Choosing the right battery size starts with understanding what you want the battery to do.
A battery intended to keep a refrigerator and a few lights running during an outage may be much smaller than a battery designed to operate most of a home overnight. A system intended primarily for time-of-use savings may be sized differently from one designed for several days of outage resilience.
For backup systems, start with the loads you want to support and how long you want to support them. Do not size a battery simply as a percentage of the solar-array size.
Two battery ratings are especially important:
A successful design needs enough of both.
Common goals include:
Keep a small group of important loads operating during an outage, such as:
This usually requires less battery power and energy than backing up the entire house.
Support essential loads plus selected larger equipment, such as:
Larger motor loads need particular attention because their starting power can be much higher than their normal running power.
Keep most or all household circuits available during an outage.
Whole-home backup does not necessarily mean every appliance can operate simultaneously without restriction.
Large loads such as:
can create substantial power demand.
Whole-home systems may therefore need to use manual or automatic load management to temporarily disable or prioritize certain appliances when the home is operating from batteries.
Store excess solar energy during the day and use it later when the solar array is producing less than the home is consuming.
For this goal, battery sizing depends heavily on:
Charge the battery when electricity is less expensive or when excess solar is available and discharge it during higher-cost periods.
For this application, sizing should consider:
The battery with the greatest capacity is not automatically the most economical choice.
This distinction is critical.
Kilowatts (kW) measure power.
Suppose your home is running:
The simultaneous running load would be:
200 + 300 + 200 + 1,500 + 800 = 3,000 watts
or:
3.0 kW
The battery system must be capable of supplying at least the required simultaneous power, subject to the equipment manufacturer's design requirements.
Kilowatt-hours (kWh) measure energy over time.
For example, a 1,000-watt load operating for 5 hours consumes:
1,000 W × 5 hours = 5,000 Wh
or:
5 kWh
A battery's kWh capacity therefore helps determine runtime, while its kW rating determines how much load it can support at one time.
The U.S. Department of Energy similarly distinguishes storage energy capacity, measured in kWh, from power capacity, measured in kW.
For each appliance or circuit you want available during an outage, record:
Do not assume that the circuit-breaker rating tells you how much energy the appliance normally uses.
Whenever possible, use:
Actual measured household load data can provide a much better sizing estimate than assumptions based only on breaker sizes.
A basic energy calculation is:
Energy used (kWh) = Load power (watts) × Hours of operation ÷ 1,000
Suppose you expect the following loads during one night:
Estimated energy:
1.5 + 1.0 + 1.0 + 1.0 = 4.5 kWh
That means you need approximately 4.5 kWh of delivered energy for those assumed loads and operating times.
This is only the starting point.
A battery advertised as “10 kWh” does not necessarily mean that every 10 kWh will always be available to household loads.
Consider:
For example, a homeowner may intentionally maintain part of the battery as an emergency reserve rather than allowing normal self-consumption to discharge it fully.
Always use the manufacturer's usable-energy specifications and system settings when estimating runtime.
A battery does not have one universal runtime.
A battery that can support a small essential-load panel for many hours might support a heavily loaded home for only a short period.
A simple conceptual estimate is:
Approximate runtime = Available battery energy ÷ Average load
For example, if approximately 9 kWh is available and the home averages 1.5 kW while operating from backup:
9 kWh ÷ 1.5 kW ≈ 6 hours
But actual runtime will vary because household loads cycle on and off and battery operating conditions change.
This calculation should therefore be treated as an estimate, not a guaranteed outage duration.
Having enough kWh is not enough.
The battery system must also be capable of supplying the required instantaneous power.
Imagine a battery has enough stored energy to operate your home for 12 hours, but the battery system can provide only 5 kW of continuous output.
If the home simultaneously requests 8 kW, the energy capacity does not solve the problem. The system must either:
This is why battery sizing requires both:
Energy analysis (kWh)
and
Power analysis (kW).
Some appliances briefly require substantially more power when they start than while they are running.
Examples can include:
For these loads, check both:
Do not assume that a battery capable of running a motor can necessarily start it.
Battery systems and associated inverters may have separate ratings for continuous output and short-duration peak output.
Use the manufacturer's current equipment specifications and design guidance when evaluating these loads.
Heating and cooling loads deserve their own review.
A central air conditioner or heat pump can represent one of the largest electrical loads in the home.
Whether it can operate during battery backup depends on factors such as:
Do not promise HVAC backup based only on battery kWh.
Electric resistance auxiliary heat can be particularly demanding and needs to be excluded during backup operation.
A backup system does not necessarily need to power every load at the same time.
Load-management equipment can automatically prevent certain appliances from operating when total demand becomes too high.
For example, a system might temporarily prevent an EV charger from operating while the home is on battery power, or prevent multiple large appliances from operating simultaneously.
Potential managed loads include:
Load management can make whole-home backup practical without requiring the battery system to support the theoretical maximum demand of every household load simultaneously.
However, load-control capability depends on the specific battery and backup system.
For an outage lasting only a few hours, stored battery energy may be the main concern.
For a multi-day outage, solar recharge becomes critical.
During daylight hours, available solar energy may need to do two jobs:
For example, if the solar array produces 15 kWh during an outage day but the house consumes 10 kWh during daylight hours, only a portion of the remaining solar energy may be available to recharge the battery after system losses and operating constraints are considered.
The larger the daytime household load, the less solar energy may remain for battery charging.
Solar production varies significantly.
During an extended outage, production can be reduced by:
A battery sized to survive one night after a perfect summer day may perform very differently during several cloudy winter days.
If multi-day resilience is an important goal, model the system using conservative solar-production conditions rather than only annual-average or best-day production.
Solar plus storage can provide resilience during grid disruptions, but the available duration depends on the balance between stored energy, household demand, and available generation.
This is an important point.
A conventional grid-tied solar system normally shuts down when the utility grid fails unless the system includes equipment specifically designed to create and operate a safe backup electrical system.
This prevents the solar system from unintentionally energizing utility lines during an outage.
To operate solar during a grid outage, the system generally needs a compatible configuration that can:
Do not assume that adding a battery to any existing solar system automatically provides outage backup.
Confirm that the solar, battery, inverter or microinverters, system controller, transfer/isolation equipment, and software are designed to operate together in backup mode.
Many battery systems allow the homeowner to reserve a percentage of stored energy for outages.
For example, if a battery is being used for daily self-consumption, the system might stop normal battery discharge after reaching a selected reserve level.
A larger reserve generally means:
The appropriate setting depends on the homeowner's priorities.
If the battery's primary goal is reducing electricity costs rather than outage backup, simply adding up emergency loads is not enough.
Economic sizing should consider:
A larger battery does not automatically produce larger savings if there is not enough excess solar or high-cost consumption available to use the additional capacity.
Once you know approximately how much storage is required, verify that the equipment can actually be installed at the site.
Consider:
A design requiring several battery units will need substantially more wall and working space than a single-battery installation.
Battery sizing can also affect other parts of the project.
Depending on the system, evaluate:
Follow the approved plans and the manufacturer's design requirements for the specific equipment being installed.
For a backup system, a practical design sequence is:
Do not size a home battery by kWh alone.
A successful battery system must answer two different questions:
Can it supply enough power to operate the desired loads?
That is primarily a kW question.
Can it supply those loads for the desired amount of time?
That is primarily a kWh question.
For extended outages, there is a third question:
Can the solar array produce enough energy to run daytime loads and recharge the battery for the next night?
Large loads, simultaneous demand, starting requirements, load management, available solar energy, and actual household behavior can all determine whether the system performs as expected.