Size home solar battery storage from backup loads, usable capacity and inverter power. Compare energy needs, surge limits and capacity degradation assumptions.
Quick answer
Nameplate kWh = daily backup kWh x autonomy days / (usable depth fraction x assumed delivery efficiency).
Notes updated 2026-10-07
Best for
Homeowners pressure-testing solar proposals before signing.
Inputs needed
electric usage, local rate, system size or quote assumptions
Formula used
Nameplate kWh = daily backup kWh x autonomy days / (usable depth fraction x assumed delivery efficiency).
Method references
System Advisor Model, Tesla Powerwall specifications
Author profile: Marcus Vance . Read the methodology and recheck local tariff, quote, and incentive details before deciding. Attribution is not a record of independent technical review.
kWh/day
0.5 = back up half your load (essentials).
kWh
Powerwall ≈ 13.5 kWh.
kW
Running load of the circuits you want backed up.
kW
Starting surge for pumps, compressors, and HVAC.
Wattcrunch Solar Planning Report
Generated 10/25/2026 · Verify assumptions with your utility and installer.
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Save the scenario or download a branded print/PDF report.
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Should I size battery energy or inverter power first?
Check both. Daily backup energy determines kWh while simultaneous and startup loads determine kW. A battery can have enough energy but insufficient power to start a pump or compressor.
How should I interpret capacity degradation?
The model uses a disclosed annual capacity fade assumption. It does not reproduce a manufacturer warranty curve or temperature-dependent aging. Verify usable capacity, reserve settings and cycle limits in the actual data sheet.
Is one home battery enough to run my house?
Energy capacity alone cannot answer this. Add the energy used by backed-up circuits during the outage, then separately check simultaneous kW and motor startup surge. A battery with enough kWh may still lack the inverter power to start a compressor.
Battery capacity for three daily backup-load budgets
Daily backup energy
Delivered energy target
Nameplate requirement
5 kWh
5.0 kWh
6.17 kWh
10 kWh
10.0 kWh
12.35 kWh
20 kWh
20.0 kWh
24.69 kWh
Illustrative assumptions: One day without recharge, 90% usable-depth fraction and 90% delivery-efficiency planning allowance. This is not a manufacturer battery specification. If capacity is already stated as usable, do not apply its depth limit twice; round-trip efficiency is not a measured discharge-only efficiency.
What to check on your bill or installer proposal
List essential circuits and actual duty cycles rather than whole-home average kWh.
Verify continuous output, surge duration, phase compatibility and transfer equipment.
Check reserve settings and warranted end-of-life capacity, not only new-battery nameplate capacity.
Example and calculation notes updated . Method references are listed below; example rates are not verified utility tariffs.
How does this solar battery sizing estimate work?
Nameplate kWh = daily backup kWh x autonomy days / (usable depth fraction x assumed delivery efficiency).
A worked example with explicit assumptions
For 10 kWh of backup loads over one day, 90% usable depth and an assumed 90% delivery-efficiency allowance, the model requires 12.35 kWh nameplate. A 7 kW surge requirement still needs a compatible inverter. This conservative sizing allowance differs from the interval simulator's separate charge/discharge efficiencies.
Reproducible example generated from the same calculator functions used by the tool. Inputs are illustrative, not measured customer results.
What should I verify before using the result?
Manufacturer usable capacity may already account for depth limits. Do not derate that specification twice; verify power and backup wiring independently.
Use your bill or written equipment quote, run a conservative scenario, and compare the result with the same inputs in another model.
Treat the result as a planning range, not a promise. Re-run it with a lower and higher production estimate, your actual utility tariff, and the written installer quote.
Check roof shade, orientation, tilt, and equipment losses.
Separate cash price, financing fees, battery cost, and roof work.
Verify export credits, fixed charges, and interconnection rules.
Confirm incentive eligibility, deadlines, and tax treatment.
Sources behind this estimate
Use the method references below to check the formula and its limitations. Input defaults and illustrative scenarios are not current tariffs, guaranteed output or live installer quotes.
Utility export rates, eligibility rules, and installer pricing can change. Confirm the applicable tariff, interconnection requirements, and tax treatment before making a purchase decision.
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Frequently asked questions
What is the difference between battery kW and kWh?+
kWh measures stored energy; kW measures how much power the inverter can deliver at once. A battery can have enough energy for a day but still fail to start an air conditioner if continuous or surge power is insufficient.
Should I apply depth of discharge to usable battery capacity?+
Not twice. If the manufacturer quotes usable capacity after its operating reserve, use that basis consistently. Confirm continuous output, surge duration, backup wiring, warranty throughput and end-of-warranty capacity separately.
How do I use the solar battery sizing calculator?+
Start with your actual usage, equipment or quote inputs, keep the displayed units, and change one assumption at a time. Manufacturer usable capacity may already account for depth limits. Do not derate that specification twice; verify power and backup wiring independently.
What calculation does this tool use?+
Nameplate kWh = daily backup kWh x autonomy days / (usable depth fraction x assumed delivery efficiency).
Can I check the arithmetic with an example?+
For 10 kWh of backup loads over one day, 90% usable depth and an assumed 90% delivery-efficiency allowance, the model requires 12.35 kWh nameplate. A 7 kW surge requirement still needs a compatible inverter. This conservative sizing allowance differs from the interval simulator's separate charge/discharge efficiencies.
What could change the accuracy of my result?+
Manufacturer usable capacity may already account for depth limits. Do not derate that specification twice; verify power and backup wiring independently.
Can I use the estimate outside the United States?+
Physical kW and kWh calculations use the same units internationally. Dollar results, regional defaults, emissions factors, incentives and utility guidance are US-oriented. Replace rate and production assumptions with local values; do not apply US tax or export rules abroad.
Can I print or save results without registering?+
Use Print / PDF to save this page from your browser without an email address. The solar plan stores supported scenario inputs in your browser; private files should be checked and redacted before uploading for extraction.
Compare the decision, not just the number
Compare cash, loan, lease, and battery scenarios using the same production, rate, and quote assumptions before choosing a financing path.