The Myth of Whole-Home Battery Backup
Battery backup systems coupled with solar are being hailed as the best solution to public safety power shutoffs in California — not to mention our archaic electric grid.
Not only are these systems ideal for powering a home when the power goes out, but they also help reduce electricity costs and provide grid support services when needed by the local utility. For emissions and cost reasons, conventional gas or diesel generators are not an option.
So there is no surprise that demand for these systems is outstripping equipment supply as well as availability of qualified installation labor.
Limits to whole-home battery backup
But there's a catch to this. We like to believe the myth of whole-home backup or the notion that our 21st century lifestyle will continue unabated despite fire hell or high water. The reality is different: Typical battery backup systems work best when they are designed to ration battery capacity and minimize the use of major appliances.
Myths often have origins in fact: Whole-house battery systems do indeed work for off-grid applications. There are an estimated 180,000 such homes in the U.S.
But these homes were designed for off-grid living: they are typically smaller and well insulated; use combustion heating with propane backup; incorporate active and passive solar thermal systems; and do not have power-hungry air conditioning systems, Level 2 EV chargers or swimming pools.
There are two fundamental engineering limits that make it impractical to run a whole house on battery power alone. First, the energy capacity of typical lithium-ion battery systems is insufficient to power an entire house through a nighttime blackout. Second, battery backup inverters are not powerful enough to start and run many large appliances.
Of course, multiple batteries and inverters can address these energy and power limitations. But the cost of 20+ kilowatts of inverters and 40+ kilowatt-hours of batteries is prohibitive for the typical homeowner.
A more practical approach is to design a battery backup system to power critical loads only: no large appliances such as air conditioning, 240-volt EV chargers or electric stoves. Instead, just four to eight smaller circuits in the house for refrigeration, lighting, entertainment, communications and convenience outlets.
Our current housing stock uses a lot of electricity, and because of a plethora of plugged-in devices, newer homes often use even more.
High-power-use appliances are most challenging for whole-home backup systems. Power consumption for a large central air conditioner is 5,000 watts, an EV charger is 7,000 watts, an electric stove is 10,000 watts and pool pumps are 2,200 watts.
Battery energy limits
So how long does a typical solar and battery system operate at night while operating these larger appliances? Answer: not very long at all.
The math is simple. If the battery is down to an energy capacity of 2.5 kilowatt-hours at night (typical if the battery is used during the evening to maximize self-consumption savings), there is only enough battery energy to run pool pumps for 60 minutes, a central AC for 30 minutes, an EV charger for 20 minutes or an electric stove for 15 minutes.
With any of these appliances running — after only a relatively brief interval of automatic whole-home backup — the battery will be soon dead and unable to power critical loads. In lyrical terms: No lights. No phone. No electric car. Not a single luxury. Like Robinson Crusoe, as primitive as can be.
One possible solution is to manually shut off large appliance loads during a blackout. Unfortunately, many blackouts occur during the day when no one is home or at night when people are asleep. Customers who have tried to manually shed loads usually end up being disappointed with their backup system.
Another solution (if a homeowner’s budget and wall space allows) is to add a second storage battery — effectively doubling the energy storage duration.
Over the past few months, we have worked with customers who have had a range of good and bad battery backup experiences. During the first blackout in our area, which happened at about 10:30 p.m., one customer who uses a continuous positive airway pressure (CPAP) machine depleted his storage battery at about 2 a.m. (he started snoring and his wife told him to sleep on the couch). Another customer used the backup system to power one of the subpanels in his home, and he did not realize there was a power failure until the battery was depleted.
The solution for both customers was to remove a few discretionary circuits from their backup subpanels so the battery would last through the night.
Inverter power limits
The battery inverter’s maximum power output (in kilowatts) is the second reason for the whole-home backup myth.
Most battery backup inverters were designed for 200-amp home electric services, implying a maximum AC output of 7,600 watts when grid-connected. When powered from the battery (which has a limited peak discharge rate), these inverters can typically provide 5,000 watts of steady-state power or 6,000 watts of peak power (about 25 amps).
However, the momentary startup surge current requirements of an AC or pump motor is often two or three times the normal current draw — meaning that the inverter simply will not switch over to backup mode. Even if the battery is fully charged on a sunny day, the AC and pool pump will not start, and none of the critical loads will get power.
Designing solar-coupled battery backup systems
Regardless of these energy, power and financial limitations, a well-designed solar and backup system can provide power almost indefinitely. Three design elements are critical.
First, the battery’s energy capacity (kilowatt-hours) and inverter output (kilowatts) should be matched to the needs of the home at night when the battery is partially discharged. Second, the number of backup circuits should be strictly limited to prevent powering too many small devices or any large appliances. Third, the size of the solar system should be sufficient to partially recharge the battery even on a cloudy winter day.
Upcoming smart home electric system technologies will address these practical limitations by automatically shedding loads during a blackout. At the 2019 Solar Power International show, companies exhibited smart appliance controls and circuit breakers that could automatically disable large appliances. Smart electric panel technology was also on display that could automatically manage all the circuits in a house.
By the end of 2019, there will be over 10,000 homes and businesses in California equipped with combined solar and battery backup systems. As these systems become less expensive (both through equipment cost reductions and incentives), they will become the most expedient and effective way for people to adjust to the new normal of public safety power shutoffs.
Not to mention the cleanest, safest and economical way to rebuild our archaic electric grid.