Watt-hours (Wh) measure energy; amp-hours (Ah) measure charge at a specific voltage. For solar sizing, Wh is the unit that actually lines up with your daily loads, panel output, and usable battery capacity. Ah is still useful on battery labels and wiring specs—but you must know the system voltage to convert Ah to Wh before comparing anything. Use Wh = V × Ah whenever you mix chemistries, voltages, or brands.
What This Guide Covers
This page explains Wh and Ah for off-grid and battery-backed solar—conversion, when each unit appears on specs, and how to size a bank without comparing apples to oranges. It does not cover panel nameplate watts (power) or charge-controller amp ratings in depth; those sit in separate sizing guides. If you already have a daily load total in Wh, jump to the off-grid solar sizing calculator and work backward to Ah at your system voltage.
What Wh and Ah Actually Measure
Both units describe how much a battery can deliver, but they answer different questions.
- Watt-hour (Wh): Energy = power × time. One Wh is one watt drawn for one hour. A 1,200 Wh battery can run a 100 W load for 12 hours, or a 400 W microwave for 3 hours. Wh is voltage-independent—1,200 Wh at 12 V is the same usable energy as 1,200 Wh at 48 V.
- Amp-hour (Ah): Charge capacity at a stated voltage. Ah tells you how many amps the battery can supply over time, but not how much energy that represents until you multiply by voltage: Wh = V × Ah.
A 100 Ah label on a 12 V bank is 1,200 Wh. The same 100 Ah at 48 V is 4,800 Wh—four times the energy. That is why comparing Ah ratings alone misleads buyers.
Why Wh Wins for Solar Sizing
Your daily load list is built in watts and hours, which sum to Wh per day. Panel daily harvest, inverter throughput, and battery usable capacity all compare cleanly in Wh.
When vendors quote batteries in Ah—as most RV, marine, and off-grid banks still do—you convert to Wh at your nominal system voltage (12 V, 24 V, or 48 V), then size against your daily Wh need and days of autonomy. The WattSizing off-grid calculator works in Wh and kWh and can show Ah at your chosen voltage so you do not have to convert by hand.
Quick Conversion Reference
| System voltage | 100 Ah in Wh | 200 Ah in Wh |
|---|---|---|
| 12 V | 1,200 Wh | 2,400 Wh |
| 24 V | 2,400 Wh | 4,800 Wh |
| 48 V | 4,800 Wh | 9,600 Wh |
Formulas:
- Wh = V × Ah
- Ah = Wh ÷ V
Always use the battery bank's nominal voltage, not the float voltage (e.g. use 12 V for a 12 V system, not 13.8 V).
What Most Guides Skip
Peukert Effect Changes Real Ah
Lead-acid batteries deliver fewer amp-hours at high discharge rates. A 100 Ah AGM might only behave like 70–80 Ah when you run a 1,500 W inverter hard. Wh-based load planning helps, but heavy surge loads still matter—especially on 12 V systems where current is highest.
Inverter and Cable Losses Are in Wh, Not Ah
A 2,000 Wh/day load list does not account for inverter efficiency (~85–95%) or wiring voltage drop. Size the bank 10–20% above your calculated Wh need, or your "100 Ah" bank will feel short even when the math looked right.
kWh vs Wh Is Just Scale
Utility bills and large home batteries use kWh (kilowatt-hours). 1 kWh = 1,000 Wh. Off-grid cabin math is the same—just watch the decimal.
Series vs Parallel Changes Voltage, Not Total Wh
Two 100 Ah 12 V batteries in parallel → 200 Ah at 12 V = 2,400 Wh. The same two in series → 100 Ah at 24 V = still 2,400 Wh. Wiring changes voltage and current, not total energy stored.
Illustrative Worked Example
Goal: Size a 12 V off-grid battery bank for a small cabin.
- Daily loads (from load list): 1,800 Wh/day
- Days of autonomy: 2 cloudy days → 1,800 × 2 = 3,600 Wh usable needed
- Battery chemistry: LiFePO4 at 80% depth of discharge → rated capacity = 3,600 ÷ 0.80 = 4,500 Wh
- Convert to Ah at 12 V: 4,500 ÷ 12 = 375 Ah rated (e.g. three 12 V 125 Ah batteries in parallel, or one 48 V 94 Ah pack through a DC-DC setup)
If you had stopped at "I need about 200 Ah because my friend has 200 Ah," you would be roughly half short—because you compared Ah without checking Wh, DoD, and autonomy together.
Practical Checklist
- Build your load list in Wh/day before shopping for batteries.
- Convert every Ah rating to Wh using your system voltage before comparing brands or chemistries.
- Apply depth of discharge for your chemistry (see depth of discharge for solar batteries).
- Add margin for inverter efficiency, cold weather, and battery aging (10–20% is typical).
- Run the numbers in the WattSizing calculator and cross-check Ah at your target voltage.
Frequently Asked Questions
Can I add Wh from batteries at different voltages?
Yes. Wh is additive regardless of voltage. Two 600 Wh packs—one 12 V and one 24 V—store 1,200 Wh total, though you would need separate converters or a single system voltage to use them together.
Why do some batteries list both Ah and Wh?
Marketing convenience. The Wh number is usually Ah × nominal voltage. Always confirm which voltage they used; a "1,200 Wh" label on a 12 V 100 Ah battery is consistent, but cross-brand comparisons still need the same voltage basis.
Is a higher Ah always a bigger battery?
Only at the same voltage. 100 Ah at 48 V (4,800 Wh) stores more energy than 200 Ah at 12 V (2,400 Wh). Compare Wh first.
Should I size panels in Wh or W?
Panel nameplate power is in watts (W); daily harvest is in Wh (watts × peak sun hours). Your battery bank must store enough Wh to cover load minus what panels deliver that day.
Do charge controllers care about Ah or Wh?
Controllers are rated in amps (A) and volts (V). You convert your array's expected charging current to pick a controller, but system energy balance is still planned in Wh.
