Turning kWh Into Hours of Backup
A home battery is rated in kilowatt-hours (kWh) of storage, but during an outage what you feel is hours of runtime, and the bridge between them is your load. The formula is simple: usable battery kWh ÷ load in kW = hours. A 13.5 kWh battery powering a 1.5 kW essentials load runs about 9 hours; double the load to 3 kW and runtime halves to roughly 4.5 hours. This calculator lets you plug in your loads and your battery so you know how long the lights stay on before you spend a dollar.
Usable vs Rated Capacity, and Why It Bites
The kWh printed on the box is the rated capacity, but batteries hold back a slice to protect their cells — a reserve of 5-10% is typical. A Tesla Powerwall lists 13.5 kWh usable, an LG RESU rated at 16 kWh delivers closer to 14.5-15 kWh, and a 5 kWh Enphase unit gives about 4.5 kWh at the outlet. Sizing off the rated figure quietly overstates your runtime by that reserve, which is why a battery that looks like a 10-hour solution on paper often comes up an hour short in practice. Always run the math on usable kWh.
Essentials, Whole-Home, and Stacking
Most homeowners start with critical-loads backup — the fridge, a few lights, the Wi-Fi router, and phone chargers, averaging well under 1.5 kW — where a single battery can cover most of a day. Whole-home backup is a different animal: central AC, an electric range, or a water heater can push the average draw to 5-8 kW and drain that same battery in a couple of hours, so it usually takes two or three units. Stacking batteries adds usable kWh (two Powerwalls hold 27 kWh, doubling runtime at the same load) and raises combined power output so bigger loads can actually start. Solar changes the picture again by refilling the bank during daylight outages.
Home Battery Calculator
Size your home battery system and estimate costs including the 30% federal tax credit (IRA 2022, valid through 2032).
Home Battery System Comparison (2026 Pricing)
Equipment cost only — installation adds $3,000-$6,000
| System | Capacity | Power Output | Cost | After 30% ITC | Warranty |
|---|---|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW | $8,500 | $5,950 | 10 years |
| Enphase IQ 5P | 5 kWh | 3.84 kW | $5,000 | $3,500 | 15 years |
| LG RESU Prime | 16 kWh | 7 kW | $11,000 | $7,700 | 10 years |
| Generac PWRcell | 18 kWh | 9 kW | $12,000 | $8,400 | 10 years |
| Franklin WH aPower2 | 15 kWh | 10 kW | $10,000 | $7,000 | 12 years |
How to Use This Calculator
- Enter Your Daily Energy Usage (kWh): Pull this from a recent utility bill — the average US home uses about 30 kWh/day. The calculator uses it to gauge whole-home demand and how far a battery bank stretches.
- Pick Your Backup Mode: Choose critical-loads only (fridge, lights, Wi-Fi, medical devices — usually 30-40% of daily use) or whole-home. Critical loads keep the average draw low so a single battery lasts far longer.
- Set Backup Duration and Battery System: Enter how many hours you want to ride through, then pick a battery. Note the usable kWh and kW output — a Powerwall 3 stores 13.5 kWh at 11.5 kW, an Enphase IQ 5P only 5 kWh at 3.84 kW.
- Add Solar, Then Calculate: If you have panels, enter the array size so daytime production can offset battery drain. Hit Calculate to see batteries needed, total usable capacity, and cost after the 30% tax credit.
How It Works
Battery runtime comes down to one ratio: usable kWh ÷ load in kW = hours. A Powerwall holds 13.5 kWh usable, so a steady 1.5 kW essentials load (fridge cycling, LED lights, Wi-Fi, phone charging) runs it about 9 hours. Push the load to 3 kW and the same battery empties in roughly 4.5 hours. This calculator sizes your bank around the loads you actually plan to back up rather than a nameplate number.
The basic rule:
- Runtime hours = usable battery kWh ÷ average load in kW — a 13.5 kWh battery at a 2 kW load lasts about 6.75 hours
- Usable kWh is less than rated kWh: most lithium home batteries reserve 5-10%, so a 16 kWh rated pack delivers roughly 14.5-15 kWh before the inverter shuts off
- Essentials backup (fridge + lights + Wi-Fi + a few outlets) usually averages 0.5-1.5 kW; whole-home with HVAC and an electric range can peak at 5-8 kW
- Stacking batteries adds kWh, not kW-per-hour: two Powerwalls hold 27 kWh usable, doubling runtime at the same load
- Solar recharge during a daytime outage resets the clock — a 6 kW array producing 4-5 peak-sun hours can refill much of a mid-size bank before nightfall
The gap between rated and usable capacity is where most sizing mistakes happen. A 5 kWh Enphase unit rated on the box gives you closer to 4.5 kWh at the outlet, so a 500 W critical load (fridge and a router) runs about 9 hours per unit — not the 10 the nameplate implies. Size for the load profile that hurts most during an outage, then decide whether you're buying hours of runtime or days.
Tips & Considerations
- Size runtime off usable kWh, not the number on the box — a battery rated at 16 kWh that reserves 8% only gives you about 14.7 kWh.
- Measure your real essentials load: a fridge, LED lights, a router, and phone chargers together often average under 1 kW, which is why one battery can cover a full day of critical backup.
- Check power output (kW), not just capacity (kWh) — a well pump or AC compressor with a high startup surge can stall a low-output battery no matter how many kWh it holds.
- Each stacked battery adds hours at the same load: two 13.5 kWh Powerwalls run a 2 kW load about 13.5 hours instead of 6.75.
- For overnight outages, plan on the battery alone and ignore solar after sunset — daytime production only helps while the sun is up.
Frequently Asked Questions
How long will a home battery run my house?
Divide usable kWh by your average load in kW. A 13.5 kWh Powerwall backing up a 1.5 kW essentials load (fridge, lights, Wi-Fi, a few outlets) runs about 9 hours. Back up more of the house so the average climbs to 3 kW and the same battery lasts only about 4.5 hours. Whole-home use with HVAC running can pull 5 kW or more, dropping runtime under 3 hours per battery. The number that matters is your real average draw during an outage, not the peak on the panel.
What is the difference between usable and rated kWh?
Rated (nameplate) kWh is the total energy in the cells; usable kWh is what the battery will actually deliver before it stops to protect itself. Most lithium home batteries hold back 5-10%. A Powerwall 3 is rated and usable at 13.5 kWh, an LG RESU rated at 16 kWh gives roughly 14.5-15 kWh usable, and a 5 kWh Enphase unit delivers about 4.5 kWh. Always size runtime off the usable figure — using the rated number overstates how long you'll last.
How many batteries do I need?
Add up the loads you want to keep on, multiply by the hours you want to cover, then divide by usable capacity per battery. Example: a 1 kW critical load for 24 hours needs 24 kWh, so two 13.5 kWh Powerwalls (27 kWh usable) cover it with margin. Whole-home backup for a typical 30 kWh/day house through a full day usually takes 2-3 Powerwalls; homes with electric heat, a pool, or EV charging often land at 3-4. Solar shrinks the count by recharging during daylight.
Can a home battery run my air conditioning?
It depends on the AC and the battery's power rating, not just its kWh. A central AC compressor draws 3-5 kW while running and spikes higher at startup, so it needs a battery with enough continuous output — a Powerwall 3 puts out 11.5 kW, an Enphase IQ 5P only 3.84 kW. Running a 3.5 kW AC continuously would drain a single 13.5 kWh battery in under 4 hours of pure cooling. Most homeowners either cycle the AC, back up a single mini-split, or stack batteries to run cooling for a meaningful stretch.
How does stacking multiple batteries change runtime?
Extra batteries add usable kWh, which stretches how long you last at a given load — they don't reduce the load itself. One 13.5 kWh Powerwall at a 2 kW load lasts about 6.75 hours; two give you 27 kWh and about 13.5 hours; three reach roughly 20 hours. Stacking also raises combined power output, which matters when you want to start big loads like a well pump or AC compressor that a single unit can't handle.
Does solar extend how long my battery lasts in an outage?
Yes, when the sun is up and your system supports it. During a daytime outage a 6 kW array might produce 4-5 peak-sun hours of energy, refilling much of a mid-size bank so it carries you through the night on the same essentials load. Without sun the battery is your only source, so plan overnight runtime on usable kWh alone. Note that many grid-tied solar setups shut down in an outage unless the battery inverter is configured for islanding — confirm yours can recharge off-grid.