What Size Home Battery Do I Need? A Backup Sizing Worksheet
Use a worked home battery sizing worksheet to separate backup energy, continuous power, motor startup, reserve and outage recharge assumptions.
“What size home battery do I need?” has two parts. You need enough usable energy for your chosen loads and duration. You also need enough output power to start and run them together. Build a load worksheet, then have an installer choose a compatible battery/inverter system.
Do not size from floor area or the electricity bill's dollar total alone. The home battery storage guide separates outage goals from daily bill shifting. This worksheet uses invented numbers to show the method, not a system recommendation.
Define the period and loads you want to support
Choose a scenario to plan for, such as an overnight interruption or a longer cloudy outage. List essential appliances and tasks you can delay. A full-year average won't show what you need on a winter night with a heat pump or a hot afternoon with cooling.
Use owner-accessible energy records or a qualified person's measurements. Note each appliance's voltage, running demand, cycling pattern and startup requirement.
🛑 Electrical measurements can expose you to hazardous voltage. Do not open an electrical panel or connect test instruments to energized equipment to fill out this sheet; leave that work to qualified personnel.
| Hypothetical load | Assumed power | Use in a 12-hour period | Energy |
|---|---|---|---|
| Refrigerator, average over its cycling | 0.09 kW average | 12 hours | 1.08 kWh |
| Router and modem | 0.02 kW | 12 hours | 0.24 kWh |
| Selected lights | 0.04 kW | 5 hours | 0.20 kWh |
| Small pump, illustrative only | 0.60 kW while running | 0.25 hour total | 0.15 kWh |
These loads total 1.67 kWh. Add an invented 0.015 kW of system overhead for 12 hours, or 0.18 kWh, and the scenario needs 1.85 kWh at the AC loads. Actual appliances, pumps and standby consumption can differ substantially. The refrigerator's average power is not its instantaneous or startup demand.
Find out what the capacity figure includes
Ask which capacity the quote lists: gross cell energy, usable battery-side energy or delivered AC energy. Check the state-of-charge window and temperature/test conditions. If the manufacturer has already excluded a hidden buffer, don't subtract it again.
Next, ask how much energy you can expect at the start of an outage. An owner reserve that limits daily on-grid dispatch differs from the physical discharge limit. Have the installer explain how your exact controls use that reserve when the grid fails.
The Department of Energy's storage explainer distinguishes energy capacity from the power available to deliver it. Its older utility-scale prices and market figures are not used here as residential cost estimates.
Calculate energy and runtime with stated assumptions
Suppose a hypothetical battery has 12 kWh of usable DC energy over its stated full operating window. For this scenario, start at 80% and plan to finish at 20% on that same usable-energy scale. The planned window is 60 percentage points, so 12 × 0.60 = 7.2 kWh DC is available within it.
At an assumed 90% discharge conversion efficiency, that becomes 6.48 kWh AC. With a constant total AC demand of 0.35 kW, including system overhead, the arithmetic gives 6.48 ÷ 0.35 = about 18.5 hours. These percentages are planning inputs, not recommended settings or product specifications.
Looking in the other direction, the example's 1.85 kWh overnight need divided by (0.60 × 0.90) gives about 3.43 kWh of DC usable capacity as an arithmetic minimum under those assumptions. That is not an installation size: power, startup, product increments, uncertainty and the chosen contingency margin still need assessment.
Check what the capacity figure already includes before applying losses. Round-trip efficiency covers charging and discharging; discharge-only measurements cover a different boundary. Don't subtract another discharge loss if the figure already represents delivered energy. Ask for consistent definitions.
Check simultaneous power and motor startup separately
Add the loads that may run together and compare them with continuous backup output. Then check whether the system can start the largest relevant motor with the other loads still running. A pump that uses little total energy may still be hard to start.
Some expansion modules add energy without adding output power. Tesla's Powerwall 3 design guidance gives a specific example of that distinction. Other systems need their own model-by-model confirmation.
Use the air-conditioner and heat-pump backup guide for compressor input, startup and auxiliary heat. Keep nameplate heating/cooling capacity separate from electrical input.
Model recharge and a less favorable outage
Repeat your load plan for a colder, hotter or longer outage. Include necessary pumps, controls and communications. Ask how their duty cycles, the share of time they run, may change. Decide what you'd stop using if the battery emptied faster than expected.
A grid-only battery can't recharge from a failed grid. Solar recharge needs an installed system that can operate while islanded, or separated from the grid. It also needs available sunlight after allowing for household use. Request a low-solar scenario instead of relying on average daily generation.
The backup coverage guide helps turn the worksheet into a protected-load schedule. The coupling guide covers integration questions when solar already exists.
Take the worksheet to bidders
- Provide the same load list, period and future-upgrade assumptions to each bidder.
- Request usable-energy and output definitions, not just the number of battery cabinets.
- Ask for starting-charge, recharge and reduced-capacity scenarios.
- Identify which loads are excluded or automatically shed.
- Keep the installed scope, warranty and commissioning demonstration with the proposal.
🛑 Do not rely on a generic runtime calculation for medical equipment that cannot tolerate an interruption. Arrange a separate continuity plan with the equipment provider and appropriate professionals.
Conclusion
Work out energy needs from realistic loads and duration. Then check simultaneous power, startup and recharge. More kWh helps only if the complete installed system can do the job you've planned.
Read more Energy Efficiency guides for connected home planning.
Scott Harding
Scott is the main author of DIY Home Comfort. Learn more about the team and our approach on the About page.
