A 3-Day Outage
Needs a Real
Backup Plan.
Protect the essentials for 72 hours. Set a daily energy budget, size the battery, and decide whether reliable recharging is part of your plan.
Plan for food, water, communication and an alternative safe location as well as electricity. A portable battery alone does not establish that staying home is safe.
72-HOUR BACKUP PLANNER
Starting watts and hours are illustrative placeholders. Replace them with measured or manufacturer data. Hours/day mean equivalent operating hours; cycling loads do not run continuously.
Starts full. One recharge after day 1 and one after day 2. Input is net energy actually stored after charging losses, not panel watts or gross energy. This daily model does not verify overnight or hourly feasibility.
No-recharge target = daily raw kWh × 3 ÷ 0.90 × 1.20.
* Daily battery budget uses the same efficiency and margin. Assumptions: 90% efficiency + 20% planning reserve.
Three repeated daily schedules. A device used for five hours daily is counted for 15 hours across the outage.
Planning target for the router, lights, charging and laptop example. Device compatibility must be checked separately.
Includes the illustrative 1.20 kWh/day refrigerator load. Your actual fridge and environment may differ.
Three Days. Two Energy Strategies.
Carry the full 72-hour budget.
Start with a fully charged battery sized for all three days. This is the calculator’s default. No solar, generator or grid recovery is assumed.
- Reduce daily use before buying more capacity.
- Include inverter standby draw in your measured budget.
- Check rated capacity, usable output and temperature limits.
Replenish between use cycles.
A smaller battery can work only if it bridges the interval between dependable recharge sessions. The optional calculator checks a simplified day-by-day budget and caps each refill at battery capacity.
- Verify charging input limits and actual net stored energy.
- Check whether loads can run while the system charges.
- Keep a fallback for missed or reduced recharge.
What Goes Into the Battery Target?
Connection + light + laptop.
20 W router × 24 h; 20 W lights × 5 h; 10 W phone charging × 2 h; 60 W laptop × 3 h. Total: 0.78 kWh/day.
Essentials + cycling fridge.
Add 150 W × 8 equivalent run hours/day = 1.20 kWh/day. Total raw energy: 1.98 kWh/day or 5.94 kWh over three days.
Essentials + fridge + fan.
Add 45 W fan × 8 h/day = 0.36 kWh/day. Total raw energy: 2.34 kWh/day. Fan power and use are placeholders.
All three targets assume no recharging, divide raw energy by 0.90 and add a 20% planning margin. They do not include water pumps, HVAC, cooking, medical equipment or other unlisted loads.
Small Loads Add Up.
Heat Adds Up Faster.
Make every operating hour deliberate.
A 20 W router running all 72 hours uses 1.44 kWh before losses. An illustrative 1,500 W heater used eight hours per day consumes 36 kWh over three days, or a 48 kWh battery target with this method. Verify your equipment before planning.
Panel Watts Are Not
Stored Kilowatt-Hours.
A recharge plan must work with your hardware, site and weather. Enter net stored energy in the calculator, not a panel’s rated power.
400 W × 3 equivalent sun hours.
That equals 1.20 kWh of theoretical energy before losses. A hypothetical 70% net yield would store 0.84 kWh. Both the sun-hours and yield values are examples, not forecasts.
The essentials plan spends a 1.04 kWh daily battery budget with the planning margin included. A 0.84 kWh recharge would not fully replace it. Clouds, shading, input limits and charging downtime can reduce recharge further.
Do not subtract an optimistic three-day solar total from battery capacity without checking when the energy arrives. Test a zero-recharge case as your fallback.
Reassess at Every Day Boundary.
Run the priority schedule.
Check outage information and local instructions. Record battery state and device usage. Keep refrigerator doors closed and track food temperatures. Prepare recharge safely if part of your plan.
Use the measured budget.
Compare actual depletion and recharge against the model. Cut optional runtime when the energy budget falls short. Reassess refrigeration, water, indoor temperature and continuity of medical needs.
Keep a workable fallback.
If restoration is uncertain, budget for more time or move to a safe location. Follow local emergency instructions. Electricity alone does not address food, water or unsafe heat and cold.
A Closed Door Is
Not a 72-Hour Solution.
FDA guidance gives about four hours for an unopened refrigerator, approximately 48 hours for a full closed freezer, and 24 hours for a half-full closed freezer. These are general guidelines, not guarantees for your appliance or food.
For a three-day outage, plan active refrigeration, safe alternative cold storage or non-perishable food. Use appliance thermometers and follow official temperature and disposal guidance. Do not taste food to judge safety.
Read FDA food-safety guidance ↗The Math, Visible.
72-hour raw = 0.78 × 3 = 2.34 kWh
No-recharge battery target = 2.34 ÷ 0.90 × 1.20 = 3.12 kWh
Track the battery, not just the total.
Daily battery budget D = daily raw ÷ 0.90 × 1.20. Start with capacity C. For each day, require the starting balance to cover D, then subtract D. After days 1 and 2, add recharge R, capped at C. No recharge is credited after day 3.
A daily check is not a guarantee.
The model repeats the same load schedule and recharge value. It does not simulate hourly loads, partial starting charge, solar variability or charging overlap. Verify nighttime autonomy, hardware and an alternative plan separately.
90% efficiency and the 20% margin are Zaron planning assumptions. They are not universal constants or agency recommendations. Measured standby draw, battery degradation, temperature and conversion pathways may require a different budget.
Energy Covers Time. Power Starts the Load.
Check overlapping loads.
The planner shows the sum of selected operating watts as a conservative overlapping-load check. Confirm continuous inverter output and the loads that can run together.
Verify manufacturer specifications.
Compressor and motor loads can require startup headroom. No surge figures are invented here. Enter a verified peak total that includes other operating devices and check the supported peak duration.
Use appropriate connections.
Match voltage, connectors and output ratings. Household circuits require an appropriate professionally installed transfer arrangement. Never backfeed a wall outlet.
A Power Plan Is Part of a Safety Plan.
Temperature over assumptions.
Use official food-safety guidance and temperature measurements when an outage extends beyond the refrigerator or freezer holding period.
Fuel-powered recharge needs care.
Operate fuel-powered generators outdoors only, at least 20 feet from homes, with exhaust directed away. Follow manufacturer instructions and CPSC guidance.
Cover the wider outage.
Prepare supplies for several days, communication alternatives and plans for equipment that depends on electricity.
Straight Answers.
Can a 2 kWh battery cover three days?
It falls below the 3.12 kWh no-recharge target for the essentials example. It may fit a lower daily load budget or a dependable recharge plan, but verify actual usable energy, output ratings and recharge timing.
Does solar make a smaller battery sufficient?
Only if actual recharge and battery capacity support your load schedule through the intervals without input. Enter net stored energy in the optional model. A passing daily budget cannot establish weather reliability or overnight autonomy.
How does the optional recharge check work?
It starts at full proposed capacity, subtracts the daily budget, then adds one net recharge after day 1 and after day 2. Each recharge is capped at battery capacity. If a day begins with too little energy, it reports a shortfall before that day's recharge.
Should I enter 24 hours for my refrigerator?
Only if your entered watts represent a measured average over time. For operating watts while the compressor is on, use equivalent run hours. Do not apply both averaged watts and another duty-cycle reduction. This example uses 150 W × 8 equivalent hours/day as an illustrative placeholder.
Does this include pumps, heating or medical devices?
No. Add unlisted loads in the Full Backup Calculator. Medical equipment needs a separate continuity plan approved by your care team and equipment supplier, including required runtime and an emergency alternative.
What if the outage goes beyond three days?
Recalculate your budget for the longer duration and review actual recharge. Keep supplies and a relocation option ready. Follow local emergency instructions, especially when water, temperature or essential medical support is affected.