more consistent supply if it could reduce the peak load. Just reminded me of a brilliant BBC documentary from a while back - The Secret Life of the National Grid. I think you probably have to piece it together from clips on youtube these days.
CHY872 11y ago
If you get 90% charge/discharge efficiency, you're multiplying the economy 7 tariff by 1/0.81 = 1.23456790 (so close) so it's more like 12.5 years, and a bit less compelling.
- lsaferite 11y ago
  That is of course ignoring the other aspects, like battery backup for the house, better for the environment (?), the ability to do solar tie in, and any possible government incentives.
  - CHY872 11y ago
    In the UK power cuts are very rare in most places (and generators are cheap), so it's not a huge advantage. The enviroment factor is at best tenuous due to 20% of energy stored lost as heat and high energy requirements for producing the batteries; and in any case unilateral green action makes no difference. The solar tie in is quite nice, but current feed-in tariffs are huge in the UK (30p per kWh perhaps), so you're getting more out of paying it back than you would for storing. Government incentives obviously change the calculations, but there aren't any at the moment.
danielweber 11y ago
You are planning to charge the thing overnight and use it exclusively during the day, nearly exhausting it each time? The battery is going to wear out well before 10 years. If you oversize your battery so you are only pulling out half the charge in the battery[1], you can probably get 1500 cycles out of it[2]. That's still less than 5 years. (This is a common pattern if you are trying to price-compare an electric car to a gas car. For the electric car, the electricity cost is a rounding error compared to the capital cost of the battery, so you can just about assume electricity is free, and worry instead about how long the battery will last.) You will get a much bigger bang-for-the-buck by demand-shifting: if the biggest load is cooling your building in the heat of the day, run the A/C overnight to superchill a heat sink that can be accessed during the day. No battery installation needed. [1] Toyota did this with the Prius. They purposefully did not use the full range of the battery because they wanted the battery to last 7+ years. [2] http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries http://batteryuniversity.com/learn/article/how_to_prolong_li...
- kbenson 11y ago
  It comes standard with a 10 year warranty, with an optional 10 year extension. If they don't have different numbers than you, I don't see why they would even attempt that.
  - danielweber 11y ago
    Warrantied to what, though. That you can still get 20% capacity levels in year 10, or that it will still turn on? This might be considered "normal wear and tear." 3000 deep cycles is really pushing what the industry knows to be state of the art. Maybe they are just taking the economic chance that most people won't be deeply cycling these batteries, and planning to do replacements for those who actually put it through its paces.
    - kbenson 11y ago
      > Warrantied to what, though. That's a good question, and it makes a big difference. > That you can still get 20% capacity levels in year 10, or that it will still turn on? This might be considered "normal wear and tear." Given that you can optionally warranty it for an additional 10 years, I doubt it's nearly that bad. If people are confused as to how they how they hope achieve 10 years reliability, and they are willing to warranty 20 years, they must have something up their sleeves. > 3000 deep cycles is really pushing what the industry knows to be state of the art I suspect that the powerwall's true capacity is higher than it's rating, and it uses that reserve so it's not doing deep cycles, similar to another commenter's assertion to how the Prius gets it's 7+ year rating, > Maybe they are just taking the economic chance that most people won't be deeply cycling these batteries, and planning to do replacements for those who actually put it through its paces. I think a combination of most users not fully cycling every day, extra reserve capacity to keep it from deep cycling, and some subset of people not using failing warranty conditions may all contribute.

deleted 11y ago

[deleted]