Whole home backup is a practical option if you want every circuit in your house, not just a handful of essentials, to keep running automatically during an outage, but it comes at a real premium over essentials-only or generator setups. Expect quiet, automatic switching and no emissions at the point of use, offset by a higher installed cost and a sizing process that takes real planning. The rest of this guide walks through how to estimate the size you need, what a fair proposal looks like, and how to vet an installer.
TL;DR:
- Whole home backup systems typically require 20 to 40 kWh of capacity, depending on climate, home size, and included loads like HVAC or well pumps.
- Proper sizing must consider both daily energy use in kWh and continuous power in kilowatts to handle surge loads without faults or delays.
- Utility interconnection rules and local permits can limit system operation and affect how much reserve power your home can actually access during an outage.
- Larger systems usually cost less per kWh than smaller ones, but the total installed price depends heavily on inverter ratings, capacity, and electrical upgrades.
- The most important step before purchasing is obtaining a detailed load calculation and load assumptions to avoid oversizing or underperforming your backup system.
Table of Contents
- What whole home backup means and when you actually need it
- How to size a whole home backup system step by step
- System components and common configurations
- Costs, incentives, and what a battery saves over its lifetime
- Permitting, interconnection, and safety you need to plan for
- How to choose an installer without getting burned
- Design tradeoffs worth understanding before you buy
- What installers see homeowners get wrong most often
- How Indigo Energy can help you get started
- Sources
- FAQ
What whole home backup means and when you actually need it
Whole home backup means every circuit in the house, HVAC, water heater, kitchen appliances, and outlets, stays powered during a grid outage without you flipping a single switch. Essentials-only backup, by contrast, covers a defined subset: refrigerator, some lighting, medical equipment, maybe a well pump, wired into a dedicated subpanel. The batteries and inverters can be identical between the two; what changes is how much capacity you buy and how the panel is wired.
The decision usually comes down to how much of your home’s normal operation you can live without for a day or more. A battery storage system switches over automatically within a fraction of a second when the grid drops, which matters for anyone running medical devices, sump pumps, or home offices that cannot tolerate a gap in power.
Whole home backup tends to make sense when:
- Someone in the household depends on medical equipment, refrigerated medication, or a powered mobility device.
- Your region sees outages that regularly stretch past a day, and losing heating or air conditioning is not tolerable.
- You work from home and need internet, computers, and lighting to stay on without interruption.
- You want to eliminate a portable or standby generator entirely, including its fuel storage and maintenance.
If your outages are short and rare, or your budget is tight, essentials-only coverage or a generator may get you through with less spent upfront. Whole home backup is the choice for households that want the outage to be invisible rather than merely survivable.
How to size a whole home backup system step by step
Two numbers drive every backup proposal: kilowatt-hours (kWh), which measure total energy stored, and kilowatts (kW), which measure how much power the system can deliver at any instant. A battery rated at 10 kWh with a 5 kW continuous inverter can run a 5,000-watt load for two hours, or a 1,000-watt load for ten hours. Both numbers matter because a battery can have plenty of stored energy but still trip if you ask it to start a large motor it cannot supply.
Sizing a system correctly follows a repeatable process:
- List your critical and desired loads in watts, including refrigerator, furnace blower, well pump, garage door opener, and any medical equipment.
- Estimate hours of use per day for each load; a refrigerator might cycle for 8 hours of actual compressor run time, while lighting might run for 5.
- Multiply watts by hours for each load, then sum them to get your daily kWh requirement.
- Decide your target outage duration, whether that is one evening or three full days, and multiply your daily kWh figure accordingly.
- Add a buffer for depth-of-discharge and inverter losses, since most lithium batteries are not run to 100% depletion and inverters lose some power to heat during conversion.
As a rough guide, essentials-only setups commonly land in the 10 to 20 kWh range, while whole-home coverage often calls for 20 to 40 kWh or more, depending on climate, home size, and whether HVAC and well pumps are included. The spread exists because a small home in a mild climate with gas heat needs far less than a large home in a hot climate running central air continuously.
Surge power deserves separate attention. Motors in air conditioners, well pumps, and garage doors draw two to three times their running wattage for a second or two at startup, and an undersized inverter will fault instead of starting the load. A good installer accounts for this surge headroom rather than sizing only to continuous wattage. During an extended outage, many households also shed high-draw loads on purpose, delaying laundry or EV charging, to stretch stored energy further rather than buying enough capacity to run everything simultaneously.
Pro Tip: Ask your installer about modular, stackable battery units. Starting with one unit and adding another later, rather than buying maximum capacity upfront, can help you avoid a costly main panel upgrade if your usage grows.
System components and common configurations
A whole home backup system is built from a handful of core parts, and understanding them helps you read a proposal instead of just trusting the total price. The battery itself is almost always lithium-ion today, chosen for its energy density and cycle life over older lead-acid chemistries, though it requires a battery management system to monitor temperature and charge state and to prevent overcharging.
The inverter and charger convert stored DC power into the AC power your home’s outlets and appliances use, and it works in reverse to charge the battery from solar or the grid. How that inverter connects to solar panels determines whether the system is AC-coupled or DC-coupled:
- AC-coupled systems connect the battery inverter to the same AC bus as an existing solar inverter, which simplifies retrofits onto homes that already have solar.
- DC-coupled systems route solar power directly into the battery before conversion, which can reduce conversion losses but usually requires the battery and solar equipment to be installed together.
- Hybrid inverters combine solar and battery inverter functions into one box, common in new installations designed from the ground up for storage.
- Automatic transfer switches detect the grid outage and isolate your home’s wiring from the utility line, which is what allows the battery to power circuits safely without back-feeding the grid.
Some homeowners pair batteries with a backup generator for very long outages, using the battery for the instant, quiet switchover and the generator to recharge the battery or carry extended loads once fuel is available. This hybrid approach adds complexity and cost but extends runtime well past what battery capacity alone provides.
Site conditions matter more than many homeowners expect. Batteries are commonly installed in garages, utility closets, or on exterior walls, and each location has ventilation and ambient temperature requirements that affect performance and lifespan. Extreme heat degrades lithium batteries faster, so an installer should specify a location that keeps the unit within its rated temperature range rather than tucking it wherever is convenient.
Costs, incentives, and what a battery saves over its lifetime
Installed cost for battery storage depends heavily on capacity, with manufactured battery costs and total capital expenditure both contributing to the final number a homeowner sees on a quote. The EAC Biennial Energy Storage Review cites manufactured battery costs around a benchmark manufactured cost per kilowatt-hour around this level as a benchmark figure, though the installed price a homeowner pays includes inverters, labor, permitting, and electrical work on top of that raw battery cost.
A benchmark manufactured cost per kilowatt-hour around this level, cited in the EAC Biennial Energy Storage Review, reflects the battery cell cost alone and is a useful reference point when comparing how much of an installed quote is hardware versus labor and equipment.
When comparing proposals, expect line items covering the battery units, one or more inverters, an automatic transfer switch, electrical permitting and labor, and any panel upgrades needed to accommodate the new equipment. Larger systems generally cost less per kWh than smaller ones, since fixed costs like permitting and labor spread across more capacity.
A few factors shape your net cost:
- Battery capacity and inverter power rating, since larger continuous and surge ratings cost more.
- Whether the battery pairs with new or existing solar, which changes wiring complexity and potential incentive eligibility.
- Panel and electrical upgrades, needed if your existing service cannot support the new equipment without modification.
- Available tax credits and local incentives, which can apply when batteries are paired with solar and should be confirmed with a tax professional or local utility program, since eligibility rules vary by location and change over time.
Compared with a standby generator, a battery system has no fuel to store or replenish, no combustion engine to maintain, and produces no emissions or noise while running. A generator typically costs less upfront and can run indefinitely with fuel resupply, but it requires regular maintenance, periodic exercise cycles, and fuel logistics that a battery does not.
Permitting, interconnection, and safety you need to plan for
Battery installations are not a plug-and-play project. Most jurisdictions require an electrical permit, and many require a separate fire or building inspection depending on where the battery is installed and its capacity. Your installer typically files these permits and schedules the inspections, but you should know they are part of the timeline and not an optional step.
Interconnection rules with your utility can also limit how the system behaves. Depending on your utility’s requirements, a battery tied to solar may face constraints on when it can export power to the grid or participate in demand response programs, and these rules can affect how much reserve capacity is actually available to you during a real outage. Ask your installer directly whether your system is designed to prioritize a full reserve for outages or to also participate in grid services, since a battery configured for both purposes may not deliver the runtime you expect.
Safety review matters just as much as capacity. Ask about the battery’s UL certification, specifically UL 9540 for the overall energy storage system and UL 9540A for fire propagation testing, since these standards govern how a battery is expected to behave under fault conditions. Confirm ventilation requirements and clearances for the installation location as well.
- Electrical permits and inspections are standard for any battery installation regardless of size.
- Interconnection agreements with your utility can limit export and affect how much capacity is reserved for outages.
- UL 9540 and UL 9540A certifications are worth confirming directly with your installer before signing.
- A realistic timeline from signed contract to full commissioning commonly spans several weeks, driven mostly by permitting and utility approval rather than the physical installation itself.
How to choose an installer without getting burned
Picking the right installer matters as much as picking the right hardware, since even a well-sized system underperforms if it is poorly installed or unsupported afterward. Start with credentials: confirm the installer is licensed and insured for electrical work in your area, and ask which battery and inverter manufacturers they are authorized to install, since manufacturer partnerships often affect warranty validity.
A short list of questions separates a serious proposal from a rushed one:
- What is my expected runtime at my estimated daily kWh usage, and how was that number calculated?
- What are the continuous and surge kW ratings of the inverter, and do they cover my largest motor-start loads?
- Can the system expand later with additional battery units, or am I locked into today’s capacity?
- What is your experience with utility interconnection in my area, and how long did recent projects take from permit to commissioning?
- What does the warranty cover, and who handles service calls if something fails after installation?
Red flags are usually easy to spot once you know to look for them: high-pressure pricing that expires the same day, a verbal estimate with no written load calculation behind it, or a contractor who cannot name the UL certifications on the equipment they are proposing. A trustworthy proposal in writing includes your load assumptions, a single-line electrical diagram showing how the battery connects to your panel, the specific equipment model numbers, and the warranty terms spelled out rather than referenced vaguely.
Pro Tip: Get at least two written proposals with load calculations before signing anything. A rushed verbal quote with no diagram is a sign the sizing was guessed, not calculated.
Design tradeoffs worth understanding before you buy
Homeowners pairing batteries with solar face a design choice that most sales conversations skip: how much inverter and battery power to buy relative to the solar array’s size. Research from the Department of Energy on PV-plus-battery configurations examines ratios like inverter loading ratio and battery-inverter ratio, finding that as the market value of exported solar power declines over time, buying more battery power capacity relative to the inverter becomes more economically attractive.
Higher battery power capacity relative to inverter size may become more economically attractive over time as the market value of exported PV generation declines.
For a homeowner, the practical translation is this: ask your designer whether they are sizing your battery and inverter for today’s conditions only, or with room to add capacity as grid rules and incentive structures shift. A system with a hybrid inverter that has spare capacity, paired with a stackable battery, gives you a path to expand without replacing core equipment. This is the same DC-coupled versus AC-coupled question raised earlier, and it is worth revisiting directly with your designer once you know your target outage duration and load list. A project-specific ROI analysis can be helpful, which is the kind of exercise these ratios are meant to inform rather than a guarantee of any particular return.
What installers see homeowners get wrong most often
The most common mistake is sizing the battery before addressing efficiency: a homeowner who replaces old lighting and an aging water heater often needs meaningfully less battery capacity than one who does not. The second is assuming interconnection rules will not affect their reserve, when in some cases they do. Prioritize efficiency first, then size the battery to what remains, and set outage-duration expectations before you shop rather than after.
— Curtis Williamson
How Indigo Energy can help you get started
If you are ready to move from research to a real quote, full-service solar contractors handle the full process for homeowners: system design, permitting, installation, and ongoing monitoring, plus a project-specific ROI analysis so you can see the financial tradeoffs before committing. Our Solar + Battery Storage Integration service is built around exactly the sizing and interconnection questions covered above, and our team walks through your load list and panel setup during the site assessment rather than guessing from a rooftop photo.
Before your assessment, have a recent electric bill on hand along with a rough list of the circuits or appliances you consider critical: this gives our designers a starting point for the same kWh and kW calculations outlined earlier in this guide. From there, we build a proposal that includes your load assumptions, equipment specifics, and warranty terms in writing.
- The installer typically handles permitting and utility interconnection paperwork directly, so you are not navigating it alone.
- An ROI analysis shows the financial tradeoffs of your specific system before you sign anything.
- System maintenance and monitoring can help keep your battery and inverter performing after installation, not just on install day.
Ready to see what a whole home backup system would look like for your house? Request a quote through our battery storage page and our team will start with the same load assessment described in this guide.
Sources
The technical claims in this guide draw on the Department of Energy’s PV-plus-battery configuration research, the EAC Biennial Energy Storage Review for cost benchmarks and resilience recommendations, ENERGY STAR’s heat pump water heater technical guide for outage behavior of common appliances, and RMI’s resilience research on automatic switching performance.
- Battery energy storage systems as a resilience solution — RMI
- Biennial Energy Storage Review — Electricity Advisory Committee (EAC), U.S. Department of Energy
FAQ
Are whole home battery backups worth it?
Whole home backup is worth it for households that need every circuit powered automatically during outages, particularly where medical equipment, HVAC dependence, or long outage history make partial coverage risky. For shorter or rarer outages, essentials-only backup or a generator may deliver similar peace of mind at a lower upfront cost.
How much does a whole home battery backup system cost?
Total installed cost depends on capacity, equipment, and labor, but the EAC Biennial Energy Storage Review cites a manufactured battery cost benchmark of a benchmark manufactured cost per kilowatt-hour around this level, which covers only the battery cell and not the inverter, labor, or permitting that make up the rest of an installed quote. A project-specific ROI analysis during the site assessment helps homeowners see the full cost breakdown before committing.
How much does a whole house UPS cost?
A whole house battery backup functions similarly to an uninterruptible power supply but at residential scale, and its cost follows the same drivers as any battery system: capacity in kWh, inverter power rating, and installation complexity. Getting a written proposal with a load calculation, as described earlier in this guide, is the most reliable way to get an accurate number for your specific home.
How long will a whole house battery backup last?
Runtime depends on your battery’s stored kWh divided by how much power your home draws during the outage, which is why the sizing steps in this guide start with a full load inventory. A household that sheds high-draw items like EV charging or laundry during an outage can stretch the same battery capacity considerably further than one running every appliance normally.