Live usable kWh and a critical-load runtime clock - plus a TOU payback sketch that does not confuse bill arbitrage with why people buy backup.
Planning sketch only. Not an installer quote, interconnection study, or fire-code review.
By Ahmet C. Toplutaş·Site owner & editor · Energy & home tools
Last updated May 21, 2026 · Reviewed May 21, 2026 · Planning estimates only - not tax, medical, or investment advice
01
Marketing quotes nameplate capacity - the gross kilowatt-hours stamped on the spec sheet. What actually reaches your fridge, sump pump, or router during an outage is smaller: you rarely discharge to empty (depth of discharge limits), and every charge/discharge cycle loses energy to heat (round-trip efficiency).
This panel uses planning defaults of 90% depth-of-discharge (DoD) and 90% round-trip efficiency (RTE). Usable energy ≈ nameplate × DoD × RTE. A flagship 13.5 kWh wall unit therefore lands near ~10.9 kWh usable - not 13.5 - before you celebrate “half a day of whole-home power.”
Inverter power (kW) is a separate ceiling from energy (kWh). A pack can run out of energy hours before it hits power limit, or stall on a single large load even with kWh left. Runtime planning needs both the usable bucket and the continuous load you assume.
Temperature matters too - cold garages and hot attics derate output and usable capacity versus lab conditions. This sketch holds fixed 90%/90% planning factors; your installer may quote different DoD limits to preserve cycle life on a 10-year warranty.
Decision framing: “How many hours of the loads I care about?” - not “the brochure says 13.5, so I am covered through tomorrow.” Start from home energy if you do not know daily kWh yet.
02
The panel’s critical-load runtime clock divides usable kWh by a continuous load profile. That is deliberately not an NFT fee waterfall, a macro plate remix, or a geothermal loop sketch - it answers “how long does this bucket last at this draw?”
Load profiles are planning stories, not smart-meter truth. Essentials (~0.8 kW) models fridge, Wi‑Fi, and lights. Essentials+ (~1.8 kW) adds a small AC or well pump. Whole-home avg derives kW from your daily kWh ÷ 24 - smooth average draw, not peak evening surges. Peak evening (~4 kW) stress-tests cooking plus AC - runtime often collapses compared with essentials-only.
Flip profiles on the same pack without changing kWh and watch hours move. That teaching moment is why resilience buyers should pick a load story before they pick a sticker size. Doubling nameplate kWh doubles runtime only if you do not also double the assumed load.
Custom load kW is there when you have a measured circuit or electrician list - e.g. 1.2 kW for medical device plus fridge. Paste measured draw when you have it; guess high when you do not. Undershooting load kW is the most common way to overstate outage hours.
03
Time-of-use (TOU) arbitrage can shrink electricity bills by shifting kWh from expensive peak windows into off-peak charging. Outage cover can justify a pack even when simple payback looks long or “n/a” on a flat rate. Households that buy “for ROI only” on a flat tariff are often disappointed; households that buy for hours of essentials are answering a different question entirely.
The panel keeps both numbers visible - runtime clock and payback sketch - so neither story hijacks the other. A steep TOU spread can make bill math attractive while a winter outage still leaves you at four hours on peak evening load. Conversely, a long payback on flat rates does not mean the clock is useless if your grid fails twice a year.
Pair storage with solar ROI when you want recharge during daylight - but remember solar production and outage weather do not always align. A battery alone does not generate kWh; it stores them.
04
Annual arbitrage ≈ daily shifted kWh × (peak rate − off-peak rate) × 365. Shifted energy is capped by usable capacity and a planning share (~35%) of daily usage - you cannot invent a second pack with spreadsheet optimism. If peak and off-peak rates are equal (flat tariff), bill savings go quiet and payback correctly reads “n/a” while the runtime clock still matters.
Paste your utility’s actual peak and off-peak $/kWh from the rate sheet - not a neighbor’s tariff. Summer super-peak adders, demand charges, and minimum bills are not modeled here; this is order-of-magnitude bill sketch, not a utility bill replica.
The incentive % dial reduces installed cost for simple payback (federal ITC / state rebates as a planning knob). Eligibility, income caps, and domestic-content rules change - replace the dial with your actual net installed quote before you sign. Permits, transfer switches, and fire clearances belong to a licensed installer and AHJ, not this page.
Demand charges and tiered blocks can dominate commercial bills but are not modeled here. Residential super-off-peak windows (midnight–6 a.m.) may not match when you can actually charge from grid or solar - real arbitrage needs a schedule, not only a rate spread.
05
Example A - Wall-class + essentials. Tap the 13.5 kWh preset; keep Essentials (~0.8 kW).
Usable ≈ 13.5 × 0.9 × 0.9 = 10.9 kWh · Runtime ≈ 10.9 ÷ 0.8 = 13.6 hours. Switch to Whole-home avg at 30 kWh/day → load ≈ 30÷24 = 1.25 kW → runtime ≈ 8.7 hours. Same sticker, very different story.
Example B - Peak evening stress. Same 13.5 kWh pack; select Peak evening (~4 kW).
Runtime ≈ 10.9 ÷ 4 = 2.7 hours - enough to finish dinner and keep partial cooling, not a full overnight whole-home plan. That gap is the point of the clock.
Example C - Steep TOU vs flat. Keep the wall-class pack; compare Steep TOU ($0.42 / $0.12) to Flat ($0.16 / $0.16).
Shifted kWh/day ≈ min(usable, 0.35 × daily kWh) → at 30 kWh/day, cap ≈ 10.5 kWh/day shifted · Spread $0.30/kWh → daily savings ≈ $3.15 → ~$1,150/yr before incentives. On flat rate, spread = $0 → payback n/a, but essentials runtime stays ~13.6 h.
06
07
08
Not fully. Usable energy is roughly capacity × depth-of-discharge × round-trip efficiency. A 13.5 kWh pack with 90% DoD and ~90% RTE is closer to ~11 kWh delivered to loads. The panel shows nameplate vs usable on purpose so backup-hour math is not inflated.
It answers how many hours the usable pack can cover a chosen continuous load - whole-home average, essentials only, or essentials+AC. That resilience readout is why many households buy storage even when simple TOU payback looks long. It is not a guarantee of inverter behavior or generator transfer timing.
Not necessarily. Bill arbitrage (charge cheap / discharge expensive) is only one job. Backup during outages, solar self-consumption under poor export rates, and rate-plan eligibility can dominate the decision. Treat payback as the bill story; treat runtime as the resilience story.
Daily shifted kWh × (peak − off-peak) × 365, capped by usable capacity and a planning share of daily usage you can actually move. Real savings depend on your tariff, weather, and how the battery is programmed. Paste your utility’s peak and off-peak rates for a tighter sketch.
No. Installed cost, federal ITC eligibility, and state rebates vary by year, installer, and whether the battery pairs with new solar. The incentive % field is a planning dial - replace it with your actual net after credits before you sign.
It sketches storage economics and outage hours. Panel count, inverter limits, interconnection, and fire code setbacks need a site survey. Pair with the solar ROI and home energy tools for generation-side planning, then get a licensed installer quote.
LIVE STORAGE SKETCH
Nameplate ≠ usable ↓Pack presets
Critical-load profile
Rate spread
Updates live. Nameplate ≠ usable. Planning sketch — not an installer quote.
Critical-load runtime
13.7 h
10.9 kWh usable · 0.80 kW (Essentials) · DoD 90% · RTE 90%
RUNTIME CLOCK (usable ÷ load)
Clock fills toward 24h of cover. Switch Essentials → Whole-home to see why “one Powerwall for the whole house” often disappoints.
NET COST
$10,850
after 30% ($4,650)
TOU PAYBACK
9.4 yr
$1,150/yr arbitrage
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