Batteries for the Home

Alan Cowe, President, bysolar

There’s a lot of hype about having an energy storage battery system at home for backup power and to take advantage of utility off-peak rates, or tied to your solar system. Let’s assume you want a battery for your home to provide backup in the event of a power outage, what size of battery will you need, how much backup power will it provide and at what cost.

First assuming you don’t have a solar system then you could use the battery to take advantage of off-peak rates, charging the battery when rates are low then using the stored energy when rates are high. The economic benefit will be dependent upon the difference in rates between off-peak and on-peak rates.

A typical house in the Northeast US consumes about 30 kilowatt-hours/day  (kWh/day), 11,000 kWh/year, and the difference between on-peak and off-peak rates is about 4 cents/kWh. So if you stored a kWh off-peak and used it during the on-peak period you would save 4 cents.

If you used 75% of your electricity during the on-peak period, then saving 22.5 kWh during the off-peak period, and using it all during the on-peak period, you would save $0.9/day or about $328/year. A 30 kWh battery that could save 22.5 kWh, using 75% capacity daily to avoid deep discharging which would be detrimental to the battery, would cost about $10,500 (using Tesla’s $350/kWh pricing).

Batteries are warranted for 10 years, the savings from differential off-peak savings would amount to $3,280 or about 33% of the battery investment. In states such as Hawaii and California, where the peak differential is higher, exceeding $0.14/kWh, then the savings over ten years would cover the investment.

A grid power outage lasting a few minutes can be handled with a battery being used for off-peak savings. However to use your battery for backup in the event of longer power outage, then you need to make sure your battery is fully charged ahead of looming storms or predictable events that may cause a grid outage. With careful conservation of energy you may be able to last a day and a half from battery power. (Don’t get rid of your fossil fuel generator.)

Now let’s assume you have a solar system which provides you with an abundance of energy that offsets your annual electrical consumption.  The solar system will produce more energy than you can use during the summer, and produce a lessor amount during winter. Typically the utility will have a net-metering plan that allows you to supply your excess electricity to the grid for credit, and later use your credit when your solar system is not producing.

Having a battery with a solar system allows you to have a supply of power in the event of a grid outage. Typically the batteries will remain charged ready to supply power in the event of a grid outage. During a power outage, as power is used from the battery it is supplemented next time the sun shines. Works fine during sunny summer days!

During winter the daily amount of solar production will be about 25% less than the home’s consumption.  Also, there may be extended periods of cloudy days or snow and ice forming on the solar panels that will further reduce the solar production. In the event of a utility outage during winter the solar system will not be able to provide sufficient power to maintain the battery charge or supply the home. Energy storage may be increased by adding more batteries providing for more days of autonomy with little or no solar production. Assuming a cost of $10,500 for each day of battery storage then providing for four days of autonomy would require an investment of $42,000. With energy conservation measures this system may provide for six days of battery power without recharging from the solar system.

A battery backup system has the advantage of automatic changeover to battery power during an outage, it is non-polluting and quiet however over an extended period without sun, it will expire (requiring power from a fossil fuel generator).

The cost of batteries has been declining from $600 - $800/ kWh in 2010 to about $400/kWh 2015, to $350/kWh (Tesla 2016). Projections of battery costs of $225/kWh by 2020 -2025 are possible, assuming mass production of batteries for applications such as automobiles and utility scale storage systems.

Future battery costs will depend on innovative technologies. Every week there seems to be another recipe with lithium and other chemistries promising lower costs, denser energy storage, faster recharging, deeper discharging, lower charge/discharge losses, etc. Exotic technologies incorporating a combination of graphene (a lattice of carbon atoms one layer thick) with a lithium compound, offers the potential for lower cost batteries.

Regardless of the technology, battery costs will remain expensive, above $200/kwh for the next decade. To achieve a truly resilient renewable energy resource will require an alternate solution – which will be the subject of another blog.



1 Response

  1. Thanks for the great share! I also like the idea of Home Energy. The best part I like is this: The reliability and availability of modern energy sources cause people to tend to assume that it will always be accessible. And as for the case of non-renewable energy sources, most people do not know or maybe even refuse to accept that it will eventually run out.

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