OFF-GRID PLANNING / BATTERY STORAGE
Turn a list of everyday loads into a storage target you can discuss with an installer or compare against a power station.
The battery should fit the life you want to live. Buying the biggest box you can afford is one approach. Knowing whether it covers the refrigerator, laptop and lights through a quiet night is a better starting point.
This guide helps you build an energy budget. It does not choose cable sizes, protection devices or a battery configuration for your property. Those decisions depend on the exact equipment and installation.
For each device, multiply its average watts by its hours of operation. Add the results to get daily watt-hours. Divide by 1,000 for kilowatt-hours. If those units are still getting acquainted, start with watts versus watt-hours.
Measure cycling equipment across a representative day when practical. A refrigerator does not normally draw its nameplate power for every hour it is plugged in. Include background loads, networking equipment and inverter idle consumption rather than assuming the cabin goes to sleep when you do.
These invented loads make the arithmetic visible. They are not typical consumption claims or a recommended cabin specification.
| Load | Assumed use | Energy |
|---|---|---|
| Laptop | 50 W × 6 hours | 300 Wh |
| Internet equipment | 20 W × 8 hours | 160 Wh |
| Lights | 20 W × 4 hours | 80 Wh |
| Fan | 30 W × 6 hours | 180 Wh |
| Refrigerator | Assumed measured daily total | 600 Wh |
| Daily appliance total | 1,320 Wh | |
Suppose you want two days without useful charging. You need 2,640 Wh delivered to those appliances. Now suppose, only for this exercise, the design permits use of 80% of nominal battery energy and delivers 90% of that usable energy to the loads.
That is about 3.67 kWh before a separate allowance for anything omitted, aging or additional reserve. The 80% and 90% values are assumptions, not universal lithium or AGM ratings. Replace them with equipment-specific limits and design allowances. Do not count the same loss twice if a manufacturer already specifies delivered usable energy.
Nominal volts multiplied by amp-hours gives a nominal watt-hour estimate. A 12.8 V, 100 Ah battery represents approximately 1,280 Wh; a 25.6 V, 100 Ah battery represents approximately 2,560 Wh. Equal amp-hours do not mean equal stored energy at different voltages.
Do not turn the 3.67 kWh result into a shopping instruction by rounding down. Check supported system voltage, permitted series or parallel arrangements, compatible charging settings and battery-management requirements first. Different batteries cannot automatically be combined.
The daily budget answers how much energy you need. Continuous discharge current, startup demand and the battery-management system answer whether the bank can supply it quickly enough. A large inverter connected to an unsuitable battery can still shut down.
Temperature limits also matter. Confirm the exact battery’s charging and discharge temperature ranges, enclosure requirements and any heating provisions. A protected porch and a freezing battery compartment are different environments.
A larger bank gives you more stored energy. It does not create more sunshine. The charging plan must serve daily use and recover from a depleted period within an acceptable time. Check seasonal solar production and charger limits alongside battery size.
Use these fields while comparing options. Entries stay only in this open page and are not submitted or saved. Print the page to keep your notes.
Compare our battery guide and supported expansion batteries. An expansion pack is not necessarily a standalone battery for another inverter. For the full planning sequence, return to building an off-grid solar system.
That depends on your location, loads, acceptable interruptions and alternative charging. Two days is an example here, not a reliability standard. Design for the consequences of running short.
Only when the manufacturer supports the combination, configuration and expansion procedure. Check model compatibility, age restrictions, cables and firmware before treating expansion as part of the plan.
The arithmetic is an original AZSP scenario. Background reading: EIA on electricity units; UAF Extension on estimating and measuring appliance energy; Victron battery installation guidance. Manufacturer instructions apply to their named equipment and should not be generalized to every battery.
AZSP educational guide. Numerical examples are hypothetical planning exercises, not measured product performance or a finished system design. Product guides may contain affiliate links that earn AZSP a commission.