How to Size a Home Battery System for Power Outages 2026

Sizing a home battery system for power outages comes down to two numbers: how much energy you need to store (kWh) and how much power the system must deliver at any moment (kW). Work out your essential loads, multiply by the days of autonomy you want, divide by the battery’s usable depth of discharge and its efficiency, then add a margin. A rough worksheet takes an hour. The hardware decision takes weeks, because a licensed installer has to check your panel, your service and local rules.

That distinction matters more than any brand on a spec sheet. A household running a fridge, freezer, router and a few lights burns roughly 2.5 to 4 kWh a day. Someone running heat pumps, a well pump, an EV charger and a water heater burns twenty times that. One of them needs a small, carefully sized system. The other needs a design conversation, not a purchase.

What You Need

You can start with nothing but a notebook. Everything below improves the accuracy of the estimate.

• Your real outage history. Ask your utility or check national outage maps for how long outages last where you live, and how often. A grid that drops out for 40 minutes once a year needs a very different system from one that goes dark for three days in a storm season.
• Appliance wattages. Nameplate labels on the back or side of each appliance give running watts. The manual or the manufacturer’s specifications sheet gives the starting or surge figure, usually several times the running wattage.
• A load list. Every circuit and appliance you want powered during an outage, with an honest note on whether it truly matters.
• Your solar production figures if you have panels: kWh per day in summer and in winter, not the panel wattage.
• Inverter specifications: continuous output in kW, peak or surge output in kW, battery voltage window, and whether it accepts an AC-coupled battery.
• Your electrical service details: main breaker rating, panel brand and age, and whether there is space for a critical-loads subpanel or a service upgrade.
• Local rules. Permitting, inspection and utility interconnection requirements differ by region. A battery on a backup circuit is a permanently installed electrical system, and the work needs a qualified, licensed installer.

If you can get a clamp meter and walk your house for a week, you will replace every guess on that list with a measurement. That is an afternoon well spent.

Step-by-Step: How to Size a Home Battery System for Power Outages

1. List the loads you need to keep running

Split your circuits into essentials and everything else before you touch a calculator. Essentials are the loads whose failure causes real damage or risk: refrigeration, medical equipment, a sump pump in a flood-prone basement, a well pump, internet access for remote work, lighting, and a freezer if you have one.

Everything else – the air conditioner, the EV charger, the electric water heater, the washer and dryer, the second fridge in the garage – goes on the discretionary list. It is not off-limits. It is simply not part of the first sizing pass, which keeps the number honest.

For each load, record three figures: running watts, starting watts and hours per day in the worst season of the year. Winter is usually the honest season for heating, summer for cooling, and the shoulder months for anything else.

2. Estimate how long each load must run

Convert each load into watt-hours by multiplying watts by hours. That gives you the daily energy draw, and it is the only number that feeds the capacity calculation.

Here is a worked example for a three-bedroom home with a fridge, freezer, a 1/3 hp sump pump and a CPAP machine, all sized for a summer outage:

Load	Running watts	Hours per day	Watt-hours per day
Refrigerator	150 W	8	1,200 Wh
Chest freezer	100 W	8	800 Wh
LED lighting (10 bulbs)	100 W	5	500 Wh
Router and modem	20 W	24	480 Wh
Phone and laptop charging	80 W	3	240 Wh
Sump pump, duty cycle	300 W	1	300 Wh
CPAP machine	40 W	10	400 Wh
Total	790 W continuous	–	3,920 Wh = 3.9 kWh

Two hours of laptop use folded into three hours of charging is fine for a sizing estimate. Be tougher with the loads you care about most, and generous with the ones you can live without.

3. Calculate usable battery capacity – where home battery sizing usually goes wrong

A battery marketed as 10 kWh does not deliver 10 kWh. It delivers its nameplate capacity multiplied by three losses and one margin, and this is the step most online sizing calculators skip.

Required capacity (kWh) = (daily load in kWh x days of autonomy) / (depth of discharge x round-trip efficiency x inverter efficiency), then add 10 to 20%.

Typical values to plug in:

• Depth of discharge (DoD): the share of stored energy the chemistry will reliably give up. Lithium chemistries, LFP and NMC alike, are typically 80 to 90%. Lead-acid – AGM, gel and tubular alike – sits near 50%.
• Round-trip efficiency: charge plus discharge losses. Modern lithium systems run 90 to 97%; lead-acid is closer to 75 to 85%.
• Inverter efficiency: 94 to 97% for a good hybrid inverter.
• Design margin: 10 to 20% for real-world variance in how hard your home actually runs.

Note the 20% rule some installers use: it is a sizing buffer, not a chemistry limit. A 30% DoD would mean drawing only 30% of the nameplate capacity per cycle – roughly a quarter of the usable energy a lithium battery offers, which is why lithium systems need far less capacity for the same job.

Running the numbers on the example above for one day of autonomy: 3.9 kWh / (0.90 x 0.90 x 0.95) = 5.06 kWh of nameplate capacity. Add a 15% margin and you land at roughly 5.8 kWh. For three days of autonomy the same formula gives 3.9 x 3 = 11.7 kWh of load, about 15.2 kWh before margin, so a 17 to 18 kWh installation.

Two practical caveats. First, capacity falls as the state of charge drops, so a battery you are relying on at the end of day three delivers less than its nameplate. Second, if your loads grow – a new freezer, an electric vehicle, a heat pump replacing a gas unit – the whole worksheet needs redoing.

4. Check inverter and electrical-system limits

Energy capacity is only half the test. The inverter has to start motors, and plenty of systems with plenty of stored energy still shut down because the inverter cannot deliver the peak.

Appliance	Typical running watts	Typical starting or surge watts
Refrigerator compressor	100-200 W	500-900 W
Chest freezer	80-150 W	400-600 W
1/2 hp well pump	350-500 W	1,500-2,500 W
3/4 hp well pump	750 W	2,500-3,500 W
1/3 hp sump pump	250-350 W	900-1,400 W
Central air conditioner (3 ton)	1,200-3,500 W	3,000-5,000 W
Mini-split heat pump	900-1,500 W	2,000-3,000 W
Washing machine	300-500 W	700-1,000 W
Microwave (1,200 W setting)	1,200 W	1,200 W (no motor start)
Gas furnace blower and inducer	300-600 W	1,400-1,800 W

Add the surge of any two motors that might start at the same moment, and require the inverter’s peak rating to exceed that figure with headroom. The continuous rating must exceed your essential-load running total with the margin applied. If either fails, more battery capacity does not help – you need a different inverter.

Then check the house. The inverter’s output has to fit within the limits of the electrical panel, or you need a panel upgrade, a load-shedding device or a critical-loads subpanel that isolates a smaller set of circuits. A critical-loads panel makes a modest battery practical, but it also means the loads outside that panel genuinely do not run during an outage.

One load surprises nearly everyone: a gas furnace. The flames come from gas, but the blower motor, the control board and the igniter are electric, and during a winter outage they become your largest continuous load by a wide margin. If your heating is gas with electric accessories, budget for it as if it were a heat pump.

5. Add solar and utility operating considerations

Solar changes the sizing arithmetic only if it can run during the outage. Standard grid-tied inverters shut down when grid power fails – that anti-islanding protection exists so a crew working on a downed line is not electrocuted. A hybrid inverter, or a battery system with an islanding-capable design, lets the solar charge the battery and serve loads independently.

Once solar can run, size the array to refill what you spend. As a working figure, a 400 W panel produces around 1.6 to 2.0 kWh on a clear summer day in a sunny region, and closer to 1.0 to 1.4 kWh in midwinter or under overcast skies. Two panels barely replace one day of essential consumption; a properly sized array replaces the day’s draw and slowly restores a depleted battery.

For cloudy weather, apply a haircut. Overcast output can fall to a fraction of full-sun production, so if your outage risk is a multi-day storm, plan for two to three days of autonomy or plan on a generator. Grid-charging through the outage is not an option – that is the whole point of the outage.

Check the charge temperature limits too. Many LFP batteries refuse to charge below freezing unless a heater is fitted, and a garage or unheated utility room in winter is exactly where they get installed. Ask how the system behaves at 0 degrees Celsius and below.

Cheap tricks stretch a small system further: keep the freezer closed and rely on thermal mass (a full freezer holds safe temperature roughly twice as long as a half-full one), run the dishwasher only when solar is generating, and set the water heater to a lower temperature.

6. Validate the design with a qualified installer

Hand the finished worksheet to a licensed electrician or a qualified battery or solar-storage designer. They will verify the numbers against your panel, your service rating and your local electrical code – none of which a worksheet can check.

Ask them to confirm, in writing: the calculated daily load, the days of autonomy assumed, the nameplate and usable capacity of the proposed battery, the inverter’s continuous and peak ratings against your largest motor loads, the solar array’s production estimate for the worst month, the permitting and interconnection steps, and what the system will not be able to run.

Then compare your figure against what the installer is proposing. Whole-home systems for moderate loads typically land in the 20 to 30 kWh range; essentials-only systems for a few kWh a day are a fraction of that. If the proposed design comes back far above your calculation, ask which assumption it is based on.

Household profile	Realistic daily load	Nameplate capacity for one day of autonomy
Apartment, essential loads only	1.5-3 kWh	3-4 kWh
Three-bedroom home, essentials plus sump pump	3-5 kWh	5-8 kWh
Home with a well pump and medical equipment	5-8 kWh	9-13 kWh
Large home, whole-home including HVAC and water heating	20-30 kWh	35-50 kWh

Those nameplate figures already include the derating chain and a margin, which is why they sit so much higher than the raw daily load.

Common Mistakes

Treating nameplate kWh as usable kWh. A 10 kWh battery typically holds 7 to 8 kWh you can actually draw, after depth of discharge and efficiency. Homeowners consistently expect double the runtime they get.

Sizing on energy and forgetting power. A large battery with a small inverter cannot start a well pump or a compressor, so the load trips out even at full charge. Check continuous and surge ratings as carefully as capacity.

Assuming every circuit can be backed up. Without a critical-loads subpanel or a service upgrade, some circuits stay dead. Decide which circuits those are before the installer does.

Skipping winter electrical loads. The furnace blower, control board and igniter, the sump pump in a wet season, the well pump – these appear on nobody’s sales sheet and often dominate a cold-weather outage.

Forgetting multi-day weather. A storm that knocks out power usually kills the solar too. If you are sizing for three cloudy days, size for three cloudy days, not for clear-sky production.

Ignoring degradation. Capacity falls year over year and, within a single event, falls as the state of charge falls. A battery you plan to empty to 5% on day three delivers less than its spec sheet says.

Leaving solar out of the plan entirely. If the inverter cannot island, your battery refills only from the grid – which means it never refills during the outage.

Buying without a load audit. One-size-fits-all quotes are the norm and they are rarely right for your house. A one-hour audit is what separates a system that works from one that does not.

Frequently Asked Questions

Conclusion

To size a home battery system for power outages, write down the loads you truly need, measure each one’s running and starting watts, convert them to watt-hours per day, choose how many days of autonomy your grid reliability justifies, then divide by depth of discharge and the efficiency chain before adding a 10 to 20% margin. Check the inverter’s continuous and peak ratings against your largest motors, and remember that a 10 kWh nameplate is closer to 8 kWh usable.

Start tonight with the essential-load inventory. Take that finished list, and the surge numbers, to a licensed installer for an electrical design review before anything is ordered or connected.