DeviceWattsHours/dayUnits
Days without powerhow many days you want to last
Daily use0 Whwhat your devices use in 24 h
Minimum battery0 Whwith a 20% margin for losses
Output power0 Weverything on at once, with margin
Solar panel0 Wto recharge one day of use (average of 4 h of useful sun in Spain)
Approximate calculation. The watts are typical figures: check the label on your devices and change them if needed. A fridge starts up with a peak of up to 4-6 times its average consumption. For medical equipment (CPAP, oxygen concentrator) confirm the consumption with the manufacturer and always have a plan B.
How it’s calculated
- Daily use (Wh) = watts × hours per day × units, added up for all devices.
- Battery = daily use × days + a 20% margin: the inverter loses energy converting to 230 V and it’s best not to drain the battery completely.
- Output power (W) = what they all use at once + a 25% margin. If you include a fridge, its start-up peak is added.
- Solar panel = daily use ÷ (4 hours of sun × 0.75). We use 4 hours of useful sun, the average in Spain between winter (about 3) and summer (about 5); the 0.75 covers clouds, angle and charging losses.
Torn between models? See the portable power station comparison, with the pros and cons of each.
Keep it on paper for when there's no power or internet.