Why a 72-hour model is different from a simple runtime estimate
A battery-only runtime estimate answers one useful question: how long stored energy lasts at an average load. A multi-day solar outage is more complicated because the battery can discharge overnight, support part of the daytime load, recharge from surplus solar and then enter the next night at a very different state of charge.
The simulator below uses 15-minute calculation steps. You can give each outage day a different solar multiplier, so a weak storm day followed by partial recovery does not have to be treated like three identical days.
Will the battery make it through three changing solar days?
This simplified time-step model follows battery state of charge across 72 hours. It serves the backed-up load first, uses solar directly when available, charges from surplus solar subject to the system charge-power limit, and records the first time the battery can no longer fully support the modeled load.
Start with a battery from the Home Solar Atlas catalog
Selecting a model loads its usable capacity and, where directly verified, its current per-unit continuous charge-power limit. Other outage assumptions remain unchanged.
Stress-test changing weather instead of assuming every day is identical
Each percentage scales the entered base solar power during that day's effective solar window. 100% means the full base assumption; 25% means one quarter of it.
How many full battery units pass this exact weather scenario?
The same load, starting SOC, efficiencies, solar timing and three daily weather multipliers are re-run from one through ten full battery units. If a verified per-unit catalog charge limit is active, charging power scales with the candidate battery count; a manual system limit stays fixed.
| Battery units | Usable storage | Charge limit used | 72-hour result | Lowest SOC | End SOC |
|---|---|---|---|---|---|
| 1 | 13.5 kWh | 5.0 kW | Interrupts at 8 h 6 min | 0% | 0% |
| 2 | 27.0 kWh | 5.0 kW | Interrupts at 23 h 26 min | 0% | 0% |
| 3 | 40.5 kWh | 5.0 kW | Interrupts at 31 h 32 min | 0% | 0% |
| 4 | 54.0 kWh | 5.0 kW | Interrupts at 44 h 11 min | 0% | 0% |
| 5 | 67.5 kWh | 5.0 kW | Interrupts at 52 h 17 min | 0% | 0% |
| 6 | 81.0 kWh | 5.0 kW | Interrupts at 69 h 47 min | 0% | 0% |
| 7 | 94.5 kWh | 5.0 kW | Pass | 10% | 10% |
| 8 | 108.0 kWh | 5.0 kW | Pass | 22% | 22% |
| 9 | 121.5 kWh | 5.0 kW | Pass | 30% | 30% |
| 10 | 135.0 kWh | 5.0 kW | Pass | 37% | 37% |
“Minimum passing” means only that this simplified energy simulation serves the modeled average critical load for 72 hours. It does not verify continuous output, motor-start surge, inverter compatibility, code requirements or manufacturer limits on the number and configuration of batteries.
Which current catalog battery needs the fewest full units for this exact outage scenario?
Every battery is rerun against the same starting SOC, average critical load, solar window, three-day weather sequence and efficiencies. Verified per-unit charge limits scale with battery count. The comparison also shows how many units are needed to keep a user-selected minimum SOC floor instead of merely reaching the end of the 72-hour scenario.
| Battery | Minimum pass | Units for SOC floor | Usable storage at minimum | Charge limit at minimum | Installed continuous output | Lowest SOC | Charge-data basis |
|---|---|---|---|---|---|---|---|
| Tesla Powerwall 3 | 7 | 8 for ≥20% | 94.5 kWh | 35.0 kW | 80.50 kW | 10% | Verified per-unit limit |
| Enphase IQ Battery 5P | >10 | >10 for ≥20% | - | - | - | - | Explicit comparison fallback |
| FranklinWH aPower 2 | 6 | 8 for ≥20% | 90.0 kWh | 48.0 kW | 60.00 kW | 6% | Verified per-unit limit |
What do the minimum-pass and reserve-floor configurations cost using your installer quote?
Enter the installed price for the first battery and, when multiple units may be needed, the installed incremental price for each additional battery. Home Solar Atlas does not prefill MSRP or market-price assumptions here.
| Battery | Minimum-pass config | Minimum-pass cost | Cost / usable kWh | ≥20% SOC config | Reserve-floor cost | Added cost for reserve margin |
|---|---|---|---|---|---|---|
| Tesla Powerwall 3 | 7 units · 94.5 kWh | Enter quote pricing | - | 8 units · 108.0 kWh | Enter quote pricing | - |
| Enphase IQ Battery 5P | No pass ≤10 | - | - | No ≥20% floor ≤10 | - | - |
| FranklinWH aPower 2 | 6 units · 90.0 kWh | Enter quote pricing | - | 8 units · 120.0 kWh | Enter quote pricing | - |
Pricing entered here stays only in this page state and is not added to the shareable outage URL. Imported Quote Analyzer, Quote Compare, Battery Quote Calculator or Compare Battery Quotes package pricing stays in browser session storage and is not extrapolated to other battery counts. The configurable estimate below assumes the first-unit installed price plus the same entered incremental installed price for each additional full battery. If an installer quote uses a different multi-unit discount, gateway allocation or bundled scope, enter figures that reflect that quote instead of treating this as a market-price estimator. Incentives, financing, tax treatment and future operating costs are not modeled here.
The SOC-floor count is still a simplified energy screen, not a guaranteed emergency reserve. This comparison is not a product ranking and does not establish total installed cost unless you enter your own quote pricing below, expansion architecture, code compliance, inverter compatibility, whole-home transfer design, motor-start capability or warranty suitability. A row using the fallback assumption should be rerun with current manufacturer or installer data before making an equipment decision.
How much base solar power is needed for this exact battery and load scenario?
The simulator searches from 0 to 30 kW of average available AC solar during a 100% solar window in 0.1 kW steps. Battery capacity, starting SOC, load, efficiencies, solar timing, charge-power limit and the three daily weather multipliers stay fixed.
| Weather stress test | Daily multipliers | Minimum base solar | Threshold lowest SOC |
|---|---|---|---|
| Stormy stretchVery weak solar for two days, then partial recovery. | 20% · 25% · 40% | No pass ≤30 kW | - |
| Mixed weatherA moderate first day, cloudy second day and stronger third day. | 60% · 35% · 80% | No pass ≤30 kW | - |
| Clear weatherUses the full entered solar assumption on all three days. | 100% · 100% · 100% | No pass ≤30 kW | - |
The weather table uses a coarser 0.25 kW search step for quick sensitivity screening. The custom sequence above uses 0.1 kW. A lower threshold does not establish a final PV array size because real outage PV follows a changing irradiance curve and is constrained by roof, inverter, islanding and weather conditions.
How much average backed-up load can this battery + solar scenario sustain?
The calculator searches average critical load from 0.1 to 20 kW in 0.1 kW steps while keeping storage, starting SOC, solar power, solar timing, weather multipliers, efficiencies and the charge-power limit fixed.
The load budget is a constant-average screening result. Real refrigerators, HVAC, pumps and other circuits cycle and surge, so a 1.0 kW average load is not equivalent to a flat 1.0 kW electrical demand at every moment.
Which backup circuits could turn a failed three-day scenario into a pass?
The advisor reruns the same 72-hour simulation after removing combinations of selected planning circuits. It first minimizes the number of shutdowns, then prefers the passing option that removes the least average load.
Choose which loads are actually eligible for shedding
Generic values are screening assumptions. When this page is opened from a saved outage-planner scenario, adjusted preset values and any custom load are restored from the URL.
Circuit shedding is an energy-planning screen. Turning off a circuit may be impractical or unsafe depending on weather, medical needs, water systems, refrigeration, building conditions and household priorities. The generic average and peak values are not a substitute for measured loads or equipment documentation.
Where the battery gains and loses energy
Solar serves the critical load first. Only surplus solar can charge the battery, and charging is capped by the entered system power limit.
| Outage day | Solar assumption | Start SOC | PV generated | Solar direct to load | Stored from solar | Surplus not stored | Battery to load | Lowest SOC | End SOC | Unserved load |
|---|---|---|---|---|---|---|---|---|---|---|
| Day 160% of base solar | 60% | 100% | 12.0 kWh | 6.0 kWh | 5.4 kWh | 0.0 kWh | 17.0 kWh | 0% | 0% | 13.0 kWh |
| Day 235% of base solar | 35% | 0% | 7.0 kWh | 6.0 kWh | 0.9 kWh | 0.0 kWh | 0.8 kWh | 0% | 0% | 29.2 kWh |
| Day 380% of base solar | 80% | 0% | 16.0 kWh | 6.0 kWh | 9.0 kWh | 0.0 kWh | 8.1 kWh | 0% | 0% | 21.9 kWh |
This is a simplified 15-minute time-step screening model, not an outage forecast or equipment dispatch controller. Solar is represented as a flat average power level during each effective solar window rather than an irradiance curve. It does not model temperature, battery degradation, high-SOC charge taper, dynamic household loads, inverter clipping, generator operation, manufacturer-specific reserve logic or every multi-battery power-sharing rule. A simulated pass is not a guarantee of real outage performance.
Compare real batteries under the same 72-hour scenario
The catalog comparison reruns Tesla Powerwall 3, Enphase IQ Battery 5P and FranklinWH aPower 2 with the same starting SOC, average load, solar window, efficiencies and three-day weather sequence. It finds the minimum full-unit count that maintains the modeled average load and shows usable storage, charge limit used, installed continuous output at that count, lowest SOC and ending SOC.
This is not a product ranking. Tesla and FranklinWH currently have directly verified charge-power records in the Home Solar Atlas dataset. For Enphase IQ Battery 5P, the current source review has a separate charge-power data gap, so the comparison visibly uses its explicit comparison fallback rather than inferring a manufacturer value.
Test the battery that is actually in your installer quote
Quote Analyzer can now hand a supported battery model and full-unit count directly into this resilience tool. The imported configuration is evaluated as quoted: the page shows its usable kWh, pass/fail result, lowest SOC and unit gap versus both the minimum-pass count and the currently selected reserve-floor count.
If Quote Analyzer has a positive battery portion price, that total stays out of the URL. It is carried only through browser session storage in the current tab when available and is shown as the quoted package total and quoted cost per usable kWh. Home Solar Atlas does not split that total into first-unit and incremental-unit prices or extrapolate it to a different quantity.
Use your installer quote instead of a made-up battery price
The resilience economics section does not contain a Home Solar Atlas MSRP, market-price estimate or hidden default. You enter the installed first-battery price and the installed incremental price for each additional battery from the quote you actually received. The calculator then applies those values to the minimum-pass configuration and to the stricter configuration that retains your selected SOC floor.
This first-unit-plus-incremental model is useful when the first installed battery carries gateway, controller, setup or labor costs that do not repeat at the same level for every additional unit. If your quote uses a different bundled structure, enter figures that reflect that specific quote. User-entered pricing stays in page state and is not added to the shareable outage URL.
Four ways to make the same outage scenario pass
The simulator deliberately separates four decisions. Battery sizing asks how many full units are needed at the current load and solar assumptions. Solar sizing keeps the current storage fixed and searches for the modeled average AC solar threshold. Critical-load budgeting keeps the equipment and weather fixed and shows how much average backed-up load the system can sustain. Circuit shedding translates part of that load budget into named planning circuits that can be tested as temporary shutdown candidates.
This makes demand reduction visible as a real resilience option instead of assuming the only answers are buying more storage or adding more PV.
What “minimum passing battery count” means
The battery sizing table reruns the exact selected 72-hour scenario from one through ten full battery units and identifies the first count that never leaves the modeled critical load unserved. For catalog batteries with a directly verified per-unit charging limit, charge power scales with full battery count. If you enter a manual system-level charge limit, that limit stays fixed while storage capacity changes.
A pass here is intentionally narrower than a complete battery-system recommendation. The count must still be checked against continuous output, short motor-start surge, inverter and gateway compatibility, manufacturer expansion limits, electrical code and installer design requirements.
What “minimum passing base solar” means
The solar sizing screen keeps the selected battery system and outage assumptions fixed, then searches from 0 to 30 kW of modeled average AC solar power during a 100% effective solar window. It reports the first value that lets the 72-hour simulation maintain the critical load. The calculator also repeats a coarser version of that search for stormy, mixed and clear three-day weather sequences.
This value is not a rooftop PV system-size recommendation. A real solar array is rated in DC nameplate kW and its AC output changes continuously with irradiance, roof orientation, temperature, shading, inverter limits and weather. Use the result only as a resilience-screening threshold for the assumptions entered in the simulator.
From a load budget to specific circuits
The load-budget screen first tells you whether the current average backed-up demand is above the modeled 72-hour threshold. The circuit advisor then tests combinations of named planning loads — such as HVAC, EV charging, a washer, extra freezer or other selected circuits — and lists combinations that make the same 72-hour scenario pass.
When the 3-day simulator is opened from a saved outage-planner URL, it restores selected preset circuits, edited average/peak values and a custom load. On the standalone page it starts with generic essential-load examples, and you can add other planning circuits. Any portion of the current average load that is not represented by named circuits remains in the simulation as unassigned load.
What “maximum sustainable critical load” means
The load-budget screen searches average backed-up load from 0.1 to 20 kW while keeping the current battery, starting SOC, solar power, solar timing, weather sequence, efficiencies and charge-power limit unchanged. If the entered load fails, the difference between the current load and the sustainable threshold is a simple estimate of how much average demand would need to be removed from the 72-hour energy model.
This is an average-energy budget, not an electrical power rating. Refrigerators, HVAC systems, pumps and other loads cycle and can have startup surges, so use the full outage planner to verify continuous and short-duration power separately.
How to build a conservative outage scenario
- Use the battery's usable capacity rather than its nominal cell capacity.
- Set the starting SOC to what the battery may realistically have when the outage begins.
- Model only the loads you truly plan to keep powered during a long outage.
- Use a conservative effective solar window rather than full sunrise-to-sunset hours.
- Reduce Day 1 and Day 2 solar assumptions for a storm-resilience test.
- Use the current battery-system charging limit when it is known instead of assuming all surplus PV can enter storage instantly.
- Compare battery count, catalog alternatives, modeled solar threshold and load reduction rather than treating one lever as the only answer.
- Mark only circuits that are realistically eligible for temporary shedding, and replace generic values with measured or manufacturer data where possible.
- Use actual installer-quote pricing if you compare configuration cost; do not treat the economics inputs as market-price guidance.
- After finding a passing energy scenario, use the full outage planner to check continuous power and motor-start surge separately.
3-day solar battery outage FAQ
Can a home battery last through a three-day outage?
It depends on installed usable battery capacity, starting state of charge, the average backed-up load, solar production during the outage, conversion losses and whether the battery can recharge fast enough during each solar window. A three-day simulation is more informative than dividing battery kWh by one constant load when solar is available.
Why does the first night matter so much for solar battery backup?
An outage that begins after useful solar production has ended may require the battery to carry the entire critical load until the next solar window. If the battery reaches zero before morning, strong solar later in the day cannot prevent that earlier interruption.
Does more solar always recharge a home battery faster during an outage?
No. Solar must first serve the backed-up household load, and the remaining power can still be limited by the battery or energy-storage system maximum charging power. Extra PV above that limit may not increase battery charging speed in the simplified scenario.
How does the calculator estimate the minimum number of batteries?
The sizing screen repeats the same 72-hour load, solar, efficiency and weather assumptions with one through ten full battery units. It reports the first unit count that maintains the modeled critical load for the entire simulation. This is an energy-resilience screen and does not replace continuous-power, motor-start surge or electrical-design checks.
Can I compare Powerwall 3, Enphase IQ Battery 5P and FranklinWH aPower 2 in the same outage scenario?
Yes. The catalog comparison reruns all three products against the same 72-hour load, starting SOC, solar timing, weather sequence and efficiency assumptions, then reports the minimum full-unit count that passes for each. Verified per-unit charging limits are used where available; a clearly labeled manual system charging fallback is used instead of inventing a value when the current source review has a charge-power data gap.
Does the battery comparison use estimated market prices?
No. Home Solar Atlas does not prefill MSRP or market-price assumptions in the resilience economics screen. You can enter the installed price for the first battery and the installed incremental price for each additional battery from your own installer quote. The tool then compares the cost of the minimum-pass and selected SOC-floor configurations using only those user-entered values.
Can I send a battery from Quote Analyzer into the 3-day outage test?
Yes. After reviewing the quote, choose the exact supported battery model and full-unit count in Quote Analyzer. The 3-day tool opens with that equipment configuration and evaluates the quoted unit count against the current 72-hour scenario. The battery package price is kept out of the URL and, when browser session storage is available, is carried only in the current tab so the imported quote block can show package cost per usable kWh without extrapolating that total to another battery count.
How does the calculator estimate the minimum solar power for three days?
The solar sizing screen holds the current battery capacity, starting charge, load, efficiencies, charging limit, solar timing and three daily weather multipliers constant, then searches upward from zero average AC solar power until the 72-hour model first maintains the critical load. The result is an outage-screening AC assumption, not a recommended rooftop DC array size.
How much critical load can a solar battery support for 72 hours?
The critical-load budget keeps the selected battery, starting charge, solar assumptions, weather sequence, efficiencies and charging limit fixed, then tests average backed-up load in 0.1 kW steps. It reports the highest average load that completes the 72-hour energy simulation without unserved demand. Instantaneous continuous power and motor-start surge still require separate checks.
Can the calculator tell me which backup circuits to turn off?
Yes, as a planning screen. The circuit-shedding advisor tests combinations of selected named backup loads against the same 72-hour simulation and prefers the smallest number of shutdowns that creates a passing energy scenario. Generic circuit values should be replaced with measured or manufacturer values when available, and essential or safety-critical loads should not be removed solely because a calculator suggests it.
What this simulator intentionally does not promise
The model is a screening tool, not an outage forecast. It uses a flat average solar level during each effective solar window rather than a location-specific irradiance curve. Real results can change with clouds, shading, temperature, inverter behavior, battery controls, changing appliance demand, charge taper near high SOC, backup reserve rules and whether the solar-plus-storage system is actually configured to operate while the grid is down. A suggested circuit shutdown is not a safety recommendation; household priorities and essential loads remain the homeowner's responsibility. Quote-based economics do not model incentives, tax treatment, financing, maintenance or future replacement cost.