How far does a 1.8 kW off-grid solar system go? Month-by-month modeled output on Hawaii Island vs. the mainland

Disclosure: Case study is the site owner's own system. The equipment (1,780 W of solar panel, a 60 A charge controller, a 314 Ah lithium bank and a 3,000 W inverter, on Hawaii Island) belongs to MiniGridMap's owner. It does not log production, so all production figures here are modeled with NREL PVWatts v8 for a generic 1.78 kW array.

Key findings (modeled)

Steadiest sun of the group: Hilo. In the model, Hilo's best month (Aug, 7.41 kWh/day DC) is only 1.26x its worst month (Dec, 5.86 kWh/day). Seattle swings 4.3x, from 8.73 kWh/day in Jul to 2.01 kWh/day in Dec. Hilo's worst month is about the same as Denver's worst month (5.79 kWh/day in Dec) and 2.9x Seattle's.
The two sides of Hawaii Island model differently. For the same 1.78 kW array, PVWatts gives Kailua-Kona 2,062 kWh/yr AC, 12% less than Hilo's 2,337 kWh/yr. The low month also moves: Kona's is Jun (5.02 kWh/day DC) and Hilo's is Dec. One island can't be sized from one number. (The model uses satellite-derived typical-year weather, so check it against local experience.)
The case-study 12 V bank and its 60 A controller. The site owner's 314 Ah lithium bank is 12 V (12.8 V nominal), so it stores 4.02 kWh, about 3.2 kWh usable at 80% depth of discharge. That covers an illustrative 2 kWh/day load for about 1.6 days without sun. The system's PowMr HHJ-60A controller lists a maximum PV input power of 720 W on a 12 V system in PowMr's own manual (1,440 W at 24 V and 2,800 W at 48 V), against an array nameplate of 1,780 W at STC. The panel wiring isn't documented, so we make no claim about clipping. Check your own controller's manual.

Chart 1: Modeled energy per day, by month

Average modeled DC kWh/day (monthly PVWatts DC output / days in month). Solid lines are Hawaii Island and dashed lines are mainland sites. Hover over a point for its value.

Modeled average DC kWh per day
Month	Hilo, HI	Kailua-Kona, HI	Phoenix, AZ	Denver, CO	Seattle, WA
Jan	6.03	7.13	7.14	6.03	2.42
Feb	6.11	6.07	8.15	7.46	3.69
Mar	6.44	6.12	9.31	8.81	4.95
Apr	6.97	5.90	10.69	9.26	7.26
May	7.36	5.15	10.86	9.36	8.36
Jun	7.12	5.02	10.80	9.96	8.23
Jul	7.35	5.57	9.48	9.55	8.73
Aug	7.41	5.67	9.31	9.02	8.67
Sep	7.23	5.96	9.25	8.74	6.42
Oct	6.87	6.26	8.51	7.42	4.16
Nov	5.91	6.35	7.69	6.72	2.41
Dec	5.86	6.13	7.05	5.79	2.01

Modeled monthly DC energy, kWh/month
Month	Hilo, HI	Kailua-Kona, HI	Phoenix, AZ	Denver, CO	Seattle, WA
Jan	187	221	221	187	75
Feb	171	170	228	209	103
Mar	200	190	289	273	154
Apr	209	177	321	278	218
May	228	160	337	290	259
Jun	214	151	324	299	247
Jul	228	173	294	296	271
Aug	230	176	288	280	269
Sep	217	179	278	262	193
Oct	213	194	264	230	129
Nov	177	191	231	202	72
Dec	182	190	219	180	62
Year	2,455	2,170	3,292	2,985	2,052

Modeled monthly AC energy, kWh/month (default 96% inverter efficiency, DC/AC ratio 1.2)
Month	Hilo, HI	Kailua-Kona, HI	Phoenix, AZ	Denver, CO	Seattle, WA
Jan	178	211	211	178	70
Feb	163	162	218	199	98
Mar	190	180	276	261	146
Apr	199	168	307	264	207
May	217	151	322	277	247
Jun	203	143	310	285	235
Jul	217	164	280	283	258
Aug	219	167	275	267	256
Sep	207	170	265	250	183
Oct	203	185	252	220	122
Nov	169	182	220	192	68
Dec	173	181	209	171	58
Year	2,337	2,062	3,145	2,848	1,949

Embed Chart 1 (copy and paste)

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<text x="56.0" y="354" font-size="12" text-anchor="middle" fill="#555">Jan</text>
<text x="104.5" y="354" font-size="12" text-anchor="middle" fill="#555">Feb</text>
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<text x="444.4" y="354" font-size="12" text-anchor="middle" fill="#555">Sep</text>
<text x="492.9" y="354" font-size="12" text-anchor="middle" fill="#555">Oct</text>
<text x="541.5" y="354" font-size="12" text-anchor="middle" fill="#555">Nov</text>
<text x="590.0" y="354" font-size="12" text-anchor="middle" fill="#555">Dec</text>
<text x="14" y="178" font-size="12" fill="#333" transform="rotate(-90 14 178)" text-anchor="middle">kWh per day (modeled, DC)</text>
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<circle cx="104.5" cy="160.5" r="2.5" fill="#0b7a75"><title>Hilo, HI Feb: 6.11 kWh/day</title></circle>
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<circle cx="444.4" cy="128.2" r="2.5" fill="#0b7a75"><title>Hilo, HI Sep: 7.23 kWh/day</title></circle>
<circle cx="492.9" cy="138.6" r="2.5" fill="#0b7a75"><title>Hilo, HI Oct: 6.87 kWh/day</title></circle>
<circle cx="541.5" cy="166.3" r="2.5" fill="#0b7a75"><title>Hilo, HI Nov: 5.91 kWh/day</title></circle>
<circle cx="590.0" cy="167.6" r="2.5" fill="#0b7a75"><title>Hilo, HI Dec: 5.86 kWh/day</title></circle>
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<circle cx="56.0" cy="131.1" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Jan: 7.13 kWh/day</title></circle>
<circle cx="104.5" cy="161.7" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Feb: 6.07 kWh/day</title></circle>
<circle cx="153.1" cy="160.3" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Mar: 6.12 kWh/day</title></circle>
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<circle cx="250.2" cy="187.9" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI May: 5.15 kWh/day</title></circle>
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<circle cx="395.8" cy="173.2" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Aug: 5.67 kWh/day</title></circle>
<circle cx="444.4" cy="164.8" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Sep: 5.96 kWh/day</title></circle>
<circle cx="492.9" cy="156.2" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Oct: 6.26 kWh/day</title></circle>
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<circle cx="590.0" cy="159.9" r="2.5" fill="#e07a1f"><title>Kailua-Kona, HI Dec: 6.13 kWh/day</title></circle>
<line x1="604" x2="626" y1="56" y2="56" stroke="#e07a1f" stroke-width="3"/><text x="632" y="60" font-size="12" fill="#222">Kailua-Kona, HI</text>
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<circle cx="56.0" cy="131.0" r="2.5" fill="#c0392b"><title>Phoenix, AZ Jan: 7.14 kWh/day</title></circle>
<circle cx="104.5" cy="101.8" r="2.5" fill="#c0392b"><title>Phoenix, AZ Feb: 8.15 kWh/day</title></circle>
<circle cx="153.1" cy="68.6" r="2.5" fill="#c0392b"><title>Phoenix, AZ Mar: 9.31 kWh/day</title></circle>
<circle cx="201.6" cy="28.8" r="2.5" fill="#c0392b"><title>Phoenix, AZ Apr: 10.69 kWh/day</title></circle>
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<circle cx="56.0" cy="162.7" r="2.5" fill="#6c5ce7"><title>Denver, CO Jan: 6.03 kWh/day</title></circle>
<circle cx="104.5" cy="121.6" r="2.5" fill="#6c5ce7"><title>Denver, CO Feb: 7.46 kWh/day</title></circle>
<circle cx="153.1" cy="82.8" r="2.5" fill="#6c5ce7"><title>Denver, CO Mar: 8.81 kWh/day</title></circle>
<circle cx="201.6" cy="70.0" r="2.5" fill="#6c5ce7"><title>Denver, CO Apr: 9.26 kWh/day</title></circle>
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<circle cx="444.4" cy="84.9" r="2.5" fill="#6c5ce7"><title>Denver, CO Sep: 8.74 kWh/day</title></circle>
<circle cx="492.9" cy="122.8" r="2.5" fill="#6c5ce7"><title>Denver, CO Oct: 7.42 kWh/day</title></circle>
<circle cx="541.5" cy="142.9" r="2.5" fill="#6c5ce7"><title>Denver, CO Nov: 6.72 kWh/day</title></circle>
<circle cx="590.0" cy="169.5" r="2.5" fill="#6c5ce7"><title>Denver, CO Dec: 5.79 kWh/day</title></circle>
<line x1="604" x2="626" y1="100" y2="100" stroke="#6c5ce7" stroke-width="3" stroke-dasharray="6 4"/><text x="632" y="104" font-size="12" fill="#222">Denver, CO</text>
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<circle cx="56.0" cy="266.6" r="2.5" fill="#2d6cdf"><title>Seattle, WA Jan: 2.42 kWh/day</title></circle>
<circle cx="104.5" cy="230.0" r="2.5" fill="#2d6cdf"><title>Seattle, WA Feb: 3.69 kWh/day</title></circle>
<circle cx="153.1" cy="193.7" r="2.5" fill="#2d6cdf"><title>Seattle, WA Mar: 4.95 kWh/day</title></circle>
<circle cx="201.6" cy="127.6" r="2.5" fill="#2d6cdf"><title>Seattle, WA Apr: 7.26 kWh/day</title></circle>
<circle cx="250.2" cy="95.9" r="2.5" fill="#2d6cdf"><title>Seattle, WA May: 8.36 kWh/day</title></circle>
<circle cx="298.7" cy="99.4" r="2.5" fill="#2d6cdf"><title>Seattle, WA Jun: 8.23 kWh/day</title></circle>
<circle cx="347.3" cy="85.2" r="2.5" fill="#2d6cdf"><title>Seattle, WA Jul: 8.73 kWh/day</title></circle>
<circle cx="395.8" cy="86.9" r="2.5" fill="#2d6cdf"><title>Seattle, WA Aug: 8.67 kWh/day</title></circle>
<circle cx="444.4" cy="151.5" r="2.5" fill="#2d6cdf"><title>Seattle, WA Sep: 6.42 kWh/day</title></circle>
<circle cx="492.9" cy="216.4" r="2.5" fill="#2d6cdf"><title>Seattle, WA Oct: 4.16 kWh/day</title></circle>
<circle cx="541.5" cy="266.7" r="2.5" fill="#2d6cdf"><title>Seattle, WA Nov: 2.41 kWh/day</title></circle>
<circle cx="590.0" cy="278.3" r="2.5" fill="#2d6cdf"><title>Seattle, WA Dec: 2.01 kWh/day</title></circle>
<line x1="604" x2="626" y1="122" y2="122" stroke="#2d6cdf" stroke-width="3" stroke-dasharray="6 4"/><text x="632" y="126" font-size="12" fill="#222">Seattle, WA</text>
<text x="56" y="378" font-size="10" fill="#777">Source: NREL/NLR PVWatts v8 API, modeled typical-year weather (NSRDB PSM v3), 1.78 kW, tilt 20°, azimuth 180°, 14% losses.</text>
<text x="56" y="392" font-size="10" fill="#777">Accessed 2026-10-03. Modeled, not measured. Chart: MiniGridMap.</text>
</svg>
<figcaption>Modeled with NREL PVWatts v8 by MiniGridMap (https://minigridmap.com/guides/off-grid-solar-hawaii-1-8kw/). Case study is the site owner's own system; figures are modeled, not measured.</figcaption>
</figure>

The case-study battery and controller: a 12 V bank

Equipment (as provided by the site owner, brand and model only):

Solar panels: 4 x REC Alpha 72 XV 445 W (REC445AA 72 XV), 1,780 W total at STC
Charge controller: PowMr HHJ-60A MPPT, 60 A, on the 12 V bank
Battery: 4 x EVE MB31 314 Ah LiFePO4 cells in series (4S): 12.8 V nominal, 4.02 kWh
Battery management system: JK-BMS B2A24S15P (150 A model)
Inverter: BELTTT 3000 W 12 V pure sine wave
Main battery fuse: Blue Sea Systems 5118 Class T, 300 A

The BMS, inverter and fuse are listed for completeness. This page makes no performance claims about them.

The bank is 314 Ah of LiFePO4 at 12 V (12.8 V nominal: 4 cells x 3.2 V). kWh = Ah x V / 1000, so it stores 4.02 kWh, about 3.2 kWh usable at 80% depth of discharge.

80% is a planning assumption: the Dry Power 12LFP100PS datasheet rates LiFePO4 for at least 2,000 cycles at 100% DoD and at least 3,000 at 80%. Cycle life for any specific cell depends on its maker's specification and how it's used.

Case study: 314 Ah at 12 V with a 60 A controller
Item	Value	How it is worked out
Bank	314 Ah, 12 V (12.8 V nominal LiFePO4)	Site owner, confirmed
Stored energy (nameplate)	4.02 kWh	314 Ah x 12.8 V / 1000
Usable at 80% DoD	3.22 kWh	4.02 kWh x 0.80
Days at 1 kWh/day*	3.2	usable kWh / load
Days at 2 kWh/day*	1.6	usable kWh / load
Days at 3 kWh/day*	1.1	usable kWh / load
PowMr HHJ-60A max PV input power, 12 V system	720 W	PowMr HHJ-60A user manual V1.3
Array nameplate	1,780 W at STC	4 x 445 Wp, REC Alpha 72 datasheet
60 A x 14.4 V	about 864 W	Plain arithmetic: 60 A x 14.4 V (the manual's 12 V absorption setting for sealed and gel batteries). Not a PowMr power spec

* Illustrative loads only (1, 2 and 3 kWh/day). These are not measured and are not the case-study household's use. For scale, EIA reports the average U.S. residential customer bought 10,791 kWh in 2022 (about 29.6 kWh/day) and the average Hawaii customer 6,178 kWh (about 16.9 kWh/day). Days are counted with zero solar input, at the battery, before inverter losses.

What the controller's manual says. The system's PowMr HHJ-60A is a 60 A MPPT controller. PowMr's user manual (V1.3) lists:

Maximum PV input power on a 12 V system: 720 W (1,440 W at 24 V, 2,800 W at 48 V).
PV input range on a 12 V system: 20–80 V DC.
Never more than 160 V DC on the PV input. The manual says to check panel open-circuit voltage (Voc) "when connecting panels together".
Charge current: 60 A, with current-limit protection at 61 A.

For context, 60 A x 14.4 V (the manual's 12 V absorption setting for sealed and gel batteries) is about 864 W of charge power. That's plain arithmetic, not a PowMr power spec. The manual's 720 W figure is lower, and it's the one to follow.

The array's nameplate is 1,780 W at STC, more than the manual's 720 W listing. We are not saying the array is clipped or wired wrongly. Its series/parallel layout isn't documented here, and the system doesn't log production. The manual says an over-sized array "will not operate at maximum power point" because the controller limits charging power. Each 445 W panel's REC datasheet Voc is 53.2 V at STC, and a series string's voltage adds up and rises in cold weather.

Check your own system first. Before you wire or add panels, look up your controller's maximum PV input power and voltage range at your battery voltage in its manual. Add up each series string's Voc with a cold-weather margin. If your array is bigger than the manual allows, or anything is unclear, ask the manufacturer or a qualified electrician first.

What changes at higher voltage

Chart 2: the same 314 Ah at 12 V (the case study), 24 V and 48 V. Light bar is nameplate kWh and dark bar is usable at 80% DoD.

What changes at higher voltage (same 314 Ah, same 60 A rating)
Bank voltage	Stored kWh	Usable kWh (80%)	Days @ 2 kWh/day*	HHJ-60A max PV input (manual)
12 V (12.8 V) (case study)	4.02	3.22	1.6	720 W
24 V (25.6 V)	8.04	6.43	3.2	1,440 W
48 V (51.2 V)	16.08	12.86	6.4	2,800 W

Worst modeled month vs. an illustrative 2 kWh/day load
Location	Worst month	DC kWh/day	Ratio to 2 kWh/day*
Hilo, HI	Dec	5.86	2.9x
Kailua-Kona, HI	Jun	5.02	2.5x
Phoenix, AZ	Dec	7.05	3.5x
Denver, CO	Dec	5.79	2.9x
Seattle, WA	Dec	2.01	1.0x

Methodology

Data source: the NREL/NLR PVWatts v8 API (endpoint https://developer.nlr.gov/api/pvwatts/v8.json; documentation), version 8.5.0, accessed 2026-10-03. NREL now publishes as the National Laboratory of the Rockies (NLR). Anyone can reproduce these runs with the inputs below.
Same inputs for every site:

Input	Value
API	PVWatts v8 (8.5.0), https://developer.nlr.gov/api/pvwatts/v8.json
system_capacity	1.78 kW DC (array nameplate)
module_type	0 = Standard
array_type	0 = Fixed, open rack
tilt	20° (all sites, for comparability; not optimized per latitude)
azimuth	180° (true south)
losses	14% (PVWatts default system losses)
dataset	nsrdb (NSRDB PSM v3 GOES TMY-2020)
radius	0 (nearest available weather cell)
dc_ac_ratio	1.2 (API default, not set)
inv_eff	96% (API default, not set)
gcr	0.4 (API default, not set)
timeframe	monthly
Access date	2026-10-03

Sites and NSRDB weather cells
Location	Lat	Lon	Weather cell	Distance	POA kWh/m²/day	Capacity factor %
Hilo, HI	19.7297	-155.0900	19.73, -155.10	1,083 m	4.73	15.0
Kailua-Kona, HI	19.6400	-155.9969	19.65, -155.98	2,121 m	4.22	13.2
Phoenix, AZ	33.4484	-112.0740	33.45, -112.06	1,305 m	6.51	20.2
Denver, CO	39.7392	-104.9903	39.73, -104.98	1,351 m	5.62	18.3
Seattle, WA	47.6062	-122.3321	47.61, -122.34	730 m	3.92	12.5

PVWatts doesn't model batteries, charge-controller limits, or energy wasted once the battery is full, so real off-grid harvest is usually lower. It uses typical-year weather, so any real year will differ.
A 20° tilt is used everywhere so sites compare like-for-like. It isn't optimized for latitude.
The battery figures are arithmetic on nameplate values and a cited DoD, not a measured test.
All production figures are modeled. The case-study system does not log production.

Sources

NREL/NLR PVWatts v8 API, endpoint https://developer.nlr.gov/api/pvwatts/v8.json (all modeled production figures; documentation with input definitions and defaults at this link): https://developer.nlr.gov/docs/solar/pvwatts/v8/ (accessed 2026-10-03)
PowMr HHJ-60A user manual V1.3, linked as "User Manual of HHJ-60A" from powmr.com (https://powmr.com/search?q=HHJ-60A). It gives max PV input power of 720 W (12 V system), 1,440 W (24 V), 2,100 W (36 V) and 2,800 W (48 V); a 12 V-system PV input range of 20-80 V DC; "Ensure input voltage does not exceed 160 VDC"; "MPPT 60A"; limited current protection 61 A; and "The controller will limit charging power in rated charge power. An over-sized PV array will not operate at maximum power point.": https://cdn.shopify.com/s/files/1/0746/0415/1079/files/HHJ-60A_User_Manual_V1.3.pdf?v=1702968165 (accessed 2026-10-03)
REC Alpha 72 Series product datasheet (Ref PM-DS-12-03-Rev-D 02.20, copy hosted by a distributor). The 445 Wp class has Voc 53.2 V at STC and a Voc temperature coefficient of -0.24 %/C: https://www.solarelectricsupply.com/media/custom/upload/ds_rec_alpha_72_series_rev_a_ul_web_en.pdf (accessed 2026-10-03)
Dry Power 12LFP100PS LiFePO4 datasheet (12.8 V nominal; cycle life >=2,000 at 100% DoD, >=3,000 at 80% DoD, >=4,000 at 50% DoD): https://www.drypower.com.au/files/Datasheets/LiFePO4/12lfp100ps-datasheet-un38-3.pdf (accessed 2026-10-03)
U.S. EIA FAQ "How much electricity does an American home use?" (2022: 10,791 kWh/yr average U.S. residential customer; Hawaii 6,178 kWh/yr): https://www.eia.gov/tools/faqs/faq.php?id=97&t=3 (accessed 2026-10-03)
Case-study system description and equipment list (brand and model as provided by the site owner), the site owner's own system: https://minigridmap.com/ (accessed 2026-10-03)

Related tool

If you've recorded your own equipment in GRID COMMAND, its Ask about my system feature (part of the $7 plan) answers from your own equipment list, shows the arithmetic, and says when something needs an electrician. Open the app.

Case study is the site owner's own system. All production figures are modeled, not measured.