Battery sizing for backup
Estimate runtime, usable capacity, and the real limits of residential battery backup.
Part of the: Solar battery and brownout planning
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SolarGabay Editorial Team — independent consumer-focused writers who translate technical sources into practical homeowner guidance.
Technical review
Reviewed against the published methodology, source hierarchy, and stated limitations. This is educational guidance, not site-specific engineering.
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Last reviewed July 20, 2026. Corrections are logged through the public corrections process and material changes update this date.
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What you should know first
Battery sizing starts by reducing the problem to critical loads. The right battery is not the biggest battery you can buy; it is the battery that can support your essential circuits for a realistic amount of time at a sensible cost. Runtime depends on usable capacity, efficiency, inverter behavior, and how disciplined your backup loads are.
Battery size should be expressed as usable kWh and supported kW, not just a marketing capacity.
Decision map
Size energy and power separately
Energy need
Watts × hours for every critical load, adjusted for efficiency losses.
Power need
Simultaneous running watts plus the largest realistic starting surge.
Usable battery
Nominal kWh reduced by reserve, depth-of-discharge, and conversion losses.
The variables that matter
Usable energy is smaller than nominal capacity because depth-of-discharge and conversion losses matter.
Inverter output and surge capacity can be the limiting factor before battery energy runs out.
Motors, compressors, and pumps need special attention because startup demand can spike.
Backup duration falls quickly when users expect the battery to power the whole house.
Energy capacity answers “how long”; power answers “what can run”
A battery’s usable energy determines approximate runtime, while inverter power and surge capability determine which loads can operate at once. Conversion losses and reserve settings mean the nameplate kWh is not fully available to the user.
A useful load audit records watts, hours, and surge behavior. It also separates essential loads from optional loads, because one air-conditioner can dominate the battery requirement.
Calculate essential-load energy in kWh.
Add conversion losses and the chosen reserve level.
Check continuous and surge kW separately.
Cycling strategy affects life and economics
Battery degradation depends on chemistry, temperature, state of charge, depth of discharge, and how often the system cycles. NREL’s residential battery work treats degradation and capacity maintenance as real operating considerations rather than assuming unchanged capacity forever.
The financially sensible battery may be smaller than the technically possible battery. Buyers should compare the value of outage protection and self-consumption against replacement cost and the number of outages actually experienced.
Ask for warranted usable capacity and retained capacity.
Keep batteries in the manufacturer’s approved temperature range.
Model at least one replacement in a long-term hybrid-system budget.
Worked example
A 10 kWh battery is not always 10 kWh of household use
A reserve setting, battery protection limit, inverter losses, and aging can reduce usable energy. A load list totaling 1 kW may therefore run for fewer than ten hours—and much less if a pump or air-conditioner starts.
Buy usable runtime for named loads, not a nominal battery number.
Decision rule
Require a load-by-load runtime table and surge check before accepting a battery size.
Practical checklist
List backup loads with running watts, surge watts, quantity, and target hours of use.
Decide what cannot run at the same time during an outage.
Ask for usable battery capacity in kWh, not only marketing capacity.
Review expected replacement timing and cost before you commit.
Evidence, scope and limitations
Battery planning is often where marketing optimism becomes expensive. Keep the load list honest and the expectations realistic.
International references are used for engineering and consumer principles. Philippine utility procedures, tariffs, permits and legal requirements must be confirmed using current local rules.
Research record
Sources and further reading
- [1]Residential Battery Storage — Annual Technology BaselineNational Renewable Energy Laboratory
- [2]Analysis of Degradation in Residential Battery Energy Storage SystemsNational Renewable Energy Laboratory
- [3]Solar and Resilience BasicsU.S. Department of Energy
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