The ultimate solution is a battery backup system with a home generator. But first, some general information…
Solar and Anti-Islanding
If you have a solar system that is connected to the grid (grid-tied), by code it is required to have a feature called “anti-islanding.” The inverters in your solar system are always monitoring the power from the grid to be within a certain voltage and frequency range. If the grid goes out of this range (including shuts down), then the inverters automatically shut down to prevent exporting power which can cause brown outs and create safety issues for utility repair persons. Thus, your typical solar system does not work during a power outage.
In order for any alternate power system to provide power to your home through your homes electrical system requires the use of a transfer switch. This physically disconnects your electrical system at the transfer switches point of connection from PG&E and enables you to provide power from an alternate source like battery back-up or a generator. These can be manual or automatic.
Most battery backup systems and home backup power generator units use an automatic transfer switch that is designed to work with their product. There is generally a delay of at least a few seconds to make this transition. For this reason NONE OF THE SYSTEMS BELOW SHOULD BE CONSIDERED FOR POWERING CRITICAL MEDICAL DEVICES WITHOUT ADDITIONAL EQUIPMENT (Not discussed here).
Note, in most cases, adding a transfer switch to an existing solar system will not enable the solar to work unless there is a battery back-up system. Enphase IQ8 microinverters, when coupled with the Enphase Smart Transfer Switch, do allow for off-grid operation during sunlight hours. This is known as “sunlight back-up” and does not require batteries. Output is limited to available power from the solar system.
Battery Backup (Energy Storage System)
There are many manufacturers of Energy Storage Systems (ESS) that are capable of providing battery backup power when the grid is down. These systems can also provide power while the grid is operating. One capability is to provide power from storage during peak rates. However, the focus here is on battery back-up and at this time, installing storage is not a financially compelling solution if your primary goal is to save on your homes utility bills (at least in PG&E territory).
Today there are many very good solutions that are available. However, there is also extensive research and investment being poured into the next generation battery technologies. It’s very possible that in the next 5 years or so, we could see battery technologies emerge that are significantly better than today’s solutions.
Lithium-Ion batteries are the most common batteries used in Home ESS’s. There are two major chemistries used in these batteries: Nickel Manganese Cobalt (NMC) and Iron or Ferro Phosphate (LFP).
NMC batteries are common in Electric Vehicles due to their high energy density to weight ratios.
However, their cycle life performance is not as good as LFP and they can suffer from thermal runaway–a situation where the failure of one cell in the pack can cause a cascading failure in the battery pack.
There are many safety measures to prevent thermal runaway, so the odds of this occurring are very low, but the result can be catastrophic. LFP batteries are heavier than NMC, but have better cycle life performance and are considered inherently safer than NMC batteries. LFP based systems tend to be more expensive than NMC based systems.
For additional information, see our energy storage systems page
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Energy Storage Systems: Two Factors to Consider
There are two major factors to consider when selecting an ESS for battery back-up: Peak power output and Energy stored. The peak power determines how much power the system can support. Most solutions run between 4 kW and 8 kW of power or approximately 15 amps to 30 amps at 240VAC.
Systems can also provide higher peak outputs for short periods of times (generally less than 10 seconds) to help start items with high initial start-up loads like motors. This varies significantly among the various manufacturers.
The energy storage of most standard or common sized systems runs between 10 and 15 kWhrs. Smaller and Larger systems are available, but this is generally the sweet spot for storage.
If your home is using 1kW of power and has a 10kWhr (usable) battery, then the system will provide power for 10 hours before it needs to be recharged. This is generally enough to support a refrigerator, some LED lighting, charge phones and computers during the night.
However, if you decide to turn on a high demand appliance like air conditioning, you can use up the battery in less than a few hours.
When paired with a solar system, the ESS can be recharged during the day. However, the rate at which the battery can be recharged depends on the home loads during the day and the size of the solar system.
If power goes out during a rainy day, late in the year with less solar access, or a smoky day, the solar system’s ability to recharge the battery will be greatly diminished. Many ESS’s do have inputs for generators as a back-up means to charge the battery when solar is not sufficient.
The main advantage of generators relative to battery back-up, is that generators can provide a lot more power, can cost significantly less, and are not impacted by weather. The main disadvantages are that they don’t enable your solar system to operate, are loud and require fuel.
The typical home backup power generator typically produces between 12 and 20 kW of power with options ranging from 5 to 60kW. 20 kW is typically more than enough for most homes.
Larger systems may make sense for estates or homes with high demands. For reference, most homes have 100A to 200A main service panels which means that PG&E provides from 24 to 48 kW of power to your home.
It’s rare to ever demand this much power at once. In addition, during a power shutdown, its good practice to reduce your demands and not operate “business as usual”. Note that once generators are over 20kW, prices tend to jump significantly as go from standard air cooled to water cooled generators.
The noise levels vary significantly across the type of backup power generator, but most will eventually annoy your neighbors. When you can, turn them off at night.
Most home backup power generators are air cooled and are placed in a sound dampening enclosure. Noise levels for these are noticeable, but much less than the typical portable generator you can find in the big box store.
Water cooled generators are a lot quieter than air cooled generators, but installed costs are almost 2X. They are also much more durable. The quietest generators are the small portable Honda generators.
Generators Require Fuel to Operate
Generators can run on natural gas, propane, gasoline, and diesel. Most home back-up generators are designed to run on natural gas or propane and are permanently connected to these sources. Most portable generators run on gasoline, but some have options to run on gasoline and natural gas and/or propane. Note that while up-front costs for generators are low compared to battery back-up, the operating costs can add up fast.
Fuel isn’t cheap, especially when you are using it 24 hrs/day for several days in a row. Also consider that access to fuel can be an issue. Gas stations runs out of gasoline and diesel during prolonged outages. Natural gas supply can be cut off as a safety measure as has happened in several cities during planned shut downs.
Natural gas lines can also be disrupted in a major disaster like an earthquake. Propane is good, but eventually your tank will need to be refilled and there is no guarantee your supplier will be able to do so during a disaster.
The size of your generator will also have a big impact on your fuel usage. A 12 kW generator at low load uses a lot less fuel than a 24 kW generator at low load, and most of the time your generator will be running at low load. So sizing for the extra capacity will cost you more in operating costs as well.
The quality of your backup power generator also generally determines the quality of the power output. $500 generators from the big box stores tend to produce fairly “dirty” power. The output is not the same as from the utilities. The better the generator, the better the power matches the output from the utility.
If you have sensitive electronics, pay attention to the power output. Look for low THD (total harmonic distortion), pure sine wave or inverter based generators. Note: if you have solar and you install a home generator, make sure the system is designed so the solar does not turn on as it can damage the generator and/or the solar system.
If you have a portable generator NEVER EVER OPERATE IT INDOORS OR IN A GARAGE. MAKE SURE THE EXHAUST CAN NOT GET IN A WINDOW OR DOOR. Carbon Monoxide is an odorless gas that can kill you without warning!!!
If you see someone doing this, alert them to the dangers of doing so immediately. There are lots of people who run out and buy generators with each planned power shut down that are not aware of these dangers.
There are also portable battery generators! These are generators that use batteries for power. They can also come with small portable solar arrays to charge the batteries. These generally don’t produce much more than 1000 watts and are pricey compared to portable generators, but they are quiet and don’t require fuel. Two good companies for this are Goal Zero and Simplifi. Both can be found on Amazon.
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There are clearly lots of choices and no one size fits all approach. If you are looking for the least expensive solution to provide power for the basics (a refrigerator, some lights, charge phones/ computers) we recommend getting a small Honda portable generator and some extension cords. This is a very quiet and cost effective solution. Do this well before you need it, so you are not racing to the store to get the last remaining generator.
If you are interested in getting a quote for a battery back-up system and/or a home generator, give us a call so we can determine the best solution for you. We install solar systems, ESS (battery back-up systems) and home generators.
Enphase Whole Home Back-Up Example (with optional generator)
Hopefully, this article will help if you’re considering energy storage systems or home backup power solutions.
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