Assessing Your Power Needs
Get your load calculation wrong and every component you buy — panels, batteries, inverter — will be the wrong size. This guide gives you the exact method: how to measure what you actually consume, how to calculate daily kWh, how to size your battery bank with the right depth-of-discharge and autonomy, and two worked examples showing a 2 kWh/day cabin versus an 8 kWh/day homestead.
Most beginners guess appliance wattage from the nameplate rating — which is peak draw, not average consumption. A refrigerator labeled 400W typically averages 100–200W because it cycles. Using nameplate figures will oversize your battery bank by 30–50% and cost you thousands in unnecessary capacity.
What Are You Sizing For?
Daily load targets and design priorities differ significantly by use case.
Target: 0.5–1.5 kWh/day. 12V compressor fridge + LED lighting + device charging. Every watt matters — start with efficiency upgrades before sizing anything.
Battery need: 1.25–3.75 kWh (2-day autonomy)
Target: 1.5–10 kWh/day depending on well pump and HVAC choices. This guide covers this size in depth with worked examples at 2 kWh and 8 kWh.
Battery need: 3.75–25 kWh (2-day autonomy)
Target: 10–20+ kWh/day. AC and well pumps are the major load drivers. Propane heat and efficient mini-splits are critical to keeping this manageable.
Battery need: 25–50+ kWh (2-day autonomy)
In This Guide
Why Load Analysis Must Come First
Every component in an off-grid system is sized from your daily energy consumption. Panels are sized to replenish what you use. Batteries are sized to store enough for 2–3 days without sun. The charge controller and inverter are sized from peak load. If you start with panels or batteries before knowing your load, every number downstream will be wrong.
The correct sequence is: load audit → battery bank → charge controller → solar array. Not the other way around. Most beginner guides skip straight to "how many solar panels do I need?" — which is the last question you should ask, not the first.
Steps 4 and 5 are covered in the Solar Sizing Guide and Power Conversion Guide. This guide covers Steps 1–3.
Three Ways to Measure Your Load: Which to Use
| Method | Cost | Accuracy | Best For |
|---|---|---|---|
| Kill-A-Watt plug-in meter | $20–30 | Highest — actual consumption | Any 120V appliance; the only reliable method for fridges, compressors, cycling loads |
| Nameplate rating (label) | Free | Low — nameplate = peak draw | Simple resistive loads only (light bulbs, toasters, space heaters); unreliable for motors |
| Utility bill average | Free | Medium for totals; no breakdown | Grid-connected homes planning to go off-grid; starting point, not a target |
Step-by-Step Load Audit
Run through these seven steps for every device you plan to power off-grid.
List every device
Go room by room. Include lights, HVAC, kitchen appliances, entertainment, connectivity (router, Starlink), tools, medical equipment (CPAP), and anything with a power cord or battery charger. Don't forget phantom loads — devices that draw power in standby.
Measure actual wattage
Use a Kill-A-Watt for variable-load devices. For simple resistive loads (light bulbs, toasters, space heaters), nameplate is reliable. For refrigerators, compressors, and motors, measure average draw over a full on/off cycle — not just the peak.
Estimate realistic daily hours
Be honest. Refrigerators run 8–12 hours/day in cycles, not 24. Lighting averages 4–6 hours. A laptop might be 4 hours. Overestimating hours is safer than underestimating. For intermittent loads like well pumps, count actual daily run time.
Calculate Wh per device
Watts × Hours = Watt-hours (Wh). A 150W refrigerator running 10 hours = 1,500 Wh = 1.5 kWh/day. A 9W LED bulb on 5 hours = 45 Wh/day.
Sum all devices
Add every device's daily Wh. Divide by 1,000 for kWh. This is your baseline daily consumption. Most off-grid households land between 1.5 and 10 kWh/day after efficiency improvements.
Identify peak load
List all devices that could run simultaneously. Sum their running watts. Then add the highest single-motor surge draw (well pump, AC, fridge). This total is your inverter minimum rating requirement.
Apply system loss factor
Multiply total daily kWh by 1.15–1.25 to account for inverter conversion losses (85–90% efficient), wire losses, and temperature derating. Use 1.25 for long wire runs or extreme temperatures. This is your final sizing target.
Appliance Power Consumption Reference Table
Running watts are what the appliance draws continuously. Surge watts are the peak draw at startup — critical for inverter sizing. These figures are verified from manufacturer specs and community measurements (DIY Solar Forum 2025–2026).
| Appliance | Running Watts | Surge Watts | Off-Grid Notes |
|---|---|---|---|
| LED bulb (9W equivalent to 60W incandescent) | 8–10W | — | Replace all incandescent first; 85% reduction in lighting load |
| 12V compressor fridge (Engel 40L, Dometic CFX3) | 30–60W avg | — | Most efficient off-grid choice; ~0.3–0.6 kWh/day |
| Full-size household refrigerator | 100–200W avg | 400–800W | Nameplate often says 400W — actual avg is 100–200W. Measure it. |
| Chest freezer | 80–150W avg | 300–500W | Often more efficient than upright; ~0.8–1.5 kWh/day |
| Window AC (5,000 BTU) | 450–550W | 1,200W | Major load; avoid if possible — use mini-split instead |
| Mini-split (9,000 BTU) | 700–900W | 1,800–2,500W | More efficient; variable-speed (inverter) models draw less at partial load |
| Well pump (½ HP) | 750W | 2,000–2,500W | Requires low-frequency inverter; kills high-frequency inverters |
| Laptop | 45–65W | — | ~0.3–0.5 kWh/day; far better than desktop (200–400W) |
| Washing machine | 500W | 1,500W | Run during peak solar hours; line-dry instead of electric dryer |
| Electric dryer | 4,000–5,000W | 5,000W | Not practical off-grid. Use propane dryer or line-dry. |
| Router / modem (always on) | 10–20W | — | 24/7 = 0.24–0.48 kWh/day; include in phantom load total |
| Starlink Standard | 50–75W active | — | ~0.6–0.75 kWh/day at 8h use; use sleep schedule to save 0.4 kWh/night |
| CPAP machine | 30–60W | — | Medical priority load — include in baseline without question |
Sources: DIY Solar Forum verified measurements, manufacturer specifications (Engel, Dometic, Starlink), AltE Store community data (accessed 2026-03-25).
Phantom Loads: The Hidden Tax on Your Battery Bank
Devices that draw power even when "off" — TVs in standby, phone chargers with nothing plugged in, smart home hubs — are called phantom loads or standby loads. Individually small, they compound into a significant daily drain.
Typical Phantom Load Sources
- Router/modem (always on)10–20W × 24h = 0.24–0.48 kWh
- Smart TV standby1–5W × 24h = 0.024–0.12 kWh
- Phone charger (idle)0.5–2W × 24h = 0.012–0.048 kWh
- Microwave clock2–7W × 24h = 0.048–0.168 kWh
- Smart home hub2–5W × 24h = 0.048–0.12 kWh
The Real-World Impact
A modest collection of standby devices can add 50–200W continuously, equating to 1.2–4.8 kWh wasted every day.
At $400/kWh battery cost, 1.2 kWh of phantom load waste = $480 in battery capacity bought purely to power devices doing nothing.
Fix: smart power strips, manual switches on non-essential circuits, Kill-A-Watt measurement before finalizing your load figure.
Calculating Your Total Daily kWh
The Formulas
The 1.15–1.25 multiplier covers inverter conversion losses (typically 85–90% efficient), wire losses, and temperature derating. Use 1.25 for long wire runs or extreme climates.
| Appliance | Running W | Hours/day | Wh/day |
|---|---|---|---|
| LED lighting (8 × 9W) | 72W | 5h | 360 Wh |
| Full-size refrigerator (measured) | 150W avg | 10h | 1,500 Wh |
| Laptop | 55W | 5h | 275 Wh |
| TV (32" LED) | 50W | 3h | 150 Wh |
| Router (always on) | 15W | 24h | 360 Wh |
| Phone charging × 2 | 20W | 2h | 40 Wh |
| Total | — | — | 2,685 Wh = 2.69 kWh/day |
| Sizing target (×1.20) | 3.22 kWh/day | ||
Surge Watts and Inverter Sizing: Why Most Beginners Get This Wrong
Every motor-driven device — refrigerators, well pumps, air conditioners, washing machines — draws 2–7× its running watts for 0.5–3 seconds at startup. This is the surge draw (inrush current). Your inverter must handle this peak, not just the continuous running load.
Surge Example: Well Pump
A 3,000W continuous inverter with 4,500W surge fails on a ½ HP well pump:
- Well pump running: 750W
- Well pump surge: 2,500W
- Refrigerator also running: +150W
- Peak demand: ~2,650W
- → Need 3,000W continuous / 6,000W surge minimum
Inverter Sizing Rule
Size for worst-case simultaneous draw, then buy an inverter rated 25% above that for headroom. For well pumps, use a low-frequency (LF) inverter — high-frequency units commonly fail on motor surge.
Battery Bank Sizing Formula
Battery capacity is not just about how much energy you use per day. Two factors reduce usable capacity below the nameplate figure: depth of discharge (DoD) and days of autonomy. Both must be factored in.
The Complete Formula
LiFePO4 (DoD = 80%)
At 3 kWh/day × 2 days ÷ 0.80 = 7.5 kWh required
~$680–860 at 48V rack pricing ($88–115/kWh)
AGM (DoD = 50%)
At 3 kWh/day × 2 days ÷ 0.50 = 12 kWh required
60% more capacity needed vs. LiFePO4 for identical usable energy
LiFePO4 batteries let you use 80% of their rated capacity safely, while AGM batteries should not be discharged below 50% without significantly shortening their lifespan. Full battery chemistry comparison with 2026 pricing →
Days of Autonomy: What the Practitioners Actually Recommend
| Autonomy | Suitable For | Risk | Practitioner Verdict |
|---|---|---|---|
| 1 day | Sunny climates, van/mobile use only | One cloudy day = empty bank | Not recommended for permanent setups |
| 2–3 days ★ Recommended | Most off-grid homes, cabins, homesteads | Low — covers typical cloudy stretches | Practitioner consensus minimum for serious systems |
| 4–7 days | Pacific NW, Alaska, monsoon India | Higher upfront cost | Appropriate for extended low-sun periods; pairs with generator |
Typical Daily Load Ranges by Use Case
Real-user data from DIY Solar Forum and r/offgrid community reports (2025–2026).
| Use Case | Daily kWh | Battery Bank (2-day LiFePO4) | Typical Loads |
|---|---|---|---|
| Van / tiny home (minimal) | 0.5–1.5 kWh | 1.25–3.75 kWh | 12V fridge, LED lights, phone charging |
| Basic cabin | 1.5–3 kWh | 3.75–7.5 kWh | Lights, fridge, laptop, device charging, router |
| Modest off-grid home (no AC, no well pump) | 3–6 kWh | 7.5–15 kWh | Above + washer, chest freezer, Starlink |
| Off-grid home with well pump | 5–10 kWh | 12.5–25 kWh | Above + ½ HP well pump (intermittent) |
| Full homestead with AC | 10–20+ kWh | 25–50+ kWh | Above + mini-split; propane heat keeps this manageable |
Load Reduction Strategies Before You Size
Every watt you eliminate has a better ROI than adding more panels or batteries. Reduce load before finalizing any component purchase.
High-Impact Changes
- Replace all incandescent/CFL with LED. 60W bulb → 9W LED = 85% reduction in lighting load.
- Switch to propane for heating, cooking, and water heating. Electric resistance heating is incompatible with most off-grid solar budgets.
- Use a 12V compressor fridge (Engel, Dometic) instead of a household unit — 0.3–0.6 kWh/day vs. 1–2 kWh/day.
- Line-dry clothes. Electric dryer at 5,000W is not viable off-grid. Propane dryer saves 3–5 kWh per load.
Medium-Impact Changes
- Replace old unrated appliances with energy-star or BEE-rated models. A decade-old fridge may draw 3–5× a new efficient unit.
- Use smart power strips to eliminate phantom loads from entertainment centers and office setups.
- Schedule high-load tasks (laundry, water pumping) during peak solar hours to draw directly from panels rather than draining batteries.
- Put Starlink on a daily power schedule — off at night saves 0.4–0.6 kWh/day.
Seasonal Load Variation: The Trap Most Guides Skip
Your load changes seasonally at the same time as your solar production changes. These two curves usually work against each other.
Load increases: More hours of lighting, heating-adjacent loads
Production drops: Shorter days, lower sun angle, more clouds
Design to winter as your worst case unless you have generator backup.
Load may spike: AC and cooling loads dominate in hot climates
Production peaks: Longest days, highest sun angle
Size AC for summer peak — solar production compensates partially.
Load moderate: Neither cooling nor heating peak
Production good: High sun angles with milder temperatures
Best time to test your system and identify inefficiencies.
Worked Example A: 2 kWh/day Off-Grid Cabin
A 400 sq ft cabin, full-time residence. No well pump, no AC, propane cooking and water heating, line-dry laundry. Based on real builds reported on DIY Solar Forum (2025–2026).
| Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lighting (6 × 9W) | 54W | 5h | 270 Wh |
| 12V compressor fridge (Engel 40L) | 45W avg | 10h | 450 Wh |
| Laptop | 55W | 4h | 220 Wh |
| Router (always on) | 15W | 24h | 360 Wh |
| Phone charging × 2 | 20W | 2h | 40 Wh |
| Starlink (8h use, sleep schedule rest) | 60W | 8h | 480 Wh |
| Subtotal | — | — | 1,820 Wh = 1.82 kWh |
| Sizing target (×1.20) | 2.18 kWh/day | ||
Battery Bank
2.18 kWh × 2 days ÷ 0.80 DoD
= 5.45 kWh
EG4 48V 100Ah (5.12 kWh) ≈ $750 ✓
Inverter
Peak running load: ~200W. No well pump or AC.
1,000–1,500W
Growatt 1000W or Victron Multiplus-II 12/1600
Solar Array
2.18 kWh ÷ 4 PSH × 1.25 = 0.68 kW
2× 400W panels
= 800W; comfortable headroom for growth
Worked Example B: 8 kWh/day Off-Grid Home with Well Pump
A 1,200 sq ft home, family of four. Well pump, chest freezer, washing machine, Starlink, home office. No AC (propane heating and cooking).
| Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lighting (12 × 9W) | 108W | 5h | 540 Wh |
| Full-size refrigerator (measured) | 150W avg | 10h | 1,500 Wh |
| Chest freezer | 100W avg | 8h | 800 Wh |
| Well pump ½ HP (3 cycles/day × 15 min each) | 750W | 0.75h | 562 Wh |
| Washing machine (3 loads/week avg daily) | 500W | 0.43h | 215 Wh |
| Home office (2 laptops + monitor) | 130W | 8h | 1,040 Wh |
| Router + Starlink (12h use) | 75W | 12h | 900 Wh |
| TV + entertainment | 80W | 3h | 240 Wh |
| Phantom loads / misc standby | 50W | 24h | 1,200 Wh |
| Subtotal | — | — | 6,997 Wh = 7.0 kWh |
| Sizing target (×1.20) | 8.4 kWh/day | ||
Battery Bank
8.4 kWh × 3 days ÷ 0.80 DoD
= 31.5 kWh
6× EG4 LifePower4 48V 100Ah ≈ $4,500
Inverter
~1.5kW running + 2,500W well pump surge peak
5,000W LF inverter
Growatt SPF 5000ES or Victron Multiplus-II 48/5000
Solar Array
8.4 kWh ÷ 4.5 PSH × 1.25 = 2.33 kW
6× 400W panels
= 2,400W; size up to 3kW for headroom
Future-Proofing Your System
Expanding an off-grid system after installation is more expensive than sizing correctly the first time. Additional panels usually fit on existing racking. Additional batteries require age-matched chemistry and often a larger charge controller.
Add 10–25% Load Buffer
Add 10–25% to your calculated load before sizing. Covers appliance additions, extra occupants, new loads (EV charging, workshop), and the reality that usage estimates run low.
Buy Battery-Expandable Systems
48V rack-mount LiFePO4 batteries (EG4, Pytes, SOK) can be added in parallel as needs grow — provided you add same-age, same-chemistry batteries. Mixed-age or mixed-chemistry banks fail early.
Oversize the Charge Controller Now
If you plan to add panels later, buy a controller that handles your future array size. Adding a second MPPT later costs $200–500 plus separate cabling. A larger controller day one is almost always cheaper.
Account for Battery Degradation
LiFePO4 batteries degrade ~2–3% per year. A 10 kWh bank at year 10 provides ~8 kWh usable. Size your initial bank to still meet your needs at 80% of original capacity in 10 years.
Free Sizing Calculators
Work through your off-grid daily energy consumption digitally — add appliances, set hours, and get your daily kWh, required battery bank size, and solar array estimate.
External tools: NREL PVWatts · AltE Calculator · BigBattery Sizing Tool
Common Load Calculation Mistakes
Using nameplate wattage for refrigerators and compressors
Nameplate = peak draw, not average. Refrigerators average 25–50% of their nameplate during normal cycling. Always measure with a Kill-A-Watt for any device with a compressor or motor.
Ignoring surge watts when sizing the inverter
A 3,000W inverter that can't surge to 6,000W will trip every time the well pump starts. Know your largest motor's surge draw before buying an inverter.
Designing for 1 day of autonomy
One cloudy day drains your bank entirely. Practitioners recommend 2–3 days minimum; 5–7 days in monsoon climates or the Pacific Northwest.
Using your grid utility bill as your off-grid sizing target
Grid households averaging 800–1,000 kWh/month almost always need to reduce to 150–400 kWh/month for a viable off-grid system. The bill is a starting point for identifying waste, not a target.
Not accounting for seasonal load variation
Winter lighting + reduced solar production is your design case. Summer AC in hot climates is a close second. Don't size for the annual average — size for the worst-case season.
Forgetting phantom loads
Router, smart TV standby, phone chargers, microwave clocks can add 50–200W continuously. Measure and eliminate before finalizing your load number.
Frequently Asked Questions
How do I calculate how much solar power I need for my house?+
Run a load audit first (Steps 1–7 above), calculate your daily kWh, multiply by 1.20 for system losses, then divide by your location's average peak sun hours (PSH). That gives you the solar array size in kW. Use NREL PVWatts for your specific location's PSH data. Example: 5 kWh/day ÷ 4.5 PSH × 1.20 = 1.33 kW of panels.
How many watts does an average household use per day off-grid?+
Real off-grid households typically use 1.5–10 kWh/day depending on whether they have AC, a well pump, and electric cooking. A basic cabin runs 1.5–3 kWh/day. A homestead with a well pump and no AC uses 5–10 kWh/day. Grid averages (800–1,000 kWh/month) are not realistic for off-grid — efficiency upgrades are required.
What appliances use the most electricity in an off-grid home?+
In order of impact: (1) Air conditioning / mini-splits — 700–2,000W running, (2) Electric water heater — 4,000W, (3) Electric clothes dryer — 4,000–5,000W (avoid off-grid), (4) Well pump — 750W running but 2,500W surge, (5) Refrigerator — 100–200W average. Propane heat, cooking, and water heating eliminate three of the top five electrical consumers.
How do you calculate battery bank size for an off-grid system?+
Use this formula: Required Battery kWh = Daily kWh × Days of Autonomy ÷ DoD. For LiFePO4 (DoD = 80%) at 3 kWh/day with 2-day autonomy: 3 × 2 ÷ 0.80 = 7.5 kWh required. For AGM (DoD = 50%), the same scenario requires 12 kWh — 60% more capacity for the same usable energy.
What is a typical daily power consumption for an off-grid cabin?+
A basic cabin (LED lights, 12V compressor fridge, laptop, router, Starlink on a schedule) typically uses 1.5–2.5 kWh/day. Adding a full-size refrigerator pushes this to 2.5–4 kWh. Adding a well pump and washing machine pushes to 5–8 kWh. These are real-user ranges from DIY Solar Forum and r/offgrid (2025–2026).
How much power does a refrigerator use off-grid per day?+
A full-size household refrigerator averages 100–200W during cycling, consuming 1–2 kWh/day. A 12V compressor fridge (Engel, Dometic) averages 30–60W, consuming just 0.3–0.6 kWh/day. Replacing a household fridge with a 12V unit is the single highest-impact efficiency upgrade for van and small cabin builds.
How many solar panels do I need for 1000 kWh per month?+
1,000 kWh/month = ~33 kWh/day. At 5 PSH with a 1.25 loss factor, you need 33 ÷ 5 × 1.25 = 8.25 kW of panels — roughly 20–21 × 400W panels. This is full grid-replacement scale, typically requiring 25+ kWh of battery storage. Most off-grid builders target 150–400 kWh/month after efficiency upgrades.
How do I reduce my electricity consumption for off-grid living?+
Priority order: (1) Replace all bulbs with LED, (2) Switch to propane for heating, cooking, and water heating, (3) Replace old unrated appliances with energy-star models, (4) Use a 12V compressor fridge instead of a full-size unit, (5) Eliminate the electric dryer — propane or line-dry, (6) Put phantom loads on smart power strips, (7) Schedule high-draw tasks during peak solar hours.
Key Takeaways
Start with a load audit. Every other component is sized from your daily kWh figure. This is not optional.
Measure with a Kill-A-Watt ($20–30). Don't guess wattage — especially for fridges, compressors, or any cycling load.
Surge watts size your inverter, not running watts. A well pump needs a low-frequency inverter rated for 2,000–2,500W surge.
Battery formula: Daily kWh × Days of Autonomy ÷ DoD. LiFePO4 = 80% DoD; AGM = 50% DoD.
2–3 days autonomy is the practitioner minimum. 1 day is not enough. 5–7 days for monsoon climates or the Pacific NW.
Grid households average 800–1,000 kWh/month. Off-grid reality is 150–400 kWh/month. Efficiency upgrades are non-negotiable.
Propane for heat, cooking, and water heating eliminates the three largest electrical loads.
Add 10–25% buffer on your calculated load for future growth and the inevitable underestimation in your first audit.
Marcus Sheridan
NABCEP-Certified Solar Installer | 12 Years Off-Grid Experience
Next
Power Conversion and Management
12 min read · Intermediate