Can Battery Backups Make Money?
Posted by Max Dunn Wed, 23 Jan 2008 18:12:00 GMT
I don’t have a lot of confidence in PG&E. It seems that every time we have a big storm, our power goes out. Once, it took 3 days for them to get the power back on! So I have been considering installing a battery backup system for my house to get through these power outages.
If I install a battery backup system, I was wondering if it would be profitable to charge up the batteries at night when electricity costs are low, at $0.05/kWh, and then use it during the day when electricity costs are high, at $0.11/kWh to $0.29/kWh [9]?
One factor in whether this makes sense is how efficient it is to store energy in a battery. In other words, if I pay for 10kWh of electricity and put it in a battery, how many kWh will I get back out? Let’s look at some numbers.
Most likely I will be using lead acid batteries, since they are the cheapest and the extra weight and size won’t matter. When doing the initial charge of lead acid batteries, they have a very good efficiency, about 95%. However, it is necessary to top-off lead acid batteries each time they are used or they will continue to give out less and less energy. But this topping-off phase is not very efficient, so overall lead acid batteries have an energy efficiency of about 85% [1].
The charger that puts the energy into the battery also has some losses as does the inverter which takes out the electricity and converts it back to AC. Decent chargers and inverters are about 85% efficient [3][4].
Combining these numbers (0.85×0.85×0.85) gives us a total efficiency of about 60%. This would mean that if I put for 10kWh of energy into my battery system, I would only get out about 6kWh.
It will cost about $0.50 to charge my 10kWh battery pack at night ($0.05/kWh x 10kWh), and it will save me $0.66 during the day ($0.11/kWh x 6kWh) for a net gain of $0.11 per day. Not much. If we look at the summer peak electricity cost of $0.29/kWh, we would save $1.74 ($0.29/kWh x 6kWh) for a net gain of $1.24. This is better, but lets see how much the batteries usage would cost.
To make lead acid batteries last a long time and get the most energy from them, they should only be discharged to 50% of the capacity. So I would need a 20kWh battery pack to cycle 10kWh out of them. Flooded lead acid batteries cost about $100 per kWh [6][7][8] so the total battery pack would cost about $2000 (20kWh x $100/kWh). They should last about 1000 cycles [10][11], so it would cost $2.00 every time the are charged and discharged.
It is not a very good deal to pay $2.00 to make $1.24! So it doesn’t look very good for my idea of charging the batteries at night and then using them during the day,
What about using lithium batteries? They are more efficient, at about 95% [2], and you can buy chargers and inverters that are 90% efficient [5]. So with these higher cost components, you could get an overall efficiency of of about 78% (0.95×0.9×0.9). And if we are using the summer peak cost of electricity of $0.29/kWh, we could save about $2.26 (10kWh * 78% * $0.29/kWh) of electricity at the peak for a net gain of $1.76.
This seems pretty good, but lithium batteries are expensive. You can charge lithium all the way down without damaging them, so we would only need a 10kWh pack. And they also last longer, usually 2000 cycles. But they are expensive, at about $1000/kWh. So a lithium battery pack would cost $5 each time we cycled it. Still losing money.
So it turns out that using a battery backup system to store energy at night when electricity costs are low and then using it during the day when costs are high doesn’t make sense since the battery usage costs even more.
[1] A Study of Lead-Acid Battery Efficiency Near Top-of-Charge and the Impact on PV System Design
[2] Battery Primer
[3] Efficiency of Power Supplies in the Active Mode
[4] Cheap Inverter Efficiency Test
[5] Vehicle to Grid Fundamentals: Calculating Capacity and Net Revenue (Page 8, Table 3)
[6] Electro Automotive Battery Page
[8] Blockbuster Golf Carts US8VGC
Are there more efficient or less costly ways of storing energy? How about if you use the cheap night electricity to pump water to a tank on your roof, then let it generate electricity on its way back down. Or run an air compressor at night to pressurize a 50 gallon drum. Or create hydrogen thru electrolysis?
Good question, let’s look at other ways of storing energy. First, let’s look at water.
Water
To determine the amount of electricity water can produce, you can use this equation from reference [1]:
watts = (Height [ft] * Flow [USGM]) / 9
or to change to kWh and solve for Gallons:
Gallons = 540,000 * kWh / Height
So if you have tank on your roof that is 10 feet high and you want to produce 10kWh of electricity you would need to 540,000 gallons of water. For comparison, the average swimming pool holds 25,000 gallons so this would be equivalent to over 20 swimming pools!
However, this is probably the most cost-effective way of storing energy being 70% to 85% efficient [2].
[1] Micro Hydro Equations
[2] Pumped-storage hydroelectricity
Air
Storing 10kWh of energy as compressed air would take only about a 300 liter tank [1], but the efficiency of compressing the air and getting it out will likely be less than 25% [2] and even in ideal situations, still less than 40% [3].
[1] Compressed Air Thermodynamics of the MDI Air Car
[2] Physics of isothermal compressed air storage
[3] Thermodynamic Analysis of Compressed Air Vehicle Propulsion
Hydrogen
Using hydrogen to store energy is only 20%-25% efficient [1]. This is about 3 times less efficient than storing the electricity in batteries [2].
[1] Does a Hydrogen Economy Make Sense?
[2] Wind-to-Wheel Energy Assessment
It’s great to see the numbers – thanks for calculating it thru. Here’s another one for you – how about a giant flywheel in your garage? I love the answer for storing water – I’m just imagining 20 swimming pools on your roof!
I’m in the USVI where the cost of electricity it $.33 KvH. I can live with that - but the government-owned power system goes off line 3-4 times per mon th in good weather (max = 1 day) and it can be really bad during hurricane season. How about your initial premise - which was not to save money, but to provide a “backup generator” capacity? We cannot have any large generator in our condo. (2500 = max) So…..I’ve looked at Gridpoint and Gaia systems—both 10 KwH total capacities. To cover my fridge, small a/c unit and bedroom (including tv, lights and computer stuff) I draw a max of 2 KwH, with actual usage closer to 1. But the systems are $12,000. Any thought or observations would be GREATLY appreciated !!!!
Carl,
Wow, paying $0.33 kWh is a lot! If you could put up solar panels, they would probably pay for themselves in 10 years. But I am guessing in your condo, this is not an option, so let’s go back to battery backups.
The first question is whether you want a system that will tie into your house power circuit and come on automatically when the power goes out. These are certainly the most convenient, and people that use them say that they often don’t even know that the power went out, until they look at their neighbor’s houses that are dark.
The downside to these systems is that they are more expensive and require an electrician to install, which adds another $1000 or so to the price.
If that is what you are looking for, then a Gridpoint, Gaia or Xantrex system would fill your needs.
For a lower cost system that doesn’t require an electrician, you could look into a UPS like the Xantrex PowerHub 1800 This lists for $900. Batteries are extra and the run time isn’t very long, but two of these each with the extra battery might be able to fill your needs and the total cost including batteries would likely be less than $4,000. This system also has the advantage that it can be connected to solar or wind generators to recharge the batteries.
You could also look into Sentinel systems. You can get a 7.2kWh system with batteries for about $4000. Most of these need to be hardwired into your home electrical system, but I recall seeing one that offered a plug outlet.
Good luck with this, and let me know what you end up doing.
Flywheels
It turns out the flywheels might be a viable way of storing energy for a house battery backup system.
They can be up to 90% efficient, can have capacities up to 133kWh, and have long lifetimes with little maintenance.
However, while some commercial and industrial flywheel storage systems are available today, there doesn’t appear to be any for sale to individual users, and the initial cost is about 50% higher1 than a battery system, although the total cost is lower over time.
1 Pentadyne VSS Brochure
See also:
Wikipedia – Flywheel Energy Storage
Beacon Power – Smart Energy 25kWh
CleanSource UPS