How to Size a Solar Generator
Buying too small means the battery dies before the outage does; buying too big means paying for weight and watt-hours you'll never touch. The good news: sizing a solar generator is simple arithmetic. You only need one unit — the watt-hour (Wh) — and three steps: add up your loads, add headroom, then check the inverter can actually run them.
Step 1 — Add up your loads in watt-hours
Watt-hours are just watts × hours. A 60W device running for 5 hours uses 300Wh. Do that for every device you want to keep alive during an outage or trip, then sum them. Most appliances list their watts on a label or in the manual; for anything with a motor, note the running watts and the higher startup surge. Here's a typical home-backup essentials list to model yours on:
| Device | Running watts | Hours/day | Watt-hours/day |
|---|---|---|---|
| Refrigerator (cycles on/off) | ~150 W avg | 24 (duty-cycled) | ~900 Wh |
| Wi-Fi router + modem | 15 W | 24 | 360 Wh |
| Phone charging (×2) | 15 W | 4 | 60 Wh |
| Laptop | 50 W | 4 | 200 Wh |
| LED lights (×4) | 40 W total | 6 | 240 Wh |
| CPAP (no humidifier) | 40 W | 8 | 320 Wh |
| Daily total | ~2080 Wh |
A fridge is the sneaky one: it only draws power while the compressor runs, so a "150W" fridge averages far less over a day — but it surges to 800–1200W for a second each time it kicks on. Budget for the average watt-hours, but size the inverter for the surge (Step 3).
Step 2 — Add ~30% headroom
Never buy a battery sized to your exact total. The label capacity is not the usable capacity. Three things eat into it:
Inverter loss. Converting DC battery power to AC wall power is only ~85–90% efficient, so you lose 10–15% off the top. Battery aging. LiFePO4 packs are rated to 80% capacity after thousands of cycles — over years, your "1000Wh" unit behaves like 800Wh. Cold weather. Batteries deliver less in the cold, exactly when a winter outage hits. Multiply your Step 1 total by 1.3 and shop for that number or higher. The example above (~2080Wh) points you toward a ~2700Wh system, or a shorter runtime on a 1000–1500Wh unit.
Step 3 — Check the inverter (continuous vs surge)
Watt-hours decide how long you run; the inverter decides what you can run at all. Two numbers matter:
Continuous watts — the sum of everything on at the same moment. If your fridge (150W), router (15W), a laptop (50W), and lights (40W) all run together, that's ~255W continuous — trivial for any unit here. Surge watts — the momentary spike when a motor starts, often 2–3× its running watts. A fridge that runs at 150W can spike past 1000W for a heartbeat. If the inverter can't cover that surge, the fridge simply won't start, no matter how big the battery is. A 1500–1800W inverter handles essentials plus a startup surge; only central A/C, electric ranges, and well pumps push past it.
One more rule that pays off for a decade: insist on LiFePO4 (LFP) chemistry. It lasts 4–6× longer than older NMC lithium (3000–4000+ cycles vs 500–800) and is far safer to run indoors. Every unit we recommend uses it.
Worked examples
Weekend camping (phones, lights, a small cooler)
Two phones (60Wh), LED lantern and string lights (150Wh over two evenings), a 45W 12V cooler running half the time (~540Wh/day × 2 ≈ 1080Wh), plus a laptop or drone charge (150Wh). Total ≈ 1440Wh over the weekend; with 30% headroom, ~1900Wh — but you can stretch a 1000Wh unit by recharging with a folding panel each sunny afternoon.
Overnight CPAP
A CPAP without the heated humidifier draws ~30–40W, so an 8-hour night is ~320Wh. Add headroom and you're at ~420Wh — a 500Wh unit covers one night, a 1000Wh unit covers two or three. Turn the humidifier off on battery: it can triple the draw.
Day-long home outage (fridge + Wi-Fi + lights + devices)
Using the table above, ~2080Wh/day. A single 1000–1500Wh portable unit carries the fridge and electronics for the better part of a day; pair it with 200–400W of solar and you refill it each afternoon, turning a one-day battery into indefinite backup. For true multi-day, whole-home coverage you want an expandable system and a transfer switch.
A right-sized starting point for most homes
If your math lands in the common 1000–1500Wh essentials range, our overall pick — the Jackery Explorer 1000 v2 — hits the balance of capacity, a ~1-hour recharge, a 1500W inverter, and a genuinely portable 10.8 lb body. Want faster recharge or a path to whole-home backup? See the EcoFlow Delta 2. On a budget? The Bluetti AC180 gives the most watt-hours per dollar.
Check the top pick's price on Amazon →Ready to compare specific units? Start with our best solar generator for home backup guide, which lines up all three picks by capacity, inverter strength, and recharge speed.
FAQ
How many watt-hours do I need?
Add each device's watts × hours, sum them, then add ~30%. A phone/laptop weekend ≈ 300Wh; a CPAP overnight ≈ 500Wh; a fridge plus Wi-Fi and lights through a day-long outage ≈ 1000–1500Wh.
What is the 30% headroom rule?
Buy a battery about 30% larger than your load total. Usable capacity is below the label because of inverter loss (~85–90% efficient), battery aging (rated to 80% after thousands of cycles), and reduced cold-weather output.
What size inverter do I need?
Continuous watts must exceed everything running at once; surge watts must clear a motor's startup spike (often 2–3× running watts). A 1500–1800W inverter covers essentials short of central A/C or an electric range.
Does a bigger battery always mean more runtime?
More watt-hours means more runtime for the same load — but only what the inverter can deliver. A big battery with a small inverter still won't start a fridge, and LiFePO4 chemistry keeps that capacity usable for far longer than older lithium.