Every sizing question starts with one number: how many watt-hours (Wh) you use per day. List each load, multiply watts by hours of use, and add them up. A fridge rated 150 W that actually cycles about 8 hours a day is 1,200 Wh. Lights at 60 W for 5 hours is 300 Wh. If you have utility bills, kWh per day × 1,000 gives you the same figure. Get this right and everything below falls out of simple math.
1. Solar Panel Sizing
Panels have to refill what you used each day using only the “peak sun hours” your location gets, the number of hours of equivalent full-strength sun. Most of the US sees roughly 3 to 6; use your worst season (usually winter) if you are fully off-grid. Then pad for real-world losses: heat, dirt, wiring, charge controller and battery losses typically eat 20–25% of nameplate.
Worked example: off-grid cabin
The cabin uses 4,000 Wh/day and gets 4 peak sun hours in winter. 4,000 ÷ 4 = 1,000 W, then 1,000 ÷ 0.75 ≈ 1,333 W. That is four 350–400 W panels. For a house using 30 kWh/day (30,000 Wh) at 4.5 sun hours: 30,000 ÷ 4.5 ÷ 0.75 ≈ 8,900 W, or about 22 400 W panels.
Things that change the answer
- Charge controller limits. Every MPPT controller (or hybrid inverter) has a max PV input in watts, a max open-circuit voltage and a max current. Size strings to stay under the voltage limit on the coldest morning, because panel voltage rises in the cold.
- Shading and tilt. A flat or shaded array can lose far more than 25%. Add panels rather than hoping.
- Overpaneling. Many inverters allow more panel watts than their rated output; that helps on cloudy days and winter mornings. Check the spec sheet before doing it.
Compare current solar panel deals and mounting hardware once you have your array size.
2. Inverter Sizing
An inverter is sized by power (watts at any instant), not energy (Wh per day). Two ratings matter: continuous, what it can run all day, and surge, what it can deliver for a fraction of a second while a motor starts.
Surge rating ≥ running loads + starting surge of the largest motor
Worked example: what size inverter for my house?
Suppose the worst moment is a fridge (150 W), well pump (1,000 W running), microwave (1,200 W) and lights/TV (300 W) all at once: 2,650 W. Times 1.25 ≈ 3,300 W continuous, so a 4,000–6,000 W class inverter. Motors often pull 3–7× running watts for a moment, so that 1,000 W pump may want 3,000–5,000 W of surge on top of everything else. Low-frequency (transformer) inverters usually handle surge better than high-frequency ones of the same continuous rating; check the surge spec and how long it lasts.
Other sizing checks
- 240V loads. Well pumps, dryers and mini-splits may need split-phase 120/240V output, or two inverters stacked.
- Idle draw. A big inverter can burn 30–100 W just being on. That is 0.7–2.4 kWh a day, so don’t oversize wildly for a small cabin.
- Battery current. Inverter DC amps = watts ÷ battery voltage. 3,000 W at 12V is 250 A; at 48V it’s about 63 A. This is the main reason big inverters are 48V.
See current hybrid and off-grid inverter deals.
3. Battery Sizing
Battery capacity is energy, so it comes from daily Wh, how many days you want to ride through without sun (“autonomy”), and how deep you can safely discharge. LiFePO4 is commonly run to 80–90% depth of discharge (DoD); lead-acid is usually limited to about 50% for decent life.
Amp-hours = Battery Wh ÷ battery voltage
Worked example: how many batteries to run a fridge and lights?
Fridge 1,200 Wh + lights 300 Wh + phone/laptop 200 Wh = 1,700 Wh/day. Two days of autonomy on LiFePO4 at 90% DoD: 1,700 × 2 ÷ 0.9 ÷ 0.9 ≈ 4,200 Wh. At 12V that is 4,200 ÷ 12.8 ≈ 330 Ah (e.g. three or four 100 Ah batteries); at 48V it is about 82 Ah, or one 5 kWh rack battery.
Worked example: whole house
10,000 Wh/day, one day of autonomy, 90% DoD: 10,000 ÷ 0.9 ÷ 0.9 ≈ 12,300 Wh. Using 5 kWh batteries, that is three (15 kWh). Add a generator for long cloudy stretches rather than buying a week of batteries.
- Check discharge current, too. The battery bank must supply the inverter’s peak DC amps. A 100 A BMS can’t feed a 6,000 W inverter at 48V (125 A) on its own; parallel batteries share the load.
- Charging. Make sure your array can actually refill the bank in your sun hours, or the extra batteries just sit partly empty.
- Cold. LiFePO4 should not be charged below freezing unless it has built-in heating or the BMS blocks it.
Browse current battery deals.
4. 48-Volt Systems
The physics is amps = watts ÷ volts. Quadruple the voltage and the current drops to a quarter for the same power. Wire losses scale with current squared (I²R), so at the same wire size a 48V system loses about one-sixteenth as much as a 12V system. That means thinner, cheaper cable, smaller fuses and busbars, and less heat at every lug.
Worked example: 5,000 W load
- 12V: 5,000 ÷ 12 ≈ 417 A, which needs very heavy cable and is impractical for most builds.
- 24V: ≈ 208 A, workable with 4/0-class cable over short runs.
- 48V: ≈ 104 A, manageable with common cable sizes and breakers.
Most whole-home hybrid inverters and server-rack LiFePO4 batteries are built for 48V (a nominal 51.2V for 16-cell LiFePO4), so that’s where the widest choice of compatible gear is. Trade-offs: 48V still deserves respect (use proper DC breakers and insulated tools), and it’s less convenient if all your loads are native 12V appliances, which then need a DC-DC converter.
5. 48 Volt vs 24 Volt vs 12 Volt
Choose voltage by the size of the inverter you need and what your loads run on. Rough guidelines:
| System | Typical inverter size | Good fit | Watch out for |
|---|---|---|---|
| 12V | Up to ~1,500–2,000 W | Vans, RVs, small boats, sheds: lots of native 12V loads, alternator charging | Very high current above ~1,500 W; thick cable, voltage drop |
| 24V | ~1,500–3,000 W | Larger RVs, boats, weekend cabins | Fewer 24V appliances; may need a 24→12V converter |
| 48V | 3,000 W and up | Full-time cabins, homes, backup systems, anything that may grow | Harder to use 12V loads directly; not typical for vehicle alternator charging |
Which voltage for an RV or cabin?
An RV running a fridge, lights, fans and a small inverter for a microwave or coffee maker is usually happiest on 12V, since the chassis is already 12V. If you want to run an air conditioner off batteries, the inverter gets big enough that 24V or 48V starts to pay off. For a cabin, if you might ever add a well pump, power tools or a mini-split, start at 48V; upgrading voltage later means replacing the inverter, charge controller and battery bank.
Calculators
Run the formulas above with your own numbers. Each result shows the math.
Estimates only. These calculators give planning numbers, not a design. Always check manufacturer specs, the NEC and your local code, and have a licensed electrician review anything you tie into a panel.
Solar panels · Inverter · Battery · Voltage drop
FAQ
How many solar panels do I need to run a house off-grid?
Daily Wh ÷ peak sun hours ÷ 0.75. At 10,000 Wh/day and 4.5 sun hours that’s about 3,000 W, roughly seven 400 W panels.
What size inverter do I need?
Add up everything that can run at once, multiply by 1.25 for continuous rating, and check the surge rating covers your largest motor starting.
How many batteries do I need?
Daily Wh × days of autonomy ÷ DoD ÷ 0.9, then divide by each battery’s Wh. 5,000 Wh/day for one day on LiFePO4 is about 6,200 Wh, so two 5 kWh batteries.
Should I build a 12V, 24V or 48V system?
12V for small mobile systems, 24V for mid-size, 48V for anything about 3,000 W and up or anything you might expand.