Batteryless & Battery-Based Grid-Tied Systems

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Batteryless & Battery-Based Grid-Tied Systems
Batteryless & Battery-Based Grid-Tied Systems

Most grid-tied PV systems are batteryless. They rely on moving energy to and from the grid. Under “net-billing” agreements, the utility provides billing credit for any surplus electricity produced by the system. System owners can then draw on that credit when more energy is being used than the system is producing. Depending on system size and the energy-use habits of the system owners, some systems can earn enough credit during the summer months (greater production) to make up for winter’s lower production, becoming a net-zero annual energy user.

The disadvantage of a batteryless system is that the system relies on the grid to operate, so if the utility grid stops working, the home is also without power—despite having an on-site source of power (PV system). For most people, this is no big deal because utility outages are usually short in duration. But there are situations in which a home must be able to continue using energy even when the grid is not working, such as relying on life-saving medical equipment, keeping computers running, or keeping food cold.

Some homes or businesses might be able get by with an uninterruptible power supply (UPS). But unfortunately, those normally operate only long enough to properly turn off computers or electronics without “crashing,” which can cause data loss.

Other homes or businesses might be able to get by with a new class of batteryless inverters that, when the sun is shining and the grid is down, can provide a limited amount of AC energy (see this article). These systems can be useful for running a fridge during the day, which will probably keep food cold enough to make it through the night until the next day. They can charge phones and other battery devices which otherwise could run out of battery energy. But they won’t be helpful at night or during cloudy days. Only batteries can make up for that.

So why doesn’t everyone include a few batteries for backup? First, including batteries adds complexity to a PV system, which adds expense. Second, battery-based systems are 10% to 15% less efficient compared to batteryless systems, since some energy is used to keep the batteries at a full state of charge—and that means less energy goes out to the grid. The larger the battery, the more self-discharge there will be, and the less net-billing credit you will get out of your system. Finally, batteries will likely need replacing sooner than the rest of the equipment in the system, generally two to four times over the lifetime of the inverter and other system electronics. If you decide to design and install a battery-based grid-tied system, use the smallest battery capacity that will serve your backup needs—to keep costs down and limit efficiency losses.

Michael Welch

Comments (6)

paul hoover's picture

Net metering is not available everywhere. In half of Mississippi counties not served by TVA, renewable power is purchased at the wholesale rate and extra charges are made for metering. Off-grid or hybrid systems which do not backfeed but use the grid for backup are the more attractive options. This is also true in parts of rural NC where electric co-ops refuse net metering. As more utilities add solar electric fees grid tie systems will become less desirable.

Marie Heartwood's picture

Generally correct, but there is a different approach if you are in an area with a fragile grid.

Solar panels produce DC power which must be "inverted" into AC. However, if you've ever lived in an RV, you know most everything in it runs off 12VDC, the same voltage as a battery system. If one gets higher efficiency but smaller panels, and more of them, they output about 14VDC, the charge voltage for a 12VDC battery. Using a 555-based charge controller which is about the most efficient you're going to find, you can charge the 12V batteries directly from the panel at about 99.9% efficiency. Converting 120VAC to 12VDC is quite easy and quite efficient and can be done at 98% efficiency or better. So if you target running everything, or at least everything important, off of 12VDC, you end up with a more efficient system than one that uses inverters and it will work when the sun is shining but the grid is down. Computers, refrigerators, freezers, heaters, radios, and anything designed for an RV or "in-car use" can run off of the 12VDC without the inverter. But it means re-thinking some aspects of how we live.

Michael Welch's picture

Hi Marie. One problem is that those 12 VDC devices are not designed as robust as standard AC appliances, because RV use is usually very part-time. Another is that fewer of them are available, and that means fewer features available.

Yet another is that AC appliances are often more efficient than DC appliances because of energy efficiency standards that many meet -- standards which do not apply to DC appliances.

Then there is the wiring issue. Low voltage wiring must be larger in gauge to avoid wire loss, and that means more expense.

I have my home wired for both AC and DC, and because of the efficiency of modern AC appliances, the only DC load I am currently using is a ceiling fan bought 30 years ago -- back when it made more sense to run DC appliances.

Nowadays, PV modules are so cheap that people just add a little bit more solar to make up for the little bit of efficiency losses from using an inverter.

Wayne Johnson's picture

You have a 12 volt computer? I've never heard of that....I'm heavily invested in computers and use them for my home business.
I have conventional grid-tie and have been exploring options to power things without the grid. It seems totally ridiculous that my solar panels sit there dead-in-the-water if / when the grid goes down. AC coupling is the obvious direction, but that involves another expensive installation and more complexity. I like your 12 volt way-of-life; it certainly has its merits. I'm still trying to think my around my obvious problem, and may re-wire part of my system to operate off-grid. BTW--if we have a national emergency, then everyone is on a "fragile grid".

Michael Welch's picture

Most laptops have 12 VDC power cord/supplies available so that they can easily be used and charged in vehicles.

Si99's picture

You can certainly run a PC from a 12v supply, you need a PicoPSU, or some other DC-DC converter. These small power supplies do have limited capacity but they'll happily run a modest PC. This article gives you an idea of how many watts you really need to run a PC http://www.zdnet.com/blog/ou/55w-pc...

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