Solar Batteries
Battery prices have been dropping dramatically over the past few years. This has led some people to wonder whether to buy large battery packs for their PV solar system, either to get off the utility electric grid entirely, to reduce the size of PV system they need, or for backup purposes. Unfortunately, we're not there yet.
To rely entirely on your PV system without being connected to the grid, a battery would have to be large enough to supply all your electricity during the hours each day when your PV system is not producing at all (say 8:00pm to 6:00am). It would also need to hold enough to supplement your system's production when you're using more electricity than the system can produce at that time (early mornings and late afternoons, likely, or when air conditioning use is high during the summer). In addition, it would have to store enough to power your house on cloudy days, when your system produces less electricity. Finally, it would need to carry you over during the months of November through February, when PV systems in Southern California are at their lowest levels. During these months, you generally use more electricity every day than you generate. There is no way that batteries could make up for this shortfall, which could easily be 1,200 kWh for the winter. This could only be addressed by getting a substantially larger PV system.
Let's consider what size battery would be needed for an average SCE homeowner ($200 per month average electricity bill without solar and 10,772 kWh per year of usage). Daily usage on a hot summer day can easily be 50 kWh to 70 kWh, while your correctly-sized PV system would produce only 30 kWh, leaving you short 20 - 40 kWh for just one day. On a cloudy day, you could be short 30 - 60 kWh. For example, last July we had a three-day stretch of partly cloudy days with temperatures over 100 degrees. During that time, our system production fell by 45%, leaving us with a 50 kWh shortage for the three days. It could even be much worse than that.
Therefore, if you were not connected to the grid, you'd need a battery well over 60 kWh at the very least. (The reason this shortfall doesn't leave you owing SCE money when you're connected to the grid is that you usually draw most of your electricity at off-peak or super-off-peak rates, while you send most of your PV production to the grid at on-peak rates. Solar leverage!)
A top-quality (Sonnen) 60 kWh battery could cost over $40,000 net, after the 30% ITC (Investment Tax Credit) and California's SGIP (Self-Generation Incentive Program) rebates (which are very complicated and uncertain as to amount). Even using 7 of Tesla's lower-quality Powerwall II batteries (probably producing only 63 kWh, not the 95 kWh Tesla claims) could easily cost over $15,000 net, even if you got the full SGIP rebates, which is unclear.
In addition, you'd need a substantially larger PV system than if connected to the grid, because it would have to be large enough to fill up this enormous battery during the times it produced more electricity than you use, plus supply all the electricity you need during the winter. So the cost of getting off the grid would be the extra cost of this larger PV system plus the cost of the batteries. It just doesn't make economic sense.
But maybe you could offset the cost of batteries by getting a smaller PV system, while staying connected to the grid. This doesn't come close to working either. Part of the reason is that a smaller PV system would frequently be unable to fill up the battery, so it wouldn't be charged enough to use when you needed it. Another reason is that you couldn't cut the cost of the PV system by nearly enough to pay for the batteries needed.
So right now and for the near future, the only practical application of batteries is for backup to protect you if the grid goes down when your PV system isn't producing enough to meet your needs. This doesn't happen very often, and when it does, it is likely to occur during the day, when demand is high - the very time your PV system is producing at its highest levels, so you are least likely to suffer if the grid goes down.
All in all, batteries are unlikely to be very attractive to supplement PV systems until their prices fall a great deal.
To rely entirely on your PV system without being connected to the grid, a battery would have to be large enough to supply all your electricity during the hours each day when your PV system is not producing at all (say 8:00pm to 6:00am). It would also need to hold enough to supplement your system's production when you're using more electricity than the system can produce at that time (early mornings and late afternoons, likely, or when air conditioning use is high during the summer). In addition, it would have to store enough to power your house on cloudy days, when your system produces less electricity. Finally, it would need to carry you over during the months of November through February, when PV systems in Southern California are at their lowest levels. During these months, you generally use more electricity every day than you generate. There is no way that batteries could make up for this shortfall, which could easily be 1,200 kWh for the winter. This could only be addressed by getting a substantially larger PV system.
Let's consider what size battery would be needed for an average SCE homeowner ($200 per month average electricity bill without solar and 10,772 kWh per year of usage). Daily usage on a hot summer day can easily be 50 kWh to 70 kWh, while your correctly-sized PV system would produce only 30 kWh, leaving you short 20 - 40 kWh for just one day. On a cloudy day, you could be short 30 - 60 kWh. For example, last July we had a three-day stretch of partly cloudy days with temperatures over 100 degrees. During that time, our system production fell by 45%, leaving us with a 50 kWh shortage for the three days. It could even be much worse than that.
Therefore, if you were not connected to the grid, you'd need a battery well over 60 kWh at the very least. (The reason this shortfall doesn't leave you owing SCE money when you're connected to the grid is that you usually draw most of your electricity at off-peak or super-off-peak rates, while you send most of your PV production to the grid at on-peak rates. Solar leverage!)
A top-quality (Sonnen) 60 kWh battery could cost over $40,000 net, after the 30% ITC (Investment Tax Credit) and California's SGIP (Self-Generation Incentive Program) rebates (which are very complicated and uncertain as to amount). Even using 7 of Tesla's lower-quality Powerwall II batteries (probably producing only 63 kWh, not the 95 kWh Tesla claims) could easily cost over $15,000 net, even if you got the full SGIP rebates, which is unclear.
In addition, you'd need a substantially larger PV system than if connected to the grid, because it would have to be large enough to fill up this enormous battery during the times it produced more electricity than you use, plus supply all the electricity you need during the winter. So the cost of getting off the grid would be the extra cost of this larger PV system plus the cost of the batteries. It just doesn't make economic sense.
But maybe you could offset the cost of batteries by getting a smaller PV system, while staying connected to the grid. This doesn't come close to working either. Part of the reason is that a smaller PV system would frequently be unable to fill up the battery, so it wouldn't be charged enough to use when you needed it. Another reason is that you couldn't cut the cost of the PV system by nearly enough to pay for the batteries needed.
So right now and for the near future, the only practical application of batteries is for backup to protect you if the grid goes down when your PV system isn't producing enough to meet your needs. This doesn't happen very often, and when it does, it is likely to occur during the day, when demand is high - the very time your PV system is producing at its highest levels, so you are least likely to suffer if the grid goes down.
All in all, batteries are unlikely to be very attractive to supplement PV systems until their prices fall a great deal.
