| Quick Answer: Size it to your evening consumption and your backup priorities, not to your total monthly usage or your square footage. The two numbers that matter are usable capacity in kilowatt-hours, which sets how long it runs, and continuous output in kilowatts, which sets what can run at once. Solari Storage sizes from your actual utility bill and rate plan, which is the only way to get this right for a specific house. |
Battery sizing is where most of the money is won or lost in a home storage project, and it is the part most often decided by a rule of thumb. Undersize it and the battery empties before the expensive hours end, leaving you buying peak power anyway. Oversize it and you have paid for capacity that never gets used, which is a costly mistake in a year with no federal credit to soften it. Here is how sizing actually works, and why your utility bill is the right starting point.
Why Is Your Utility Bill the Right Starting Point?
Because it contains your real consumption rather than an estimate of it. Square footage tells you almost nothing useful. Two identical houses on the same street can differ by a factor of three in evening electricity use depending on whether anyone is home during the day, how the house is heated and cooled, and whether there is an electric vehicle in the driveway.
Your bill also identifies your rate plan, which is what determines when the expensive hours fall and how steep they are. Sizing without the rate plan means sizing without knowing which hours the battery needs to cover. This is why a quote produced from an address and a satellite photo is a guess, and why we ask for a recent bill before we say anything about size.
What Is the Difference Between Capacity and Output?
Capacity, measured in kilowatt-hours, is how much energy the battery holds. It answers how long. Output, measured in kilowatts, is how much power it can deliver at any one moment. It answers how much at once. These are independent, and a battery can be strong on one and weak on the other.
The distinction becomes concrete during an outage. A battery with generous capacity but modest output might run your refrigerator, lights, and internet for two days without trouble, yet be unable to start a central air conditioner or a well pump. If those loads matter to you, output is the constraint to design around, not capacity. Most sizing conversations focus almost entirely on kilowatt-hours and skip this entirely.
How Do You Work Out Your Evening Load?
Start with the hours that matter. In most of California the expensive window runs from around 4 PM to 9 PM, so the question is how much electricity your household consumes across those five hours on a typical day. Many utilities provide hourly or fifteen-minute usage data through their online portals, which turns this from an estimate into a measurement.
If you only have a monthly total, it can be worked down, but the result is coarser. Evening consumption is rarely a neat fraction of the monthly figure, because it is driven by occupancy patterns and by heating and cooling behavior rather than by average use. Seasonality matters too: the summer evening load in an air-conditioned home can be several times the winter figure, and sizing to an annual average will disappoint in August.
Should You Size for Bill Savings or for Backup?
These are different design targets and they should be stated separately rather than blended into one number. Sizing for savings means covering your evening consumption during peak hours, day after day, and then stopping. Additional capacity beyond that point adds cost without adding much saving, because there is nothing left to displace.
Sizing for backup means carrying your essential loads through an outage of a length you choose. That is usually a larger number, particularly if you want to cover multi-day Public Safety Power Shutoffs. Most households want some of both, and the honest approach is to decide what backup duration you actually want rather than accepting a default, then check what that adds over the savings-optimal size.
What Role Does Your Solar Play in Sizing?
A large one, and it works in your favor. If you have solar, the battery is recharged each day for free, so the design question is whether your daily surplus generation is enough to fill it. A battery substantially larger than your typical daily surplus will not fill completely, and the unfilled portion is capacity you bought but cannot use. Our installation process page covers how design follows from the assessment rather than the other way around.
During an outage this same recharging is what changes backup arithmetic fundamentally. A house with solar is not counting down from full to empty over several days; it is cycling, drawing down through the night and refilling through the day. That means a smaller battery can carry a longer outage than a simple division of capacity by load would suggest.
What If Your Needs Will Change in a Few Years?
Then modularity is worth paying attention to at the design stage. Adding an electric vehicle, switching from gas to a heat pump, or converting a garage into occupied space all raise evening consumption significantly, and a battery sized precisely for today’s usage can be undersized within two years. Some platforms handle this better than others. The Lunar System scales in modules rather than whole units, so capacity is matched to your actual consumption instead of rounded up to a fixed block, and it can be expanded later. Other platforms stack whole units, which is a coarser but perfectly workable path.
There is also a timing point specific to 2026. In previous years, oversizing carried less risk because the federal residential tax credit absorbed 30 percent of the extra cost. That credit ended for systems installed after December 31, 2025, so every additional kilowatt-hour is now paid for in full. Getting the size right matters more this year than it did last year.
Frequently Asked Questions
Is there a rule of thumb for battery size based on house size?
There is, and we would encourage you to ignore it. Square footage does not predict evening electricity consumption, which is driven by occupancy, heating and cooling type, and whether an electric vehicle is charging at home. Two identical houses can differ several times over.
Can I add more capacity later if I get it wrong?
Usually yes, though how gracefully depends on the platform. Modular systems expand in smaller increments, while others stack additional whole units. Either way, expanding later generally costs more than sizing correctly at the outset, so it is worth discussing likely changes during design.
How many kilowatt-hours do I need to run my whole house?
That question usually needs reframing, because whole-house backup is limited by output as much as by capacity. Ask instead what you need running simultaneously and for how long. Once those are defined, the specification follows, and it is often smaller than people expect.
Does an electric vehicle change how I should size a battery?
Significantly. Home vehicle charging adds a large and often evening-weighted load, which raises both the capacity and the output you need. It also strengthens the case for storage, because charging from stored solar rather than from peak grid power is where much of the saving sits.
How do I get a size recommendation for my specific home?
Send us a recent utility bill and tell us what you want backed up during an outage. We read your real usage and rate plan, then specify across four platforms rather than fitting you to the one we happen to sell, including telling you if storage does not make sense for your home.
Ready for a straight answer? Send Solari Storage a recent utility bill and we will tell you whether storage makes sense for your home, including if the numbers say do not buy. Request your free assessment.
Key Takeaways
- Size to evening consumption and backup priorities rather than to monthly totals, square footage, or a rule of thumb.
- Usable capacity in kilowatt-hours sets how long the battery runs, while continuous output in kilowatts sets what can run at once.
- Savings sizing and backup sizing are different targets and should be decided separately before being combined.
- With solar, the battery recharges daily, so a smaller system can cover a longer outage than simple division suggests.
- Capacity beyond your daily solar surplus may never fill, and capacity beyond your evening load adds cost without adding savings.
- With no federal residential credit available in 2026, oversizing is paid for in full, so accurate sizing matters more this year.
