In this article
- Two numbers, two different jobs
- Estimate with breathing room
- Measure the awkward devices
- Choose what stays on
- Plan the next charge
- Make a small energy budget you can check
- Show the uncertainty instead of hiding it
- Recharging belongs in the same worksheet
- Use the calculation to remove low-priority loads
Source-based guide created with AI assistance. Calculations are illustrative; equipment selections are researched, not hands-on tests. How we work.
Two numbers, two different jobs
Watts (W) describe power at a moment in time. Watt-hours (Wh) describe an amount of energy. A steady 50 W load running for ten hours uses 500 Wh. A battery’s Wh rating does not tell you whether its inverter can start or run a particular appliance. Check continuous output, surge output, voltage, frequency, and connector compatibility separately.
Estimate with breathing room
Our calculator divides usable battery energy by a continuous device load. Its default 85% usable-energy assumption leaves a rough allowance for conversion losses and reserve. This is a planning estimate, not a measured product test. Battery age, temperature, inverter idle draw, and load behaviour change the result.
Measure the awkward devices
A refrigerator cycles, and a compressor may need a high starting surge. A charger’s label can show its maximum input rather than typical consumption. Measure energy over a representative period with a suitable meter when possible. Use manufacturer guidance for devices where interruption has serious consequences.
Choose what stays on
Separate essential communication and lighting from comfort loads. Heating, kettles, and cooking appliances can consume a small battery quickly. Build a simple schedule: which devices need to run continuously, and which can be charged in short sessions? Avoid using a planning estimate as a guarantee for critical medical equipment.
Plan the next charge
Solar recharge depends on daylight, cloud, shade, panel orientation, and compatible electrical limits. A storm can reduce output precisely when you need it. Have an alternative recharge or relocation plan. If you use a fuel generator, operate it outside, away from building openings, following its instructions and official safety guidance.
Make a small energy budget you can check
The EIA explanation of electricity units distinguishes power from energy. Apply that distinction to a daily schedule rather than adding up the biggest numbers printed on chargers. The example below uses invented steady loads to show the calculation; it is not measured performance for any product.
| Task | Assumed use | Energy |
|---|---|---|
| Network equipment | 20 W for 6 hours | 120 Wh |
| Small light | 5 W for 4 hours | 20 Wh |
| Charging session | 10 W average for 2 hours | 20 Wh |
| Total delivered energy | These tasks only | 160 Wh |
At an assumed 85% usable-energy fraction, 160 ÷ 0.85 is about 188 Wh of rated battery energy. That is a calculation input for comparison, not a recommendation to buy an exactly sized pack. Actual conversion losses, inverter idle draw, charging behavior and battery condition can change the result. Check whether all tasks can be supplied from the available ports.
Show the uncertainty instead of hiding it
For a hypothetical 500 Wh battery and a steady 50 W load, an 85% usable fraction gives 8.5 hours. At 70% it gives seven hours. Neither assumption is a measured specification. Showing both helps you see whether the plan still works when less energy reaches the device than you first expected.
A device cycling on and off adds another uncertainty. If a compatible meter records 0.8 kWh during a representative 24-hour period, that is 800 Wh for the day, equivalent to about 33 W averaged over that period. It does not mean the appliance draws only 33 W while running or starting. Its instantaneous requirement may be much higher. A battery can satisfy the energy calculation and still fail the starting-power check.
Use a meter within its ratings and follow its instructions. Observe a representative period rather than a single quiet minute. Conditions during an outage may differ from your measurement, especially for refrigeration in a warm room. Keep the original measurement and the assumptions beside your calculation so that you can revise the plan intelligently.
Recharging belongs in the same worksheet
Write down where the next charge could come from, how you will reach it and what equipment it needs. A solar panel’s rated output is not energy delivered all day. Shade, weather, orientation and the station’s input limits affect recharge. A theoretical 200 Wh of energy at a steady 100 W takes two hours before losses; changing sunlight means the real process can take longer or fail to complete.
Use only manufacturer-compatible charging connections. If your alternative involves a fuel generator, follow CDC generator safety guidance: operate it outdoors at least 20 feet from doors, windows and vents. Never run one indoors or in a garage. A generator is a separate safety and installation decision, not simply another charging cable.
Use the calculation to remove low-priority loads
When the budget is too large, first shorten or remove optional tasks. A phone can often charge in a session rather than occupying the AC inverter all day. A network connection may be unnecessary overnight or unavailable upstream. Write those choices into a schedule and practice it. Then use the runtime calculator to compare the revised load, with the same caveats. For equipment whose interruption could endanger someone, a general energy budget is only one input to a professionally informed contingency plan.
Sources & further reading
Editorial guidance, not a local emergency warning. Follow your national and local authorities.
