Automotive Calculators

Solar-to-Battery Calculator

2026-09-24 7 min

Balance solar yield against battery capacity: daily refill percent, full-charge days and usable amp-hours.

Daily Yield: 1,500; Wh/day
Daily Refill: 58.6; %
Days To Full: 1.71; days
Battery Size: 213; Ah@12V

Calculator

Array Power(W) Help

Panel watts

Sun Hours(hrs/day) Help

hrs/day

Peak sun hours

Battery Energy(Wh) Help

Usable capacity

Results update live as you type

Live statistics

Daily Yield

1,500

Wh/day

Daily Refill

58.6

Days To Full

1.71

days

Battery Size

213

Ah@12V

Driving plans — pick your profile

Plans adjust the projected fuel use so estimates match how you actually drive. Your measured MPG stays untouched.

Results

Normal

Daily Yield

1,500Wh/day

Solar energy per day

Daily Refill

58.6%

Battery share refilled daily

Days To Full

1.71days

Empty-to-full time

Battery Size

213Ah@12V

Equivalent 12 V bank

Exact arithmetic on your inputs. Verify measurements and current rates for decision-grade figures.

Visualizations

Daily Refill sensitivity

46.9x0.8

52.7x0.9

58.6x1.0

64.5x1.1

70.3x1.2

Battery refill per day

58.6%

0200

Formula

How Solar-to-Battery Calculator is computed from its inputs, with every derived figure shown.

Yield = Watts x Sun x 0.75. Refill_pct = Yield / Battery_Wh x 100

Alternative Form

Days to full = Battery_Wh / Yield. Over 100 percent daily means headroom.

Variables

Symbol	Name	Unit	Description
P	Array power	W	Nameplate
H	Sun hours	hrs	Peak equivalent
E	Battery	Wh	Usable energy

Explanation

Yield balance identical to panel math; ratios against usable battery energy give refill and timing.

Derivation

Yield balance identical to panel math; ratios against usable battery energy give refill and timing.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Set array power.

Array Power

Value:400 W

Intermediate: 400 W

Result: Accepted

Step 2

Set sun hours.

Sun Hours

Value:5 hrs/day

Intermediate: 5 hrs/day

Result: Accepted

Step 3

Set battery energy.

Battery Energy

Value:2560 Wh

Intermediate: 2560 Wh

Result: Accepted

Step 4

Compute daily yield, then ratio it against battery energy.

Yield = Watts x Sun x 0.75. Refill_pct = Yield / Battery_Wh x 100

Array Power:400 W

Sun Hours:5 hrs/day

Battery Energy:2560 Wh

Intermediate: Yield = 400.0 x 5.0 x 0.75

Result: Refill = 58.6 %/day

Final Answer

1,500

Worked Examples

See how the calculator works with different scenarios.

Scenario

400 W, 5 sun, 2560 Wh bank.

Inputs

Array Power:400 W

Sun Hours:5 hrs/day

Battery Energy:2560 Wh

Calculation

Apply the stated formula to the example inputs; the primary result follows the steps above.

Result

Daily Refill 58.6 %.

Interpretation

Near 59 percent daily.

Understanding Your Result

Your Result: Daily Yield: 1,500 Wh/day

Typical range: Five sun hours with 0.75 derate is a solid planning baseline. Daily camper loads commonly 500 to 2000 Wh; EV miles need house-scale arrays.

What This Means

Daily yield is array watts times sun times derate. Refill percent is yield over battery energy; full days invert the ratio.

Why It Matters

Mismatched arrays and banks waste money both ways: big panels on small batteries clip, small panels on big banks never fill.

Professional Interpretation

Installers audit loads with meters, size arrays to worst-season sun, and pair panels with batteries sized for nights plus storm days.

How the plan factor applies

Plans do not scale physics results here; selections are recorded while energy figures follow sun and load.

Loads first

Metered daily watt-hours precede every panel and battery decision.

Recommendations

•Watt-hour budgets end dead-battery surprises
•Sun-hour math sizes arrays honestly
•Charge-time views plan around weather
•Cost views price energy independence

Learn — fuel economy, deeply

Vehicle solar is an energy budget: panel watts times sun hours times system efficiency must cover the load. Calculators in this group size panels, time solar charging, budget EV, camper, RV and battery charging, and price solar energy against fuel and hookups.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

2 · What EPA / WLTP ratings actually mean

City cycle — stop-and-go, 21 mph average, idle included. Hybrids shine here (regen + engine-off).
Highway cycle — steady 48 mph average with gentle accelerations. Aero and gearing dominate.
Combined — 55% city + 45% highway weighting. The single number most drivers should compare.
Real-world gap — expect 5–15% below the sticker with normal driving; hypermilers can beat it, aggressive drivers miss by 25%+.
MPGe — 33.7 kWh = 1 gallon-equivalent. A 120-MPGe EV uses ~⅓ the energy per mile of a 35-MPG car.

3 · Factor-impact table — what moves your number most

Factor	Typical impact	Why
Speed above 55 mph	−5 to −25%	Aero drag rises with v². Each 5 mph over 55 ≈ +$0.30–0.50/gal equivalent.
Aggressive driving	−10 to −30%	Hard acceleration + braking wastes kinetic energy, especially in the city.
Under-inflated tires (−5 PSI)	−2 to −4%	Higher rolling resistance. Check monthly; pressure drops ~1 PSI per 10°F.
Roof rack / cargo box	−5 to −25%	Empty rails ≈ −5%; loaded box at highway speed up to −25%.
Excess weight (+100 lb)	−1 to −2%	Matters most in stop-and-go. Clean the trunk before long trips.
A/C at city speed	−5 to −15%	At highway speed A/C beats open windows (drag). Use recirculation.
Cold weather (20°F vs 77°F)	−10 to −20%	Rich warm-up mixture, thicker fluids, winter-blend fuel, heater load.
Clogged air filter / bad O2 sensor	−4 to −40%	A failed oxygen sensor alone can cut economy by up to 40%.
Idling	0 mpg	0.2–0.5 gal/hr burned standing still. Restart beats 30 s+ idling.
E10 → E85 fuel switch	−15 to −30%	Ethanol carries less energy per gallon; flex-fuel range drops accordingly.

Winter playbook

• Expect −10 to −20% on short trips; combine errands on one warm engine.
• Tires lose ~1 PSI per 10°F — re-inflate monthly.
• Idle 30 s max, then drive gently; cabin heat comes faster under light load.
• Winter-blend fuel has slightly less energy — a 2–3% seasonal dip is normal.

Summer playbook

• City: windows down under ~45 mph; highway: A/C + recirculation beats open windows.
• Park in shade; ventilate before blasting A/C.
• Hot asphalt softens tires — still check pressure, heat raises it.
• Roof cargo hurts most in summer road-trip season — move it inside or to a hitch carrier.

4 · Measure like a pro (the 3-tank protocol)

Fill to first click at the same pump, record the odometer (Tank 0).
Drive normally for 2–3 tanks; always refill to first click at the same pump.
MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

•Measure loads with a meter, not labels
•Face panels to the sun when stationary
•Clean dust; output drops fast
•Size wire and fuses with the panels
•Track sun hours by season
•Keep a generator or hookup backup

Common Mistakes

Sizing to panel watts alone

Sun hours and derate decide energy, not watts.

Forgetting inverter losses

AC loads pay 5 to 15 percent conversion tax.

Ignoring shade

Partial shade can collapse a string.

Undersizing batteries

Autonomy needs nights and storm days.

Assumptions

This calculator makes the following assumptions to provide estimates:

Stated sun hours: Peak hours match place and season.

Fixed derate: Heat, dirt and wiring losses hold.

Known loads: Daily watt-hours entered are complete.

Limitations

Be aware of these limitations when using this calculator:

Weather averaged: Storm sequences need margin.

No shading model: Trees and buildings are excluded.

Simplified electrics: MPPT and wiring detail smoothed.

Frequently Asked Questions

Answers to common questions about this calculator.

How does the Solar-to-Battery Calculator work?

Enter panel watts, sun hours, battery and load inputs. The calculator balances daily watt-hours against solar yield as stated in the formula.

What is a peak sun hour in the Solar-to-Battery Calculator?

One hour of 1000 W per square meter equivalent. Five peak hours is a strong day, not five clock hours.

How much margin belongs in the Solar-to-Battery Calculator?

Oversize 20 percent for weather, dirt and heat, plus battery autonomy for nights and storm days.

How accurate is the Solar-to-Battery Calculator?

Exact energy arithmetic. Shade, tilt, season and panel heat shift real yield around the estimate.

Can the Solar-to-Battery Calculator replace a generator?

For modest daily loads with good sun, often yes. Dense loads and air conditioning usually still need backup.

How many solar watts does my camper need?

Daily watt-hours over sun hours over derate. A 1000 Wh day on 5 sun hours needs roughly 250 W of panel.

How long does solar charging take?

Battery watt-hours needed over solar watts in. Clouds and angle stretch it.

Can solar charge an EV?

Trickle yes, full no from the car roof. House arrays can add meaningful daily EV miles.

How do I size panels for an RV?

Sum daily loads, divide by sun hours and derate, then check roof area and battery capacity.

What does solar energy cost?

System cost over lifetime kilowatt-hours, often below fuel and hookups where sun is strong.

How much battery do I need?

Night plus autonomy days times daily use over usable depth of discharge.

Do panels work in winter?

Yes at lower yield: short days, low sun and snow cut output sharply. Oversize or hybridize.

What is peak sun hour?

One hour of 1000 W per square meter equivalent. Five peak hours is a strong day, not five clock hours.

Solar versus generator for RVs?

Solar wins silent daily energy; generators win dense backup power. Most off-grid rigs carry both.

Related Calculators

Explore more calculators related to this topic.

Related Guides

Deepen your understanding with these educational resources.

Vehicle Solar Guide

Load audits, sun hours and array sizing

Off-Grid Battery Guide

Capacity, depth of discharge and autonomy

Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG: Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km: Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe: Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE: Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance: Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag: Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling: Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II: On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.
Peak sun hour: One hour at 1000 W per square meter. Sizes daily energy.
Derate: Combined loss factor for heat, dirt, wiring and conversion.
Autonomy: Days of use covered without new solar input.

Authoritative References

Trusted sources and standards referenced by this calculator.

EPA — FuelEconomy.gov DOE — Fuel-Efficient Driving Tips NHTSA — Driving Safety & Efficiency SAE International — Fuel Economy Standards Consumer Reports — MPG Truth IRS — Standard Mileage Rates

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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Page Information

Last Updated:

2026-09-24

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.

How you compare — vehicle class averages

Class	Combined MPG	City / Hwy	Fuel / 12k mi	Cost @ $3.50	CO₂ / yr
Compact sedan	34	30/40	353 gal	$1,235	3.1 t
Midsize SUV	25	22/29	480 gal	$1,680	4.3 t
Pickup truck	19	17/23	632 gal	$2,211	5.6 t
Hybrid sedan	49	50/47	245 gal	$857	2.2 t
EV (MPGe)	115	125/105	— (electric)	$550	1.1 t

Combined averages blend EPA city/highway cycles. EV row shows MPGe and home-charging cost; tailpipe CO₂ is zero, grid CO₂ ≈ 1.1 t/yr.

Maintenance checklist

Tick off what you already do — your car scores 1/6 (17%).