Solar When the Grid Goes Out: Should I Get Batteries?
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Updated 1/5/2015 – While adding batteries to a grid-tied solar PV array is still not a no-brainer in many situations, it is a lot more straightforward than it was when this article was originally published and we have updated it accordingly.
No 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 electricity from the solar panels into grid-compliant 120V alternating-current electricity 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.
How to Add Battery Backup to an On-Grid Solar PV Array
A grid-tied (and net-metered) solar electric array connects directly to the power grid and uses the grid as ‘storage’ – when there is no solar power available, it consumes electricity from the grid. When there is excess solar power, it feeds that power to the grid, and the homeowner earns a credit. That credit can be used against future electric bills. When the grid is out, this system is (generally) not able to power loads in the home, though, some inverters (such as the Pika Energy Island and the SMA TL Series with its ‘Secure Power Supply’) may be able to power some loads directly off the inverter if the sun is out.
An off-grid solar electric array has no connection with the utility grid. The system’s energy storage is limited to what is contained in the batteries. When there is no sun, the system must either run from the stored energy in the batteries, or run on an alternate power source such as a generator. Obviously, this kind of system is totally independent of the grid and grid outages are meaningless to off-grid systems. However, due to the expense of batteries, the battery storage of off-grid systems is typically quite limited and many common ‘creature comforts’ of a modern homes are omitted in the design of purpose built off-grid dwellings.
A grid-tied solar electric array with battery backup is the best of both worlds: when the grid is up, it has the advantage of buying/selling power from the grid, at rates far superior than the operating cost of a generator. When the grid goes down, it automatically fires up reserve storage in the battery bank, letting you run critical household loads off the battery bank and recharging the battery bank with solar panels.
Note that we say critical loads – unless project budget is unlimited, most responsible solar energy installers will not design a solar PV battery backup system to run an entire household’s electricity needs in an emergency situation. The power demands of a conventional home can be extreme, and while they may make sense under conventional circumstances, when the grid is out, it makes a lot of sense to conserve battery storage for critical loads, such as heating equipment, well pumps, refrigerators, freezers, etc. You can even count many electronics (laptops and cell phones) as ‘critical loads’ since their energy use is fairly small.
This chart gives you the rough idea of how long a 5 kilowatt-hour battery bank could sustain different use cases:
|Appliance||Wattage (estimate)||Runtime (5kWhr usable storage)|
|Refrigerator||1,200 (start) ; 200 (running)||24 hrs|
|Well pump||3,000||1.5 hrs|
|Oil boiler||800||6 hrs|
|Oil furnace||1,000||5 hrs|
|18kBtu Heat pump||2,000||2.5 hrs|
|40″ EnergyStar LCD TV||82||60 hrs|
|100 watt incandescent lightbulb||100||50 hrs|
|(4) 25 watt CFL lightbulbs||100||50 hrs|
|Typical coffee pot||1,000||5 hrs|
Note: This all assumes 5kWh of USABLE storage, which is not the same as total storage, due to a concept called ‘depth of discharge’ (what % of a battery’s total reserves should you deplete before recharging it). The conventional wisdom has been to cycle lead acid batteries to not more than 20% of their total capacity; other chemistries can be discharged further. Update this chart accordingly based on your expected load use and runtime. Actually, for anyone seriously considering battery backup we suggest making a spreadsheet using these inputs (wattage of appliances * runtime) to determine the realistic battery size that will meet your needs.
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 an electric rate below the utility rate 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 an IRR that is competitive with the stock market (and far safer). Further, people are leveraging the competitive cost per kilowatt-hour of solar electricity to then make additional investments such as efficient space heating and electric car charging.
It may be that resiliency and grid independence are important to you, moreso than strict economic ROI. Or you may have a home that is 100% electric and solar powered and loathe the idea of putting in a fossil fuel generator. If so, adding battery backup to a grid-tied solar electric array may be a great fit for you. As of 2015, nearly all solar arrays installed by ReVision Energy includes a grid-tied inverter that will be future compatible with battery backup, so you don’t necessarily need to make the decision right away. Of particular interest is the Pika Energy Island clean backup solution made by Maine-based Pika Energy.
Installed cost can vary quite dramatically based on equipment chosen and size (and type) of battery, but a minimum ballpark cost of $10,000 over and above the cost of a straight grid-tied PV array is a good starting point for a battery backup system.
Feel free to contact ReVision Energy to discuss your home’s energy requirements and to see whether a clean energy backup system is right for you. You might also like to read the rest of our series on batteries and solar: