Quick answer: start with 1,100 W of solar
Frost protection is possible on solar; comfort heating usually is not. Keeping a small shed above freezing with a 400 W heater for 12 hours at 50% on-time uses about 2,400 Wh a day, roughly 1,100 W of solar on a four-sun-hour day and more in midwinter. Heating the same shed to room temperature with a 1,500 W space heater can take five times that energy or more.
Planning result
At 200 W per module, the array target is 6 panels. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin. Heating is the worst seasonal mismatch in solar: the load peaks in the weeks with the fewest sun-hours, and snow on the panels can take the array to zero for days.
How a shed heater actually uses power
Planning case: about ten feet of thermostat-controlled tubular frost-protection heater (Dimplex rates its tubes at 40 W per foot), or the low setting of a compact heater, at about 400 W, keeping a lightly insulated shed above freezing for 12 winter night hours and switched on about 50% of that time.
A resistive heater turns every watt into heat, so there is no efficiency trick inside the heater: the energy is set by how fast the shed loses heat. The thermostat switches the element fully on or off, and its on-time climbs steeply as the outdoor temperature falls or the wind rises. Insulation, draught-proofing, and a lower set point change the watt-hours far more than the choice of heater.
Load calculation from running watts and duty cycle
The appliance-class estimate is 400 W running for 12 h/day at a 50% duty cycle, an average of about 200 W across those hours:
Then losses and reserve raise the energy the system must collect to about 3,200 Wh/day.
Typical manufacturer nameplate/manual range for the shed heater class, triangulated with Dimplex: Thermostatic tubular heaters specification sheet and U.S. Consumer Product Safety Commission: Keep Warm and Safe This Winter: generators, furnaces and space heaters, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's nameplate and manual before sizing.
Run the heater on a plug-in energy meter through a cold night with the thermostat at your real set point, and note the outdoor low. Repeat on a colder night; the difference shows how quickly the load grows with the weather.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 2,400 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 1,100 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 6 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 4,000 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 315 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 160 Ah | Nominal planning conversion |
Heating runs overnight, so the battery carries nearly all of it. Size storage for the coldest expected night plus at least one cloudy day, and protect a lithium battery from charging below freezing; in an unheated shed its location matters.
Build the closest passing wiring-diagram preset
The Shed Heater Planning System preset is an engine-passing starting point whose array, battery bank, continuous inverter rating, and surge rating each meet or exceed this page's stated planning allowances. Adjust it only from measured load data and verified equipment limits.
Inverter and startup current
Use at least 500 W continuous with 500 W surge for this planning case. A resistive heater has no motor surge; the element draws close to its rated power the moment it switches on. The inverter needs continuous capacity for 400 W plus any fan or light on the same circuit.
Because this load draws its full power for long stretches rather than for a moment, the battery-to-inverter cable carries heavy current continuously. A run that is too long or too small wastes energy as heat and can push the inverter to its low-voltage cutoff late in the day, even with energy left in the battery, so use the diagram machine's real run length and keep every fuse within the equipment and conductor limits. The 450 W peak estimate is a planning figure, not the manual's rating.
The common mistake
Ways to cut the load
- Insulate and draught-proof before buying panels; halving the heat loss halves the daily watt-hours.
- Set the thermostat just above freezing when nobody is in the shed.
- Heat the person, not the building: a heated seat pad or a small radiant panel at the bench uses far less than heating the air.
- For comfort heat, consider a non-electric source and keep solar for lights, ventilation, and tool charging.
- Keep any heater at least three feet from combustibles and plug it straight into a wall outlet, never an extension cord or power strip, as the U.S. Consumer Product Safety Commission advises; a shed is full of things that burn.
What to verify before buying equipment
- The exact running watts, startup current, voltage, frequency, and required circuit from the appliance label and manual.
- Actual watt-hours over a representative day; seasonal heating or cooling loads need a design-day measurement.
- Local peak-sun-hours for the worst operating season, plus shade and array orientation.
- Controller maximum input voltage at the coldest expected temperature and maximum output current.
- Battery discharge current, series/parallel permission, terminal torque, low-temperature controls, and overcurrent protection.
- Conductor ampacity, voltage drop, terminal temperature ratings, disconnecting means, grounding, local amendments, and inspection requirements.
For a whole-system load list, use the load and appliance calculator. For the system context around this appliance, continue to the Off-grid heating and cooling guide.
Frequently asked questions
How much solar do I need for a shed heater?
For the planning case on this page (400 W of thermostat-controlled tubular heat keeping a shed above freezing), allow about 1,100 watts of solar at four peak-sun-hours per day. That covers an estimated 2,400 Wh daily load plus conversion, temperature, and recharge margin. In midwinter, with fewer sun-hours and possible snow cover, plan on more array or a backup charging source.
What size inverter should I use for a shed heater?
Plan on at least 500 W continuous and 500 W surge. There is no motor surge, so continuous headroom is what matters; the heater manual and thermostat type still govern.
What battery size keeps a shed heater running for one day?
The one-day planning allowance is about 4,000 Wh nominal usable design capacity, roughly 315 Ah at 12.8 V or 160 Ah at 25.6 V for LiFePO4. Because the heater runs overnight, plan extra storage for consecutive cold, cloudy days.