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House-level battery backup now, solar panels later -- what do I need to know? Question
In the last few years my area has seen increased power outages -- usually a few hours, occasionally longer. I'd like to defend against these better so that a poorly-timed outage doesn't ruin my whole day. I'm thinking, in particular, of whole-house battery backup.
I know that this kind of battery is routinely part of solar systems (else your solar panels wouldn't be much good at night). I do want to upgrade to solar, but my roof is nearing the end of its life and I don't want to have to pay for installing panels, then lifting them for a new roof, and then reinstalling them. I'm willing to wait until it's time to replace the roof anyway and then add solar panels.
What can I do in the meantime to improve my backup-power situation? Ideally I would like to be able to power heat or cooling, the fridge, some lights, and a few computers for up to 24 hours in the event of a power outage. If I buy the battery part of a solar system now, then am I risking expensive incompatibility in a few years when it's time to add the panels? Or is the risk there something that a qualified electrician could handle for me? Is there some other UPS-like system that I could wire into my house?
I don't want to hook up a gas-powered generator. In addition to the extra maintenance hassles there, I live in a city, the neighbors are close, and generators are loud.
2 answers
The trick is getting everything to work together. Typical solar (without battery) installations actually work with the grid. If you generate more power than you need, the balance goes to the utility and you get some money for it. This is net metering.
The problem is that it is grid-tied - if the power goes out then you very likely have no power at all. That's because the system is designed to synchronize with utility power. No utility power and it doesn't know what to do and shuts off. If it stayed on, the power would be going out and zap utility workers. (The same problem happens if you use a generator and don't cut off the utility connection, which is why you need a generator interlock or a transfer switch if you connect a generator to your breaker panel.)
Battery backup solves the problem because it is designed to charge when utility power is on and discharge when utility power is off. If you use solar (off-grid) then it charges when solar is on and discharges when solar is off (night). But getting enough battery backup to power peak loads is not easy or cheap. Plus your battery backup is typically designed for a day or so of power. Power outages can last longer than that, so there is a natural inclination to pair them with some additional power source, which can include solar, generator (but then you are paying for a generator that is only used in some outages) or the latest is vehicle-to-grid if you have an EV that can do that.
So utility power + battery backup + solar is a very natural pairing. It is quite different from a totally off-grid battery + solar setup as then the batteries are cycling frequently (every night) and you size your system based on peak loads.
With utility + battery + solar, you size based on necessary loads and manage accordingly during an outage, just as you would with a small (not whole-house) generator. Which is not a big deal - refrigerator, computers, lights are all small loads. With a reasonably sized backup you can also run HVAC (except electric resistance heat, but hopefully you don't have that), some cooking appliances, even the clothes dryer - you just might need to turn off one large appliance before you turn on another. The great thing is that if you have solar and manage well, you could do just fine for an extended blackout as long as the sun still shines for several hours each day and you manage your power well. If the power goes out the day before Thanksgiving, you may have to skip the turkey (ovens use a lot of power), but at least you can leave it in the freezer and know you have enough power to keep it running.
The bottom line is in the control electronics. This sits between solar, battery and utility to manage everything. There is no fundamental reason you can't have a system which handles it all, includes a large battery pack, and does not require solar. However, a company that sells packages based primarily on solar panels may not be interested in selling you that equipment. You can't just "roll your own" - for safety reasons it all has to be done right to avoid backfeeding the utility grid, and if you want to add solar in the future the utility will likely have rules as to certain key specifications of the system that you will need to plan for at the beginning to avoid extra costs.
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You should consider a whole house standby generator rather than whole house batteries.
From a cost perspective you can get a 20KW generator installed for around $10,000 (USD). A 20KW battery and inverters are likely to cost three to four times as much.
Lifespan for a generator is also much better than for batteries. A standby generator that is used only during occasional outages may last 20 years. Whole house batteries need to be replaced every 5 to 7 years.
Generators can handle much longer power outages than batteries. It is easy to have enough fuel on-hand to last days with a generator. Batteries will usually last only hours and may last much less than that if they aren't fully charged when the power goes out.
The environmental impact of battery production is so great, that buying batteries is worse for the environment than running a standby generator occasionally.
When you get solar, the power grid acts as your energy storage. With net-metering, you send excess energy your panels produce to the grid and you later withdraw it when you need it. Batteries are not typically part of a solar installation unless it is off-grid or net-metering is not available.
When you eventually have a generator, solar panels, and net-metering, your normal usage and production will end up looking something like this where solar is the light orange and usage is the dark orange. During the day the panel produce lots of electricity and ship the extra off to your neighbors. At night you withdraw the extra power that you created during the day. Your solar installer will help you size your solar system so that your overall usage for the year matches what your panels produce.
During a power outage your standby generator provides all the power to your home and your solar panels are not active.
A standby generator will automatically start when the power goes out, so you may have about 15 seconds without electricity. You can solve that problem by using small batteries in uninterruptible power supplies (UPS) for things like computers, routers, telephony, and networking equipment that you don't want to have reboot when the power goes out.
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