The size of home battery you need comes from two numbers. Your daily critical load in kWh and the backup days you want set the target. Multiply them, divide by the usable-capacity factor of about 0.81 for LiFePO4, and round up to the nearest standard module. A home running 5 kWh of critical loads for 2 days needs about 12.3 kWh of nameplate capacity, which lands on a 15kWh pack. A home running 10 kWh for 2 days lands near 25 kWh, which points to a 30kWh pack.
The tiers below give fast answers for each home battery size. Safety essentials run 5 to 10 kWh, everyday comfort runs 10 to 20 kWh, and whole-home or multi-day coverage runs 20 to 40 kWh and above. The decision table in this guide maps your load and backup days straight to the 15/20/30/40kWh moveable LiFePO4 packs we sell. The interactive calculator below returns the required kWh, the nameplate target, and the pack that fits your numbers.
The deeper question behind the home battery calculator is the purpose of the battery. A battery sized for solar self-consumption, one sized for a 48-hour outage, and one sized for off-grid living follow different math even in the same house. This page explains that math, the four battery specifications that decide it, four worked case studies, and the off-grid and moveable scenarios the generic tools ignore.
1. The Sizing Formula: Start With Energy Demand, Then Verify Power
Home battery sizing fails when people start from the wrong number. The number that drives everything is not your monthly electricity bill, your roof area, or your inverter size. It is the daily energy of the loads the battery must actually carry during an outage or off-grid period. Home battery capacity is energy, and energy is watts times hours. Every other step in this guide multiplies off that one number, so getting it wrong makes the whole bank wrong.
Our solar battery sizing worksheet walks through the same four-step method in full detail. This guide takes the method further in four places. It adds the purpose framework, the efficiency-boundary check, the 15/20/30/40kWh moveable pack selection, and the off-grid scenarios that the US-centric tools miss.
The home battery formula itself is simple. The energy the loads need over the backup period is:
Then convert usable energy to nameplate capacity:
For a LiFePO4 home battery, a design factor of about 0.81 works when you plan with 0.90 depth of discharge and 0.90 round-trip efficiency. A 15kWh battery with that factor delivers about 12.2 kWh of usable AC energy. Never size a home battery bank from its nameplate sticker, because the sticker overstates what actually reaches your outlets.
The full process has seven steps, and section 10 lists them in order. Start with purpose, measure the load, convert to usable energy, add reserve only if the datasheet does not already include it, then verify power, charging, and recharge. Skip the power check on a home battery and you can end up with enough energy and not enough output.
2. Define the Purpose Before Calculating Anything
The same home needs very different storage depending on what the battery must accomplish. Government guidance from the Australian Energy Department makes the same point: the purpose decides the sizing inputs. Our table maps the five common goals to their main sizing consideration.
| Primary goal | Main sizing consideration |
|---|---|
| Solar self-consumption | Energy used after sunset and surplus solar available to charge the battery |
| Electricity bill reduction | Energy shifted from low-price periods to high-price periods |
| Essential-load backup | Daily energy of critical appliances and required outage duration |
| Whole-home backup | Total electricity demand during an outage, including heating or cooling |
| Off-grid operation | Energy demand, seasonal solar production, and the longest low-generation period |
A battery designed for self-consumption does not need to store a whole day of consumption. Solar panels feed the daytime load directly, so the battery mainly serves the evening remainder. For backup, the key question is different: how much electricity must stay available if the grid fails? For off-grid operation, the design must also cover consecutive cloudy days and confirm that solar can recharge the bank afterward. A system sized on an average sunny day can fail during a prolonged low-generation stretch.
This purpose step matters because it stops two classic home battery errors. The first is sizing an essential-load battery from the full utility bill, which overshoots two to three times. The second is sizing a self-consumption battery from the whole daily load, which buys capacity that rarely cycles.
3. List the Loads the Battery Must Carry
A backup home battery does not run the whole house. It runs the shortlist of loads you refuse to lose. Walk through the building room by room and write down each device, its running watts, and how many hours per day it runs on battery. Sum watts times hours and divide by 1000 to get kWh per day:
| Load | Typical running watts | Daily energy on battery |
|---|---|---|
| Refrigerator | 150W (cycling) | 1.0 - 1.5 kWh |
| LED lighting (whole home) | 80 - 120W | 0.4 - 0.6 kWh |
| Ceiling fans (3-4 units) | 60 - 75W each | 1.0 - 1.7 kWh |
| Wi-Fi router and modem | 10 - 20W | 0.2 - 0.5 kWh |
| TV plus phone and laptop charging | 60 - 200W | 0.5 - 1.0 kWh |
| Water pump | 370 - 750W | 1.0 - 3.0 kWh |
| Medical device (CPAP, concentrator) | 30 - 300W | 0.2 - 7.0 kWh |
| Window air conditioner | 500 - 1,200W | 4.0 - 8.0 kWh |
Nameplate power alone is not enough when appliances cycle on a home battery bank. A refrigerator runs about one third of the time, so its average draw sits near 150W even though the sticker says 500W. The most reliable inputs are smart-meter data, a plug-in energy meter, or actual operating measurements. One window AC unit can double or triple the whole list, so decide early whether cooling rides on the battery or on another solution.
Two methods feed the home battery load list. The bottom-up method above is the accurate one, because it forces you to name every device. The bill method is the fast one. Take monthly kWh, divide by 30, and apply a 0.25 to 0.60 factor to isolate the loads the battery will carry. The bill method works only when you already know the load split. Write the list down either way, because the list is the single source of truth for the calculator. Our 15 kWh capacity calculation guide works through a full load-matching example on one pack size.
4. Choose Your Backup Days
Backup days, sometimes called autonomy days, is how long the bank must run with no solar and no grid. The choice comes from the site, not from a default setting.
| Scenario | Backup days | Typical daily load | Fits |
|---|---|---|---|
| Short outage cover | 1 day | 3 - 8 kWh | Grid-tied homes with reliable supply |
| Regular load-shedding | 2 days | 5 - 15 kWh | Regions with daily or weekly cuts |
| Remote or weak grid | 3+ days | 8 - 25 kWh | Off-grid cabins, rural sites, farms |
| Moveable or rental use | 12 - 24 hours | 2 - 10 kWh | Rented homes, job sites, emergency kits |
Without recharge, doubling the backup days doubles the required deliverable energy from a home battery. A 5 kWh daily load for 1 day needs 5 kWh usable; for 2 days it needs 10 kWh. That compounding is why the autonomy choice deserves real outage history, not a guess. One day is enough for a home battery in a house that loses power a few times a year. Two days covers regular load-shedding and most storm seasons. Three or more days fits off-grid sites and remote locations where a cloudy stretch can outlast a small bank.
Solar changes the math. If panels keep feeding loads and recharging the battery during the outage, a time-based energy balance is more accurate than multiplying daily use by days. The ac-coupled vs dc-coupled solar guide explains how PV recharging changes the bank size you actually need.
5. Convert Usable Energy to Nameplate Capacity
A home battery nameplate rating and the energy you can actually use are different numbers. Four specifications sit between them, and each one can be misread.
Nominal capacity is the stated total energy, such as 15kWh. Usable capacity is the energy available within the manufacturer's operating limits. Depth of discharge is the fraction of nominal capacity the design cycles down to. Reserve is energy held back for emergencies or operating limits. A 10kWh battery with a 90% DoD window delivers about 9kWh of accessible energy before inverter losses, and that example is not a universal ratio.
Round-trip efficiency has a boundary problem. Round-trip efficiency on a home battery describes the energy returned during discharge relative to the energy put in during charge. The precise meaning depends on the measurement boundary: some figures cover only the battery module, while others include the inverter and system wiring. Discharge efficiency and round-trip efficiency are not automatically interchangeable. When a datasheet states a guaranteed usable AC-energy figure, compare that figure directly against the required AC load. Do not apply depth-of-discharge and inverter-efficiency deductions a second time, because that double-counting quietly inflates the bank by 10% to 20%.
For a LiFePO4 home battery planned from nominal DC capacity, the conversion is:
With 90% usable capacity and 92% battery-to-AC discharge efficiency, 10kWh delivered to AC loads needs about 12.1kWh of nameplate capacity. With a further planned reserve of 10%, the same 10kWh needs about 13.4kWh. Temperature adds another derating layer. A pack at 0°C can deliver roughly 15% to 40% less energy than at 25°C. Add margin when the battery lives in an unheated space. The winter solar battery guide covers heating options. Our LiFePO4 BMS guide explains how the protection circuits set the DoD operating window and the reserve.
The usable-capacity table below uses one consistent assumption set: 90% usable capacity, 92% discharge efficiency, and no extra reserve. These are planning assumptions, not universal battery specifications.
| AC energy needed per day | Nameplate for 1 day | Nameplate for 2 days |
|---|---|---|
| 3 kWh | 3.6 kWh | 7.2 kWh |
| 5 kWh | 6.0 kWh | 12.1 kWh |
| 8 kWh | 9.7 kWh | 19.3 kWh |
| 10 kWh | 12.1 kWh | 24.2 kWh |
| 15 kWh | 18.1 kWh | 36.2 kWh |
| 20 kWh | 24.2 kWh | 48.3 kWh |
Round up to a real module, never down, because a home battery bank that is 10% short fails on the second night of an outage.
6. The 15/20/30/40kWh Decision Table
This table maps daily load and backup days straight to the moveable home battery packs we ship. It uses the 0.81 usable factor and rounds up to the next pack size.
| Daily critical load | Backup days | Usable kWh needed | Nameplate needed | Recommended pack |
|---|---|---|---|---|
| 3 - 5 kWh | 1 day | 3 - 5 kWh | 4 - 6 kWh | 15kWh (room to grow) |
| 5 kWh | 2 days | 10 kWh | 12.3 kWh | 15kWh |
| 8 - 10 kWh | 1 day | 8 - 10 kWh | 10 - 12.3 kWh | 15kWh or 20kWh |
| 8 - 10 kWh | 2 days | 16 - 20 kWh | 20 - 24.7 kWh | 20kWh or 30kWh |
| 10 - 15 kWh | 2 days | 20 - 30 kWh | 24.7 - 37 kWh | 30kWh or 40kWh |
| 15 - 20 kWh | 2 days | 30 - 40 kWh | 37 - 49 kWh | 40kWh or two packs |
The four moveable sizes cover the full home battery range. The 15kWh moveable LiFePO4 battery fits small homes and 1 to 2 days of essential loads. The 20kWh moveable pack covers average homes with a day of comfort loads. The 30kWh moveable battery handles larger homes and 2 days of normal use. The 40kWh 48V pack targets whole-home backup and multi-day off-grid sites. Our 15kWh vs 20kWh vs 40kWh comparison walks through the trade-offs between the sizes in detail. The how to choose a solar battery guide covers the wider selection criteria.
7. Case Studies: Four Ways the Math Changes
The appliance figures below are illustrative inputs for home battery calculations, not universal consumption statistics. Replace them with measured values wherever possible.
Case 1: Essential loads during a 48-hour outage. A household wants refrigeration, lighting, internet, fans, device charging, and a few small loads. The daily budget runs about 5.0 kWh. The list covers refrigerator 1.4, router 0.3, LED lighting 0.5, fans 1.0, laptops and phones 0.5, TV 0.5, and small essentials 0.8. For two days without recharge:
C(nominal) = 10 ÷ (0.90 × 0.92) ≈ 12.1 kWh
C(with 10% reserve) = 10 ÷ (0.90 × 0.92 × 0.90) ≈ 13.4 kWh
That lands on a 15kWh pack with comfortable headroom. A smaller pack could work if the outage plan allows less energy use or solar recharge is available, but 15kWh is the defensible starting point for this scenario.
Case 2: Whole-home backup. The US Energy Information Administration reported average residential electricity purchases of 865 kWh per month in 2024, about 28.4 kWh per day. That is a national average of grid-delivered electricity, not a universal household figure. Using it as a whole-home daily requirement:
C(nominal, 2 days) ≈ 68.6 kWh
Those numbers explain why whole-home backup costs far more than an essential-load home battery. They are not recommendations for every home. A household can shrink the requirement by excluding nonessential circuits, adjusting thermostat settings, and scheduling flexible loads around solar.
Case 3: Air conditioning during outages. An AC unit drawing 1.2 kW average for eight compressor-operating hours consumes 9.6 kWh of delivered electricity in one day. That load appears before any other appliance is added. Research published in the journal Energy in 2024 modeled residential solar-plus-storage backup under US climate conditions. For a three-day interruption, median storage needs ranged from 10 kWh in temperate conditions to 90 kWh in hot climates. That is a modeled research result, not a rule. Its lesson is that the energy required to keep a home comfortable during an outage can dominate the storage calculation. Efficiency improvements plus thermostat adjustments can cut that requirement materially.
Case 4: Solar self-consumption. A home consumes 18 kWh per day. Solar feeds 8 kWh directly during daylight, 7.5 kWh of surplus is available, and the evening load after sunset is 6 kWh. The home battery target is the 6 kWh evening requirement, not the 18 kWh total. With a 90% AC-to-AC round-trip efficiency, 7.5 kWh of surplus returns about 6.75 kWh, which covers the evening load. A much larger battery stays partly empty when the solar system rarely produces enough surplus to fill it. The best size for self-consumption is the one that stores enough evening energy without creating persistent underutilization.
8. Off-Grid and Moveable Scenarios
The generic sizing tools assume a US house with a grid connection. That assumption fails for two big groups of buyers: off-grid sites with no grid at all, and households that need a battery they can move.
Off-grid sizing. An off-grid home battery carries every load every day, not just the critical list during an outage. The design must cover five inputs: daily energy consumption, seasonal solar generation, consecutive low-generation days, recharge capability, and load management. A battery that covers two days of load is not necessarily adequate off-grid, because several cloudy days with weak solar can drain it with no fallback. A cabin using 8 kWh per day with 4 days of autonomy needs 32 kWh usable, which divides to about 40 kWh nameplate and matches the 40kWh pack. Confirm that the array can recharge the bank within one good sun day, or the battery becomes the bottleneck.
Moveable sizing. A moveable home battery serves rental homes, job sites, caravans, and emergency kits, where installation is temporary and the pack may roll between locations. The load list is shorter, typically 2 to 10 kWh per day, and the backup window is 12 to 24 hours. A moveable pack with wheels makes the difference between a battery that follows the need and one that is bolted to a wall. The 15kWh battery with wheels and the 20kWh battery with wheels bring the same 51.2V LiFePO4 chemistry to a rolling platform. The full line lives in the moveable solar battery collection.
Weight changes the equation here. A 30kWh pack holds about 200 to 250 kg of cells and steel, which is fine on a floor mount but heavy to roll every week. Match the pack size to how often it actually moves. A battery that rolls daily works best at 15 to 20 kWh. A battery that moves once a season can go up to 30 or 40 kWh.
9. kW vs kWh: Check Three Power Limits
Capacity answers how long a home battery runs. Power answers what it can run at one moment, and the two are not the same. A 15kWh battery with a 5kW inverter stores plenty of energy but cannot start a 7kW load, because the inverter is the bottleneck.
Three separate limits decide what the system can supply: the battery's continuous discharge power, the inverter's continuous AC output, and the surge power available for starting motors, compressors, and pumps. The weakest applicable limit wins. A refrigerator compressor or a water pump can draw three to five times its running watts for a few seconds on startup, so the surge check is not optional.
When you size a home battery, match three numbers. They are the usable kWh for runtime, the continuous kW for the loads that run together, and the surge kW for the biggest motor start. A grid-connected system can draw additional power from the grid, but that does not mean the battery can independently support the same load during an outage.
Charging power has a ceiling too. A home battery can be too large for the charging source. If solar production is low, the system may not produce enough surplus to fill the battery each day, and the extra capacity delivers little practical value. The charging rate and available solar surplus belong in the sizing decision alongside storage capacity.
10. Common Sizing Mistakes
- Sizing from square footage. Floor area says almost nothing about home battery energy needs. A 150m2 home with gas heating and LED lights can need less than a 90m2 all-electric home.
- Using the full utility bill for a critical-load battery. The battery covers the shortlist, not the whole house. Bill-based sizing without the load factor overshoots two to three times.
- Buying nameplate instead of usable. A 15kWh sticker delivers about 12.2 kWh to the outlets. Quote the usable number.
- Counting efficiency losses twice. If a datasheet states guaranteed usable AC energy, applying an extra inverter-efficiency deduction underestimates the pack and inflates the quote.
- Oversizing relative to the charging source. Extra capacity that never fills adds cost without backup value. Check solar surplus and charging rate first.
- Ignoring temperature. A battery in an unheated shed loses 15% to 40% of its capacity in winter. Size for the coldest month.
- Assuming a battery guarantees whole-home backup. Backup depends on the inverter, the transfer arrangement, and which circuits are enabled. Confirm the configuration before promising full coverage.
- Buying a bank that cannot expand. A stackable 51.2V home battery platform lets you start at 15kWh and add a 20kWh pack later. Confirm the inverter parallel limit before promising a second home battery pack.
11. FAQ
How many kWh do I need for home battery backup?
Multiply your daily critical load in kWh by the backup days you want, then divide by about 0.81 for a LiFePO4 system. A home with 5 kWh of critical load and 2 backup days needs about 12.3 kWh of nameplate capacity, which lands on a 15kWh pack. Use the interactive calculator on this page for your own numbers.
Is a 5kWh home battery enough?
It can cover a modest set of essential loads over a limited period. After usable-capacity limits and conversion losses, the AC output is less than the nominal 5kWh. Whether it is sufficient depends on the actual load and the required duration, so run the load list first.
Is a 10kWh home battery enough to run a house overnight?
It may be enough for a household with modest evening demand and no major appliances. It is likely too small for substantial air-conditioning, electric heating, water heating, or cooking loads. Use measured nighttime consumption to decide.
How many kWh does a house use per day?
A typical US home uses about 29 kWh per day, but that number is misleading for battery sizing. The battery only carries the loads you choose, usually 5 to 15 kWh per day. Off-grid homes carry the full daily load, which can run 10 to 30 kWh depending on appliances.
How long does a 15kWh battery last?
A 15kWh home battery delivers about 12.2 kWh usable. On a 5 kWh daily load it runs about 2.4 days. On a 10 kWh daily load it runs about 1.2 days. The runtime equals usable kWh divided by the daily load.
Should a home battery be sized for one day or two days of backup?
It depends on the expected outage duration and whether solar recharge is available. Without recharge, two days of the same load need twice the deliverable energy. With solar recharge, the outcome depends on generation and consumption during the outage, so model the time balance rather than multiplying by days.
Does a larger battery always improve savings?
No. A larger battery helps only when the household can use the extra capacity and charge it economically. A battery that stays undercharged or rarely cycles its added capacity may not justify its cost. Evaluate the tariff, solar surplus, charging opportunities, and expected utilization.
Should battery capacity include future degradation?
Yes, when long-term performance matters. Check the warranty's capacity-retention terms and whether they specify nominal, usable, or delivered energy. Add initial capacity based on the product data and operating conditions, not on an assumed universal degradation rate.
Can I add more battery capacity later?
Yes, if the platform is stackable. The 51.2V moveable packs support parallel connection, so you can start at 15kWh and add a 20kWh pack later. Confirm the inverter parallel and communication limits before planning the expansion.
What size battery do I need for a 2,000 sq ft house?
Square footage does not set battery size. A 2,000 sq ft home with gas heat needs about 5 to 10 kWh for essential backup. The same home with electric heat and AC needs 20 to 40 kWh. Run the load list, not the floor plan.
How many solar panels do I need to charge a 15kWh battery?
A 15kWh battery needs about 3 to 4 kW of solar to recharge in one good sun day, assuming 4 to 5 peak sun hours and 80% system efficiency. Panels sized for the daily load, not the battery size, keep the bank full.
Why does my battery run out faster than the calculator says?
The usual causes are a higher real load than the list, winter temperature loss, a low inverter efficiency, or a BMS reserve you did not count. Recheck the load list, reapply the usable factor, and measure the actual draw with a monitoring app before buying more capacity.
