6 ms·

Forgive my ignorance, what does one do during non-insolated hours? Do solar cells typically come with battery packs or does one just pull from the grid? If the

by jvm 11y ago

Forgive my ignorance, what does one do during non-insolated hours? Do solar cells typically come with battery packs or does one just pull from the grid? If the latter then the cost savings are a bit less impressive than stated. Also can you put power back into the grid in San Francisco? Is that benefit included in the cost savings estimate?

qooleot 11y ago
Often, the most economical answer is heating a water heater rather than storing via a battery.
thrownaway2424 11y ago
Solar power systems don't necessarily come with batteries but a decent installation company should be able to propose and implement an integrated system. The Tesla battery pack is an example of some recent innovation in that area. You can sell power into the grid but PG&E only has to pay you wholesale rates, 3-4 cents per kwh, not what they are charging you to take the same amount of energy out, which is 34 cents per kwh at the top tier. It's basically a scam. You don't want to overbuild on the assumption that selling the surplus would be profitable.
- jsprogrammer 11y ago
  Is there no real solar power market then? If your local utility is a monopoly then you basically have to accept whatever the utility wants to pay you (even far below 'market' rate). You have no power to negotiate, only capitulate or try to get the local regulator to enforce fair trade.
  - thrownaway2424 11y ago
    Local conditions may vary, of course, but if just go with the completely unimaginative system, and you live in California in PG&E territory, there is no effective market.
  - _delirium 11y ago
    I suspect you'd fare pretty badly in a market, even if a better one existed. In Texas's deregulated electric marketplace, the market price for large-scale solar installations selling into the grid has fallen to around $0.05/kWh. Power from a single small-scale residential installation with less reliable delivery is probably worth less than that.
    - ZeroGravitas 11y ago
      Depends on how much "better" the market is e.g. if it priced in the greenhouse gas externalities, the avoidance of transmission losses and grid upgrades etc then solar may be worth triple the residential rate. I believe a couple of states have calculated "value of solar" to use as part of their grid regulation, though specific factors like mix of gas and coal production and projected grid expansion factor into this.
      - jsprogrammer 11y ago
        Any reasonable grid would also be wired such that the power you generate will first go to your neighbors, if they need it, instead of 'round-tripping' to the utility and back. So, the utility is taking a huge cut even though it isn't really a factor in the transaction. There are of course large-scale infrastructure costs (you get to fail-over to nuclear/coal/gas/wind/other), but it seems like all of those things should be accounted separately instead of all being lumped into two line items on a monthly bill from your local monopoly.
  - marcosdumay 11y ago
    Well, you can go buy a battery.
- bostonpete 11y ago
  > It's basically a scam. Why is it a scam that PG&E won't pay you more for power than they can buy it wholesale?
  - thrownaway2424 11y ago
    The 3-4c wholesale rate is for base power (giant infrastructure generators, hydroelectric dams and such things), but solar knocks the top off the mid-day demand curve, at which time PG&E is paying much more for "peaker" power generation. But they don't have to pay you the peaker rate, they pay you the baseline rate. Anyway, to my mind, any system in which current comes out of your PV installation at 3 cents and flows directly into your neighbor's air conditioner at 34 cents is a prima facie scam.
    - _delirium 11y ago
      Even peaker plants run ~15 cents these days, since the price of gas has fallen so low. edit: I'm more familiar with the Texas grid, but beyond the cheapish price of modern peaker plants, in that case the most expensive input sources (small peaker plants, and esp. spot-market purchases) are rarely used at all. Demand prediction and management is good enough to avoid the more expensive inputs, through a mixture of local generation management and advance wholesale purchases on the DC interconnects. On the handful of days when some issue causes a shortfall, it would be valuable to buy residential solar inputs, but this happens about 5-6 days a year. To handle those days, most utilities have a savings-rebate program, where if you agree to reduce your usage between 2-5pm on these handful of days, you get a $0.60/kwh credit. So they solve their occasional peak problem by just turning your thermostat up 2º and "buying" the unused energy at a premium price.
      - thrownaway2424 11y ago
        Yeah, but that makes PG&E's "smartrate" summer afternoon price of 75c per kWh even less justifiable. This data may not be publicly available, but I'd love to see graphs of current flowing out of PG&E substations vs. time of day. At some point the PV generation capacity is going to make them have weird upside-down diurnal cycles.
JoblessWonder 11y ago
I'm fairly certain San Francisco lets you "run the meter backwards" and put power back into the grid. That is how they are calculating 100% solar power since the amount you are sending in during the day offesets the amount you are pulling in during the evening. This also means that you don't need to worry about battery packs.
birken 11y ago
If you are already connected to the grid, then you can simply just feed the electricity back into the grid when over-producing (which is tracked by your meter), and then use electricity from the grid at night. This actually works in your benefit if you have solar panels, because electricity prices are higher during the middle of the day (more demand). So you can "sell" daytime electricity (at high prices) [1], and then buy electricity at night which is much cheaper. This actually is the main benefit of the battery if you are connected to the grid, you can do the same thing. 1: You don't want to sell past your own consumption though otherwise you get paid very little for it
- bmelton 11y ago
  Except that, as pointed out above, the utilities only pay the wholesale rate for the electricity they buy back from you, and sell the overage to your neighbors for full price. Meanwhile, you're likely losing money selling your peak day electricity for 5 cents and buying it back for 15.
  - birken 11y ago
    At least with PG&E, my understanding was that those rates only applied to a net over-production over a long period of time (I believe over the course of a year). As long as your power bill is not negative, any over-production during high value times of the day is sold at the market rate and offsets low value electricity you buy at night. This is important with solar system sizing, because not only don't you want to be a net exporter of energy, the lower tiers of PG&E's energy rates are very cheap so you don't want to generate that power either. You really only want to offset the high-tier, high-demand electricity you use (or plan to use in the future). For me it didn't really matter because my energy usage is highest during the peak times of the day, but this is my understanding of how it works. I'll know for sure in a couple of months when I see my bill :)
    - secabeen 11y ago
      This is correct. Under Net-Metering policies in effect in California for systems installed now, you get reimbursed at full retail rates for every kWh you produce and export to the grid up to the level you import from the grid when solar is insufficient to need (at night, etc.) The wholesale rate is only for exported power generated beyond imported use.
timcederman 11y ago
Very few installations use batteries. Right now in the Bay Area, almost everyone is using net energy metering (NEM). You get a bill once per year, and they balance out your usage vs generation, and any extra usage is charged. There is a minimum you have to pay each year, so most installers aim for 85-95% of your usage to be generated by your system. As other folks have stated, it's not advantageous to sell back to the grid given the low price - as such, very very few people actually sell back to the grid, and instead make a small purchase each year, and mostly at off-peak rates.

by jvm 11y ago

qooleot 11y ago

Often, the most economical answer is heating a water heater rather than storing via a battery.

thrownaway2424 11y ago

Solar power systems don't necessarily come with batteries but a decent installation company should be able to propose and implement an integrated system. The Tesla battery pack is an example of some recent innovation in that area. You can sell power into the grid but PG&E only has to pay you wholesale rates, 3-4 cents per kwh, not what they are charging you to take the same amount of energy out, which is 34 cents per kwh at the top tier. It's basically a scam. You don't want to overbuild on the assumption that selling the surplus would be profitable.

jsprogrammer 11y ago
Is there no real solar power market then? If your local utility is a monopoly then you basically have to accept whatever the utility wants to pay you (even far below 'market' rate). You have no power to negotiate, only capitulate or try to get the local regulator to enforce fair trade.
- thrownaway2424 11y ago
  Local conditions may vary, of course, but if just go with the completely unimaginative system, and you live in California in PG&E territory, there is no effective market.
- _delirium 11y ago
  I suspect you'd fare pretty badly in a market, even if a better one existed. In Texas's deregulated electric marketplace, the market price for large-scale solar installations selling into the grid has fallen to around $0.05/kWh. Power from a single small-scale residential installation with less reliable delivery is probably worth less than that.
  - ZeroGravitas 11y ago
    Depends on how much "better" the market is e.g. if it priced in the greenhouse gas externalities, the avoidance of transmission losses and grid upgrades etc then solar may be worth triple the residential rate. I believe a couple of states have calculated "value of solar" to use as part of their grid regulation, though specific factors like mix of gas and coal production and projected grid expansion factor into this.
    - jsprogrammer 11y ago
      Any reasonable grid would also be wired such that the power you generate will first go to your neighbors, if they need it, instead of 'round-tripping' to the utility and back. So, the utility is taking a huge cut even though it isn't really a factor in the transaction. There are of course large-scale infrastructure costs (you get to fail-over to nuclear/coal/gas/wind/other), but it seems like all of those things should be accounted separately instead of all being lumped into two line items on a monthly bill from your local monopoly.
- marcosdumay 11y ago
  Well, you can go buy a battery.
bostonpete 11y ago
> It's basically a scam. Why is it a scam that PG&E won't pay you more for power than they can buy it wholesale?
- thrownaway2424 11y ago
  The 3-4c wholesale rate is for base power (giant infrastructure generators, hydroelectric dams and such things), but solar knocks the top off the mid-day demand curve, at which time PG&E is paying much more for "peaker" power generation. But they don't have to pay you the peaker rate, they pay you the baseline rate. Anyway, to my mind, any system in which current comes out of your PV installation at 3 cents and flows directly into your neighbor's air conditioner at 34 cents is a prima facie scam.
  - _delirium 11y ago
    Even peaker plants run ~15 cents these days, since the price of gas has fallen so low. edit: I'm more familiar with the Texas grid, but beyond the cheapish price of modern peaker plants, in that case the most expensive input sources (small peaker plants, and esp. spot-market purchases) are rarely used at all. Demand prediction and management is good enough to avoid the more expensive inputs, through a mixture of local generation management and advance wholesale purchases on the DC interconnects. On the handful of days when some issue causes a shortfall, it would be valuable to buy residential solar inputs, but this happens about 5-6 days a year. To handle those days, most utilities have a savings-rebate program, where if you agree to reduce your usage between 2-5pm on these handful of days, you get a $0.60/kwh credit. So they solve their occasional peak problem by just turning your thermostat up 2º and "buying" the unused energy at a premium price.
    - thrownaway2424 11y ago
      Yeah, but that makes PG&E's "smartrate" summer afternoon price of 75c per kWh even less justifiable. This data may not be publicly available, but I'd love to see graphs of current flowing out of PG&E substations vs. time of day. At some point the PV generation capacity is going to make them have weird upside-down diurnal cycles.

JoblessWonder 11y ago

I'm fairly certain San Francisco lets you "run the meter backwards" and put power back into the grid. That is how they are calculating 100% solar power since the amount you are sending in during the day offesets the amount you are pulling in during the evening. This also means that you don't need to worry about battery packs.

birken 11y ago

If you are already connected to the grid, then you can simply just feed the electricity back into the grid when over-producing (which is tracked by your meter), and then use electricity from the grid at night. This actually works in your benefit if you have solar panels, because electricity prices are higher during the middle of the day (more demand). So you can "sell" daytime electricity (at high prices) [1], and then buy electricity at night which is much cheaper. This actually is the main benefit of the battery if you are connected to the grid, you can do the same thing. 1: You don't want to sell past your own consumption though otherwise you get paid very little for it

bmelton 11y ago
Except that, as pointed out above, the utilities only pay the wholesale rate for the electricity they buy back from you, and sell the overage to your neighbors for full price. Meanwhile, you're likely losing money selling your peak day electricity for 5 cents and buying it back for 15.
- birken 11y ago
  At least with PG&E, my understanding was that those rates only applied to a net over-production over a long period of time (I believe over the course of a year). As long as your power bill is not negative, any over-production during high value times of the day is sold at the market rate and offsets low value electricity you buy at night. This is important with solar system sizing, because not only don't you want to be a net exporter of energy, the lower tiers of PG&E's energy rates are very cheap so you don't want to generate that power either. You really only want to offset the high-tier, high-demand electricity you use (or plan to use in the future). For me it didn't really matter because my energy usage is highest during the peak times of the day, but this is my understanding of how it works. I'll know for sure in a couple of months when I see my bill :)
  - secabeen 11y ago
    This is correct. Under Net-Metering policies in effect in California for systems installed now, you get reimbursed at full retail rates for every kWh you produce and export to the grid up to the level you import from the grid when solar is insufficient to need (at night, etc.) The wholesale rate is only for exported power generated beyond imported use.

timcederman 11y ago

Very few installations use batteries. Right now in the Bay Area, almost everyone is using net energy metering (NEM). You get a bill once per year, and they balance out your usage vs generation, and any extra usage is charged. There is a minimum you have to pay each year, so most installers aim for 85-95% of your usage to be generated by your system. As other folks have stated, it's not advantageous to sell back to the grid given the low price - as such, very very few people actually sell back to the grid, and instead make a small purchase each year, and mostly at off-peak rates.