How Many Batteries Do I Need?

Be wary of marketing statements about how many days of power the battery offers, as that is entirely dependent on how much energy your home requires, so it can vary widely. The key to sizing is to understand how much power you will be using/drawing from the battery. Batteries are seen by some homeowners to get off the grid, but that’s an unlikely scenario, at least with lithium batteries. These types of batteries typically only hold enough energy to power an average American home for hours, not days. The amount of battery storage required is based on your home’s energy usage. Energy usage is measured in kilowatt-hours over a period of time. As an example, a home requiring 1,000 watts for 10 hours per day = 10 kWh per day. You also need to consider the battery’s performance and how much continuous output you require. It’s essential to consult with a solar energy professional to size the panels and batteries for your needs properly.

 

It would take several lithium-ion batteries to power a home for a day or more. Lead-acid batteries, which have been around for decades, are less efficient, offer less storage, are often larger, and do not last as long. Lithium batteries can also be left at a partial charge without any adverse effects. It’s difficult to escape a utility bill anytime soon unless you have the financial wherewithal to invest heavily in solar panels and batteries—combined with an extremely airtight, energy-efficient home (like this net-zero energy remodeled Victorian home in Minneapolis).

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Time of Use Home Batteries

Having stored energy could provide useful in other applications. A handful of utilities have time-of-use (TOU) rates that make customers pay more for electricity during high demand times. With a home battery, a homeowner could simply switch to stored energy during high demand periods to reduce the amount of electricity they buy from their utility, and reduce the average cost per kilowatt-hour (kWh) they are charged. If you need to recharge their electric vehicle (EV) during that same high demand time, you could use stored electricity rather than depend on expensive utility power.

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But outside of California, TOU rates are not all that common, although many utilities are testing TOU programs of their own. Currently, most utilities apply “net metering” rates to solar households, offering to buy their electricity at a per kilowatt charge equal to what they charge for power. For example, let’s say you sell electricity for 11 cents a kilowatt-hour (kWh) to your local utility. It’s likely in many states that the utility charges you the same rate when it sells you electricity. In this case, which is a more typical net metering situation, electricity storage offers no financial advantages.

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How Home Batteries Help With Avoided Costs

The good news is that utilities are evolving their pricing mechanisms. A handful of states have begun to employ “avoided costs” to pay for electricity produced by residential solar installations, among them Nevada and Hawaii. Avoided costs reimbursements are much less, as little as 4 cents kWh. At that rate, using the power you stored will save money, making storage a more desirable option. Additionally, utilities have begun marketing programs offering cheaper overnight electricity rates for EV owners. But for some EV owners looking to have the flexibility to charge at high demand times, a lithium battery can serve as a less expensive option than buying energy from their utility at, say, 6 p.m.

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Which Home Battery Is Right for Me?

While battery technology is still in its infancy, a breakthrough came with lithium-ion batteries. These batteries–the same kind found in cell phones and many other devices–capture energy from solar panels as direct current (DC) and convert it through an inverter to alternative current (AC), the kind used in American homes.

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Several flavors of battery storage exist: DC-coupled systems, AC coupled systems, AC battery systems, and hybrid converter systems. Considering the complexity of the options, ask your battery installer which system will work best with your solar array and the infrastructure of your local utility. Generally, AC battery systems such as the Tesla Powerwall 2 and the Enphase Battery are popular with homeowners who have no desire to live off the grid. The AC coupled systems offer affordability and ease of installation. The more sophisticated hybrid converter system, such as LG Chem RESU, allows for high voltage lithium batteries.

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Batteries can be sized to serve the size of your home and your solar arrays. A larger home may need a larger battery. Again, consult a clean energy expert and electrician to ensure the sizing works and the system will meet your expectations.