Solar + Battery Backup Calculator — See Exactly How Long Your Battery Lasts During a Power Outage

A solar and battery backup calculator tells you how many hours your home battery will power your selected appliances during a grid outage, and whether your solar panels can recharge the battery fast enough to sustain indefinite island operation.

Enter your battery capacity, daily solar recharge estimate, and the quantity of each critical appliance you want to keep running — the calculator returns your total load in watts, battery duration in hours, net daily energy surplus or deficit, and an island sustainability status showing whether your system achieves indefinite, stable, or limited backup power.

How to Use the Solar + Battery Backup Calculator

Step 1 — Enter your usable battery capacity.

Type your battery system’s usable kilowatt-hour capacity. Use the manufacturer’s specified usable figure — not the total nameplate capacity. The Tesla Powerwall 3 provides 13.5 kWh of usable capacity. The Enphase IQ Battery 5P provides 5.0 kWh per unit. Two stacked Powerwalls provide 27 kWh. Franklin Electric apower 2 provides 13.6 kWh.

If you have multiple batteries, add their usable capacities together and enter the combined total. Battery capacity is the single most impactful variable in the calculation — doubling your battery capacity doubles your backup duration at the same load.

Step 2 — Enter your daily solar recharge estimate.

Type how many kilowatt-hours your solar system will generate during a typical outage day. This is not your system’s peak production on a perfect sunny day but a realistic average — accounting for cloud cover, winter sun angles, and the fact that outages often coincide with storms.

A conservative estimate for backup planning uses 50–70% of your system’s normal daily production. If your 8 kW system normally produces 32–40 kWh/day in summer, use 15–20 kWh for outage planning. In the Pacific Northwest or during winter in northern states, use 5–10 kWh even for reasonably sized systems. This figure determines whether your solar can outpace your critical load consumption — the key to achieving indefinite island operation.

Step 3 — Set your appliance quantities.

The calculator pre-loads seven common household critical appliances with their typical wattage ratings and suggested default quantities. Adjust the quantity for each item to match what you actually plan to run during an outage.

The refrigerator defaults to 1 at 150W — most households have one refrigerator that must stay powered. Room lights default to 2 at 60W each — representing LED fixtures in key living spaces. Internet and Wi-Fi defaults to 1 at 30W — keeping communications online. Phones and laptops default to 1 at 100W — covering device charging for the household.

TV and entertainment, sump pump, and gas furnace fan default to 0 — these are optional loads you choose to include based on your specific resilience priorities.

For appliances not in the list, add their wattage to your quantity estimate — a 600W gas furnace fan and an 800W sump pump are both included for exactly this reason. Use the appliance’s label wattage or manufacturer specification. Set any item to 0 to exclude it from your critical load entirely.

Step 4 — Read the three result cards.

The Outage Duration card shows how many hours your battery will sustain your selected load without any solar recharging — pure battery duration based on your entered capacity and total watt load. If this exceeds 48 hours it displays as greater than 2 days.

The Island Status card shows one of three verdicts. INDEFINITE in green means your solar daily recharge equals or exceeds your daily consumption — your system can sustain island operation as long as the sun shines. Stable in blue means your battery lasts more than 24 hours even without solar — enough for most short outages. Limited in red means your battery drains faster than solar can replenish it and will eventually deplete.

The Total Load card shows your constant simultaneous power draw in watts — the combined wattage of all entered appliances at their entered quantities.

Step 5 — Study the daily energy balance visualization.

A horizontal bar shows your daily energy pool divided into two segments. The purple segment represents your battery storage capacity. The amber segment represents your daily solar recharge. Together they show your total daily energy resource.

Below the bar, three data points complete the picture: hourly consumption in kWh, 24-hour total consumption in kWh, and the critical net daily surplus or deficit. A positive surplus means solar more than covers daily consumption and your battery stays charged or grows. A negative deficit means solar falls short and your battery depletes by that amount each day — countdown to full depletion calculable by dividing battery kWh by the daily deficit.

Step 6 — Experiment with load reduction.

The most powerful insight from this calculator is understanding which appliances dominate your critical load. The sump pump at 800W and gas furnace fan at 600W each add significantly more load than the refrigerator, lights, and internet combined. Setting high-wattage appliances to 0 and recalculating immediately shows how much backup duration each appliance costs you — helping you make informed decisions about which loads are worth including in your critical backup circuit.

Step 7 — Export your analysis.

Click Export PDF Report to save a printable backup power analysis — useful when working with an electrician to design your critical load panel, discussing system sizing with a solar installer, or creating a household emergency preparedness plan.

The Battery Backup Duration Formula Explained

Total load: Total watts = Sum of (appliance watts × quantity) for all selected appliances

Hourly consumption: Hourly kWh = Total watts ÷ 1,000

Battery duration (no solar): Duration (hrs) = Battery capacity (kWh) ÷ Hourly kWh

24-hour consumption: Daily kWh = Hourly kWh × 24

Net daily surplus or deficit: Surplus/deficit = Solar recharge (kWh) − Daily consumption (kWh)

Island status thresholds: INDEFINITE = Net surplus ≥ 0 (solar covers daily consumption) Stable = Duration > 24 hours (battery lasts more than a day) Limited = Duration ≤ 24 hours and net deficit exists

Example — 13.5 kWh battery, 15 kWh solar, refrigerator + 2 lights + Wi-Fi + phones:

• Total load = 150 + (60×2) + 30 + 100 = 400W
• Hourly kWh = 400 ÷ 1,000 = 0.40 kWh/hr
• Duration = 13.5 ÷ 0.40 = 33.75 hours
• Daily consumption = 0.40 × 24 = 9.6 kWh
• Net surplus = 15 − 9.6 = +5.4 kWh/day
• Status = INDEFINITE — solar exceeds daily load

Frequently Asked Questions

Q: What is the difference between battery backup and a whole-home standby generator?

A: Both keep your lights on during an outage, but they work very differently and suit different resilience needs.

A whole-home standby generator — the natural gas or propane units from Generac, Kohler, or Briggs & Stratton — connects to your home’s main electrical panel and can power your entire home including central air conditioning, electric ranges, and other high-demand appliances. Generators turn on automatically within seconds of a grid failure and run continuously as long as fuel is available.

A typical 22 kW whole-home generator costs $5,000–$12,000 installed and runs on utility natural gas or a propane tank.

A solar-plus-battery backup system powers a defined critical load panel — only the circuits you designate as critical during outage mode. It operates silently, requires no fuel, and recharges automatically from solar during the outage.

The transition to backup power is seamless — no startup delay, no noise, no exhaust. The limitation is capacity: a 13.5 kWh battery cannot run central AC or an electric range without rapidly depleting. For most homeowners, a battery backup handles normal power outages lasting hours to a day, while a whole-home generator is better suited for extended multi-day outages in severe weather seasons.

Q: Can my solar panels charge my battery during a power outage?

A: Yes — but only if your system is specifically designed to allow it, which requires either a hybrid inverter or a dedicated backup-capable battery system.

Standard grid-tied solar inverters without battery storage shut down completely during a grid outage due to the anti-islanding safety requirement. Adding a battery system with a hybrid inverter — such as a Tesla Powerwall, Enphase IQ System, or SolarEdge Energy Hub — creates a separate microgrid on your critical load panel that allows solar to charge the battery and power critical loads even when the grid is down.

The solar recharging capability is what transforms a battery from a finite backup resource into a potentially indefinite one. Without solar recharging, a 13.5 kWh battery powering a 400W critical load lasts approximately 34 hours. With 15 kWh/day of solar recharging against a 9.6 kWh/day consumption, the battery actually gains charge each day — creating a sustainable island that can theoretically last as long as the outage continues and the sun keeps shining.

Q: How many Tesla Powerwalls do I need for whole-home backup?

A: The answer depends on what you define as whole-home backup and what loads you consider essential.

A single Powerwall 3 at 13.5 kWh can sustain basic critical loads — refrigerator, lights, internet, and device charging — for 24–48 hours without solar recharging. With average solar recharging, a single Powerwall can sustain this basic load indefinitely. A single Powerwall cannot run central air conditioning, electric water heaters, or electric ranges for meaningful durations.

Two Powerwalls at 27 kWh extend basic critical load coverage significantly and can support moderate HVAC use during daytime solar production. Three or more Powerwalls — 40+ kWh — provide genuine whole-home backup capability for most households excluding electric heating, electric cooking, and EV charging.

Tesla’s own recommendation for whole-home backup for a typical US home with gas cooking and heating is two to three Powerwalls. For all-electric homes, four or more units may be necessary for comfortable whole-home operation.

Q: What appliances should I prioritize on my critical backup circuit?

A: When designing your backup circuit with an electrician, prioritize appliances in order of safety and food security first, then comfort and communications.

Refrigerator and chest freezer come first — food spoilage begins in four hours at ambient summer temperatures and represents real financial loss and health risk. Well pump or sump pump comes next for homes dependent on well water or with basement flooding risk — an 800W sump pump running intermittently during a storm event is non-negotiable for many homeowners.

Medical equipment — CPAP machines, oxygen concentrators, insulin refrigerators — must be on the backup circuit for households with medical needs.

Next priority is communications — internet router and modem at 30W provide access to emergency information and remote work capability during extended outages. Device charging for phones and laptops at 100W total keeps the household connected. Lighting at 60W per room for two or three rooms covers safety and livability.

HVAC is the most power-hungry consideration — a gas furnace fan at 600W is feasible for backup; central air conditioning at 3,000–5,000W is extremely difficult to sustain on residential battery systems and is typically excluded from critical backup circuits.

Q: How does battery efficiency affect backup duration?

A: Round-trip efficiency — the percentage of energy you put into the battery that you actually get back out — directly reduces effective backup duration and is assumed at 90% in this calculator’s design.

A 13.5 kWh battery at 90% round-trip efficiency stores 13.5 kWh but returns only 12.15 kWh of usable energy per complete charge-discharge cycle. The remaining 1.35 kWh is lost as heat in the battery’s internal chemistry and power electronics during charging and discharging. This efficiency loss applies every cycle — meaning a battery that cycles daily for a year loses approximately 492 kWh to heat annually at 90% efficiency.

Higher-efficiency batteries — the best lithium iron phosphate systems achieve 95–97% round-trip efficiency — extend backup duration and reduce long-term energy losses. The practical difference between 90% and 95% efficiency on a 13.5 kWh battery is approximately 0.67 kWh per cycle — modest for a single cycle, but meaningful over thousands of cycles across the battery’s service life.