Grid & off-grid

Grid tie with battery backup comparison

Adding a battery to a grid-tied system means power during outages, without the huge cost of going fully off-grid. Enter your exact equipment choices to see added cost and real outage coverage.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your system
Use your quoted or planned system size.
Common residential backup batteries range from 5 to 20 kWh. Enter rated capacity, not usable.
Depth of discharge and cost per kWh vary significantly by chemistry.
Some energy is lost converting DC battery power to AC household power and back.
3 Backup needs during an outage
Fridge, some lights, and a few outlets typically run 5 to 10 kWh a day. Skip air conditioning and electric heat from this figure unless included below.
Major HVAC loads dramatically shorten how long a battery lasts during an outage.
Most grid-tied backup batteries are sized for 1 to 2 days, not extended off-grid autonomy.
Informational only, helps frame whether a bigger battery is worth the extra cost.
4 Cost assumptions Optional overrides
National average is roughly $2.50 to $3.50 per watt before incentives.
Covers mounting, wiring, and configuring a critical loads panel for the battery.

How it works

Three steps to your result

Step 1

Enter your system size and exact battery details

Use your quoted or planned solar size, plus the battery capacity, chemistry, and inverter efficiency you're considering. Common backup batteries range from 5 to 20 kWh.

Step 2

Estimate your critical load precisely

Add up what you'd want running during an outage, such as a fridge, some lighting, and a router. Add air conditioning or electric heat separately if you want that covered too, since it changes runtime dramatically.

Step 3

Check the coverage against your goal

The result shows how many hours your critical loads would run, and whether that meets your chosen goal. If it falls short, try a larger battery size or compare against a full off-grid system if outages are frequent in your area.

What a backup battery buys you

The middle ground between grid-tied and off-grid

A standard grid-tied system without a battery shuts off during a grid outage, even in full sun, because utility safety rules require it, as California's utilities regulator explains for planned outages, since a system still pushing power onto a de-energized line puts repair crews at risk. Adding a battery lets your system keep a chosen set of circuits powered when the grid goes down, without the cost of the multi-day autonomy a fully off-grid system needs. This makes it a practical option for anyone dealing with occasional outages rather than no grid access at all.

Why chemistry and inverter efficiency both matter for runtime

Two batteries with the same rated kWh capacity can deliver very different real-world runtime depending on their depth of discharge and the inverter's round-trip efficiency converting stored DC power to usable AC power. A lithium battery with 95% usable capacity and a 94% efficient inverter delivers meaningfully more usable energy than a lead-acid battery at 50% usable capacity, even before accounting for the lower per-kWh price of lead-acid. The inverter efficiency loss calculator breaks down where that round-trip loss goes if you want to see it isolated from the chemistry choice. The grid tie vs off grid comparison tool uses this same chemistry logic at full off-grid scale, where the gap becomes even more significant.

Why sizing for critical loads, not your whole house, matters

Backing up an entire home, including air conditioning, electric water heating, or an electric range, requires a battery bank closer to off-grid scale. Most backup installations instead route a handful of circuits, like the fridge, some lights, and a home office, through a dedicated critical loads panel, which keeps battery costs reasonable while covering what matters most during a short outage, and the critical loads only sizing calculator can help pin down that circuit list before you settle on a battery size here. The grid interconnection cost estimator can help you budget for the additional equipment a battery add-on typically requires.

When full off-grid makes more sense than a backup battery

If outages in your area are frequent, extended, or you have no reliable grid access at all, a backup battery sized for a day or two may not be enough, since California's utilities regulator notes that a single-battery system typically covers priority appliances for only 24 to 48 hours during a planned outage. The U.S. Department of Energy's homeowner's guide to solar frames storage as a standard add-on for exactly this kind of situation, not a niche upgrade. The grid tie vs off grid comparison tool lays out the full cost difference between a modest backup battery and true off-grid autonomy.

Questions

Frequently asked

A residential backup battery typically adds $5,000 to $20,000 depending on capacity and chemistry, on top of the solar system cost. Lithium storage runs roughly $900 to $1,050 per kWh installed, so a 10 kWh lithium battery adds around $9,500 including labor.

It depends on your critical load, battery chemistry, and inverter efficiency. A 10 kWh lithium battery running an 8 kWh per day critical load, such as a fridge, some lights, and a few outlets, covers roughly a day. Running air conditioning or electric heat off battery power shortens that significantly.

Yes. A lithium battery with 95% usable capacity delivers nearly double the runtime of a lead-acid battery with the same rated kWh but only 50% usable capacity. Factoring in inverter round-trip efficiency losses on top of that gap matters for an accurate outage coverage estimate.

No. A grid-tied system with battery backup still relies on the grid for normal operation and net metering credit, using the battery only during outages. Full off-grid systems never connect to the grid at all. The grid tie vs off grid comparison tool shows how much more a true off-grid system costs.

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