TL;DR
Sizing a solar and battery system requires three calculations: what power you need (watt-hours per day), how much storage to cover cloudy days (battery bank size), and how many panels produce that energy (panel wattage). Each step has a straightforward formula. Run the numbers before buying anything — most people over-buy panels and under-buy storage.
Step 1: Calculate Your Daily Load
List everything you need to power and how long you'll run it each day. The product of watts × hours = watt-hours (Wh) per day.
Example load calculation (emergency minimum):
| Device | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| LED lighting (4 bulbs × 10W) | 40W | 4 hrs | 160 Wh |
| Phone charging (2 phones) | 20W | 2 hrs | 40 Wh |
| AM/FM/NOAA radio | 5W | 6 hrs | 30 Wh |
| Laptop computer | 45W | 2 hrs | 90 Wh |
| CPAP machine (if needed) | 40W | 8 hrs | 320 Wh |
| Total | 640 Wh/day |
Add 20% for system inefficiencies (conversion losses, wiring resistance): 640 × 1.2 = 768 Wh/day as your actual target.
Other common devices and their power draw:
| Device | Watts | Notes |
|---|---|---|
| Mini-fridge (12V compressor) | 50-80W | 8-12 hrs equivalent cycling |
| Box fan (medium) | 75-100W | Per hour |
| Window AC unit (smallest) | 500-1,200W | Not practical on modest solar |
| Electric hot plate | 1,000-1,500W | Not practical on modest solar |
| TV (32-inch LED) | 35-50W | Per hour |
| Ham radio (receive) | 5-20W | Per hour |
| Ham radio (transmit) | 50-100W | Brief duty cycles |
Step 2: Size Your Battery Bank
Your battery bank must store enough energy to run your daily load for your chosen number of days without sun (autonomy days).
Formula: Battery bank size (Wh) = Daily load (Wh) × Autonomy days ÷ Depth of discharge (DOD)
For the example above, with 2 days of autonomy and lithium batteries (90% DOD):
768 Wh/day × 2 days ÷ 0.90 = 1,707 Wh battery bank minimum
Round up to the next available product size: a 2,000Wh battery bank (two 100Ah 12V batteries, or one 2,000Wh power station, or two 1,000Wh power stations).
DOD by battery type:
- LiFePO4 lithium: 80-90% usable
- AGM sealed lead-acid: 50% usable
- Flooded lead-acid: 50% usable (with equalization charges)
Practical note: For most residential emergency preparedness systems, a portable power station (Jackery, EcoFlow, Bluetti, Goal Zero) is simpler and more appropriate than a custom battery bank. They include the battery management system, inverter, and charge controller in one unit.
Step 3: Size Your Solar Array
Your panels must produce enough energy on an average day to replenish what you used.
Peak sun hours: The number of hours per day when solar irradiance is at approximately 1,000 W/m² (full sun equivalent). This varies by location and season:
- US Southwest (Phoenix, Las Vegas): 5.5-6.5 peak sun hours
- Pacific Northwest (Seattle): 3.5-4.5 peak sun hours
- US Northeast: 4.0-5.0 peak sun hours
- US Southeast: 4.5-5.5 peak sun hours
Check your specific location at pvwatts.nrel.gov for precise data.
Formula: Panel wattage needed = Daily load (Wh) ÷ Peak sun hours × 1.25 (system efficiency factor)
For the example, in a 4.5 peak sun hour location:
768 Wh ÷ 4.5 hrs × 1.25 = 213W of panels minimum
Round up: a 300W panel array (one 300W or two 150W panels) comfortably covers this load.
System Components
A complete system requires four components:
Solar panels: Monocrystalline panels are standard for residential and portable use. Higher efficiency than polycrystalline at the same cost. 100W to 400W per panel. Rigid panels are more efficient; portable folding panels are more flexible but slightly less efficient.
Charge controller: Regulates charging from panels to batteries. MPPT (Maximum Power Point Tracking) controllers are 10-30% more efficient than PWM and required for any serious installation. Sized in amps: divide your panel wattage by system voltage (12V or 48V) for minimum amp rating.
Battery bank: LiFePO4 lithium for most applications. Lead-acid for budget-constrained installations accepting the trade-offs.
Inverter: Converts DC battery power to AC household current. Sized at 110-125% of your maximum simultaneous AC load. Pure sine wave (not modified sine wave) is required for sensitive electronics, CPAP machines, and most motors.
Portable Power Station vs. Custom Build
| Factor | Portable Power Station | Custom Build |
|---|---|---|
| Cost (per Wh storage) | $0.50-$1.50 | $0.20-$0.80 |
| Expandability | Limited | Unlimited |
| Portability | Excellent | Poor |
| Complexity | Plug-and-play | Requires electrical knowledge |
| Warranty and support | Strong (major brands) | Self-managed |
| Best for | Emergency preparedness, small loads | Whole-home backup, large loads |
For most preparedness applications (phone charging, lighting, communications), a portable power station is the right choice. For whole-home backup or running a refrigerator, a custom system is necessary.
Common Sizing Mistakes
Under-buying storage, over-buying panels: Panels produce energy; batteries store it. A 400W panel on a 200Wh battery fills the battery in 30 minutes and wastes the remaining day's production. Match them.
Not accounting for winter: Solar production in December at northern latitudes can be 30-50% of July production. Size for your worst month, not your best.
Forgetting inverter efficiency: Inverters are 85-95% efficient. Every watt you consume from AC also includes 5-15% inverter loss. Account for this in your load calculation.
Planning for surge vs. continuous loads: Motors and compressors draw 3-5x their rated watts at startup. Your inverter must handle the surge load, not just the continuous load. A mini-fridge rated at 80W may surge to 250W at compressor start.
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Frequently Asked Questions
What's the minimum solar setup worth having for emergency preparedness?
A 100-200W solar panel plus a 500-1,000Wh lithium power station (like a Jackery, Bluetti, or EcoFlow unit) is the practical minimum. This combination charges phones, runs LED lighting, powers a small fan or radio, and can run a CPAP machine overnight. Total cost: $300-800. It won't run an air conditioner or refrigerator, but it covers communication, lighting, and critical medical devices.
What's the difference between watts and watt-hours?
Watts measure the rate of power consumption at any moment. Watt-hours measure total energy over time. A 60-watt light bulb uses 60 watts every hour it runs — 60 watt-hours per hour, or 1,440 watt-hours (1.44 kWh) over a full day. Sizing a battery bank requires thinking in watt-hours (total energy) while selecting solar panels requires thinking in watts (rate of production). Match production rate to consumption and storage to meet the gap.
Should I buy lithium or lead-acid batteries?
Lithium (LiFePO4 specifically) for most emergency preparedness applications. Lithium batteries have 80-100% usable depth of discharge versus 50% for lead-acid, meaning a 100Ah lithium battery stores effectively twice the usable energy of a 100Ah lead-acid at the same size and roughly the same cost over their lifespan. Lithium lasts 10x longer (3,000-5,000 cycles vs 300-500 for flooded lead-acid). The upfront cost is higher but the total cost of ownership is lower.
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