Solar When the Grid Goes Out: The Case Against Batteries

December 20th, 2011 by Fred Greenhalgh

Solar During Power OutagesNo one’s happy when the grid goes out. Power outages are at the minimum inconvenient, and, if a home is poorly suited to be without power, problematic or even dangerous. So it would seem obvious, why not install a grid-tied solar electric system and power your home using sunshine during an outage?

While technically this is possible, it is not so simple. Grid-tied photovoltaic systems are designed to provide decades of trouble-free electricity generated by the sun. They are simple, economical, reliable, and, as their name implies, tied to the grid. When the grid goes down, the system goes down, too.

This is by design: by shutting down, the grid-tied solar electric inverter (the component that converts direct-current power from the solar panels into grid-compliant 120V AC power used in your home or business) prevents power from backfeeding to the grid and injuring nearby line workers.

The only way to circumvent this is to add batteries, which provide a backup source of power when the power goes out. Unfortunately, batteries are almost the antithesis of a grid-tied system: they are bulky, inefficient, ecologically unfriendly, unreliable and expensive.

Grid-Tied with Battery Backup Vs. Off-Grid

Sunny Island Off Grid SolarA grid tied battery backup system uses the same principles as an off-grid solar electric system when the grid goes down. Instead of shutting off completely, the system switches to an alternate inverter, which is designed to interact with a battery bank, letting you run household loads off the battery bank and charge them with your solar panels.

The equipment to do this is quite sophisticated (in the situations where ReVision goes this route we install SMA’s Sunny Island product), and is derived from R&D for very remote locations where grid access is not feasible. While they can work smoothly, they come at a great cost, especially once you consider the cost of short-lived lead acid batteries (using similar technology as is in your car battery).

An average home with an electric bill of $100/month, for instance, consumes ~625kWh of power a month (assuming a grid price of .16c/kWh). That averages around 20kWh/day of power. Enough good-quality batteries to carry that load for just two days would cost around $25,000. Realistically, a battery-backup system is only designed to support emergency loads for short periods of time (water pump, refrigerator and maybe a freezer), but even so, the cost reaches approximately $20,000 of additional cost for a short-term grid-tied battery backup solution.

Further limitations of this system is the battery life – approximately 5-7 years – as well as the battery potential. While this system can provide backup power for a few days, for true energy security you need an option for charging the batteries should a long-term outage occur during bad solar weather (which tends to be when we lose power in Northern New England). So, even with a large battery investment you still require a generator to be totally secure.

Getting To Your Goals: Why Are You Going PV?

Most people look at a solar energy investment as a way to reduce their fossil fuel energy consumption while locking in their electric rate below the grid average for a duration of 25+ years. Cost pressure on solar panels and generous state and federal rebates make grid-tied PV an excellent investment right now, with a ‘simple’ payback of around 10 years in Maine or New Hampshire.

Adding battery backup changes this equation completely. Dollars that could be invested in more renewable energy production are invested in short-lived, ecologically unfriendly battery components. Realistically, with the grid down as infrequently as it is, a battery-backup system means you are paying a stiff premium for a system you do not need 99% of the time.

With that caveat, there are times when solar with battery backup meets specific goals, mostly in situations where no grid downtime is tolerable. For example, keeping critical loads like life support equipment online, or simply ensuring failsafe electricity for homes where one spouse travels frequently in the winter and doesn’t want their partner home alone in a power outage!

So What Am I To Do During Power Outages?

Though we are loathe to recommend the installation of a fossil-fuel burning appliance, the reality is that for the average numbers of days a typical home is without power (1-2 days a year or less), and given the cost and complexity of installing a grid-tied system with batteries, a generator is often a better investment.

Even an extremely robust home standby generator is roughly 1/4 of the price of enough batteries and equipment to maintain a home for a few days without power. While a battery bank will struggle to keep up with heavy energy hogs like a refrigerator or well pump, a properly sized generator will carry these without missing a beat.

If you just can’t stand the thought of a generator, here are some other power outage tips from our renewable energy experts:

  • Wood is good – Cord wood stoves are a tradition in New England and a wonderful appliance to have when the power goes out. They can be good for cooking and eating.
  • Stockpile water – Most people’s top complaint about power outages is running out of water. Fill up your tubs and empty milk jugs with water so you can cook dinner, wash hands and flush toilets during outages. And if you’re on city water and have a solar hot water system, you can continue to use your solar hot water even without power!
  • Keep alternative lights handy – With the sun firmly set by 5pm nowadays, a power outage can make for a long dark night. Luckily, LED flashlights are lightyears more efficient than their incadescent predecessors. LL Bean has a nice selection.
  • Make it fun – A power outage need only be as much an ordeal as you make it. Kids especially can be encouraged to make it an opportunity for adventure and find it fun to cook on the woodstove or propane grill. A bit of flexibility and patience go a long way.

Stay warm out there!


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